Soil Horizons

Soil profiles are often many meters deep, but with the majority of studies in soil microbiology focusing exclusively on the soil surface, we know very little about the nature of the microbial communities inhabiting the deeper soil horizons. We used phospholipid fatty acid (PLFA) analysis to examine the vertical distribution of specific microbial groups and to identify the patterns of microbial abundance and communitylevel diversity within the soil profile. Samples were collected from the soil surface down to 2 m in depth from two unsaturated Mollisol profiles located near Santa Barbara, CA, USA. While the densities of microorganisms were generally one to two orders of magnitude lower in the deeper horizons of both profiles than at the soil surface, approximately 35% of the total quantity of microbial biomass found in the top 2 m of soil is found below a depth of 25 cm. Principal components analysis of the PLFA signatures indicates that the composition of the soil microbial communities changes significantly with soil depth. The differentiation of microbial communities within the two profiles coincides with an overall decline in microbial diversity. The number of individual PLFAs detected in soil samples decreased by about a third from the soil surface down to 2 m. The ratios of cyclopropyl/monoenoic precursors and total saturated/total monounsaturated fatty acids increased with soil depth, suggesting that the microbes inhabiting the deeper soil horizons are more carbon limited than surface-dwelling microbes. Using PLFAs as biomarkers, we show that Gram-positive bacteria and actinomycetes tended to increase in proportional abundance with increasing soil depth, while the abundances of Gram-negative bacteria, fungi, and protozoa were highest at the soil surface and substantially lower in the subsurface. The vertical distribution of these specific microbial groups can largely be attributed to the decline in carbon availability with soil depth. q

The long-term role of fire in coastal temperate rain forest is poorly understood. To determine the historical role of fire on western Vancouver Island (British Columbia, Canada), we constructed a long-term spatially explicit fire history and examined the spatial and temporal distribution of fire during the Holocene. Two fire-history parameters (time-since-fire [TSF] and fire extent) were related to three landscape parameters (landform [hill slope or terrace], aspect, and forest composition) at 83 sites in a 730-ha low-elevation (less than ϳ200 m) area of a mountainous watershed. We dated fires using tree rings (18 sites) and 120 soil-charcoal radiocarbon dates (65 sites). Comparisons among multiple radiocarbon dates indicated a high probability that the charcoal dated at each site represented the most recent fire, though we expect greater error in TSF estimates at sites where charcoal was very old (Ͼ6000 yr) and was restricted to mineral soil horizons. TSF estimates ranged from 64 to ϳ12 220 yr; 45% of the sites have burned in the last 1000 yr, whereas 20% of the sites have not burned for over 6000 yr. Differences in median TSF were more significant between landform types or across aspects than among forest types. Median TSF was significantly greater on terraces (4410 yr) than on hill slopes (740 yr). On hill slopes, all south-facing and southwest-facing sites have burned within the last 1000 yr compared to only 27% of north-and east-facing sites burning over the same period. Comparison of fire dates among neighboring sites indicated that fires rarely extended Ͼ250 m. During the late Holocene, landform controls have been strong, resulting in the bias of fires to south-facing hillslopes and thus allowing late-successional forest structure to persist for thousands of years in a large portion of the watershed. In contrast, the early Holocene regional climate and forest composition likely resulted in larger landscape fires that were not strongly controlled by landform factors. The millennial-scale TSF detected in this study supports the distinction of coastal temperate rain forest as being under a fundamentally different disturbance regime than other Pacific Northwest forests to the east and south.

We evaluate the relationship between the carbon-to-nitrogen ratio (C:N) of the soil organic horizon and nitrate leaching in runoff or seepage water from 33 conifer forests across Europe. The sites span a geographical range covering 11 countries from Ireland to western Russia and Finland to the southern Alps, and encompass a wide range in throughfall nitrogen deposition. The aim of the study is to evaluate the hypothesis that the C:N ratio of the organic (OH) horizon can be used to estimate the level of leaching of nitrate from a forest ecosystem. The analysis suggests that C:N ratio can be an indicator of nitrate leaching for conifer forests across Europe if these ecosystems are grouped into broad categories of throughfall nitrogen deposition. At low levels of N deposition (< 10 kg N ha-' year-'), nitrate leaching is low regardless of the OH C:N ratio. At intermediate (10-20 kg N ha-' year-') and high (~20 kg N ha-' year-') N deposition, nitrate leaching increases with decreasing C:N ratio. In addition, for any given value of C:N, the level of nitrate leaching is higher at high N-deposition sites than at intermediate N-deposition sites. From the current data, OH horizon C:N ratio can give a reasonable estimate of the annual export flux of nitrate (95% confidence interval ca +5 kg N ha-' year-') for sites receiving throughfall-N up to about 30 kg N ha-' year-'. Above this level, the variability in the data increases, suggesting other factors may need consideration to refine estimates of nitrate leaching.

Interactions among plants, soil, hydrology, and microbes regulate nutrient cycling and loss in ecosystems. Variability among these factors is likely to regulate patterns of productivity and N cycling along topographic gradients in forest and other terrestrial ecosystems. Our objectives were to determine interrelations among spatial and temporal patterns in microbial biomass, N transformation rates (net N mineralization, net nitrification, denitrification potential) and soil, plant, and stream variables along an elevational gradient (525-775 m) at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in the White Mountains of New Hampshire, USA. We examined these relationships in the forest floor (O e and O a horizons) and upper mineral soil to assess the contribution of these different layers to overall microbial biomass and N cycling rates and to examine potential differences among soil layers in the spatial and temporal variation of these soil characteristics and their correlations with one another. Broad patterns in microbial biomass and N transformation rates were correlated in space and time and varied with elevation, soil horizon, season, and year. Both microbial biomass and N cycling activities were greater in summer than in fall or spring, although the magnitude of seasonal differences was much greater for the N cycling activities than for microbial biomass. Nitrification rates and denitrification enzyme activity were greatest at the highest elevation site, despite the predominance of beech (Fagus grandifolia) in the canopy at that site, which would be expected to inhibit these activities. Differences among years in precipitation may have driven annual variation in N transformation rates, which were correlated with annual variation in litter N content. Elevational patterns in nitrification were broadly correlated with elevational patterns in stream nitrate (NO 3 Ϫ ) concentration, suggesting an important link between soil N transformations and nutrients in stream water along this elevational gradient. These results indicate that interactions among plant communities, soil characteristics, and soil microbial communities determine spatial and temporal patterns of N transformations, which are potentially linked to variation in stream nutrient concentrations and outputs at the watershed scale in these northern hardwood forest ecosystems.

Wooded meadows are seminatural plant communities that support high diversity of various taxa. Due to changes in land use, wooded meadows have severely declined during the last century. The dominant trees in wooded meadows acquire mineral nutrients via ectomycorrhizal fungi. Using anatomotyping and sequencing of root tips, interpolation and extrapolation methods, we studied the diversity and community structure of ectomycorrhizal fungi in two soil horizons of both managed and forested parts of a wooded meadow in Estonia. Species of Thelephoraceae, Sebacinaceae and the genus Inocybe dominated the whole ectomycorrhizal fungal community of 172 observed species. Forested and managed parts of the wooded meadow harboured different communities of ectomycorrhizal fungi, whereas soil horizon had a negligible effect on the fungal community composition. Diverse soil conditions and host trees likely support the high richness of ectomycorrhizal fungi in the wooded meadow ecosystem. Direct sequencing integrated with interpolation and extrapolation methods are promising to identify the fungi at the species level and to compare species richness between communities of ectomycorrhizal fungi.

The preferential retention of heavy metals by soils is critical to their availability and mobility through the soils to contaminate groundwater. We examined the competitive adsorption of Cu and Zn by Bt horizon of a Savanna Alfisol from a dilute salt solution under a variety of conditions. Copper and Zinc adsorption from the Cu-Zn binary solution produced a characteristic H-type isotherm for Cu, and a linear isotherm for Zn. The distribution coefficient K d , which defines the affinity of the metals for the soil adsorbents, was five times greater for Cu than Zn. The adsorption envelope of the metals showed that maximum Cu adsorption occurred at pH 5.5 as against pH 6.8 for Zn adsorption probably reflecting difference in the first hydrolysis constant of the metals. The removal of organic matter from the soil reduced K d,Cu 40 times as compared to the natural soil, whereas K d,Zn was reduced by half as compared to the natural soil. Similarly, removing amorphous hydrous oxide by extraction with oxalate solution prior to the introduction of Cu and Zn reduced K d,Cu 100 times, and K d,Zn by 20 times as compared to the natural soil, but when both amorphous and crystalline hydrous oxides were removed prior to the introduction of Cu and Zn, the K d,Cu was on par with the natural soil, whereas K d,Zn increased by more than two times as compared to the natural soil. It seems that reactions with organic matter and amorphous oxides are the major controls on Cu, whereas Zn might be largely sorbed by cation exchange reaction when in competition with Cu for binding sites in soils. The metal binding sites in the amorphous hydrous oxides and organic matter were more selective for Cu than Zn.

Since geophysical methods are non-invasive, they can be of great help in soil studies because they disturb neither the structure nor the dynamics of the soil. Moreover, data are acquired with reliable spatial sampling. The usual ways of investigation, like augering and excavation, disturb the soil and are totally incompatible with a spatially dense sampling strategy, which would destroy the object of the study. Both approaches are complementary when excavations have a limited extent and are distributed according to the information conveyed by the geophysical investigation. A basic principle of applied geophysics is to measure different physical parameters without direct Ž . access to the studied volume. Horizontal andror vertical variations of the parameter s can be recorded. Possible soil parameters should be restricted to measurements which do not alter the Ž . Ž . medium reversible effect . To be significant, the variations of the parameter s should exhibit a wide dynamic range over different soil types and should be correlated in some way to soil parameters such as particle size or hydraulic conductivity. After summarising the soil properties, two examples are shown whereby electrical resistivity was used. The first example is a specific Ž . soil so-called hardpan sandy soil in arid area in Lagadge, North Cameroon. Using resistivitý surveys the three dimensional extension of a very coherent horizon was mapped. This horizon is Ž . delineated by low resistivities -100 V m conductivities ) 10 mSrm because of the disposition of clay particles around the quartz grains. In a second example, a ''homogeneous'' area ought to be found delimit the extent of a surface where a pesticide transfer experiment is to take place. Accurate mapping of soil horizons was not feasible by augering. Resistivity data have clearly shown the three-dimensional extension of clayey horizons in the complex delta context. q

Plant allocation patterns may affect soil C and N storage due to differences in litter quality and the depth of plant C and N inputs into the soil. We studied the dynamics of dual-labeled ( 13 C/ 15 N) Pinus ponderosa needles and fine roots placed at two soil depths (O and A horizon) in a temperate conifer forest soil during 2 y. Input of C as fine roots resulted in much more C retained in soil (70.5 ± 2.2% of applied) compared with needle C (42.9 ± 1.3% of applied) after 1.5 y. Needles showed faster mass loss, rates of soil 13 CO 2 efflux, and more 15 N immobilized into microbial biomass than did fine roots. The larger proportion of labile C compounds initially present in needles (17% more needle C was water soluble than in fine roots) likely contributed to its shorter C residence time and greater degree of transformation in the soil. A double exponential decay function best described the rate of 13 C loss, with a smaller initial pulse of C loss from fine roots (S 1 k 1 ) and a slower decay rate of the recalcitrant C pool for fine roots (0.03 y À1 ) compared with (0.19 y À1 ) for needles. Soil 13 C respiration, representing heterotrophic respiration of litter C, was much more seasonal from the O horizon than from the A. However, offsetting seasonal patterns in 13 C dynamics in the O horizon resulted in no net effect of soil depth on total 13 C retention in the soil after 1.5 y for either litter. Almost 90% of applied litter N was retained in the soil after 1.5 y, independent of litter quality or soil depth. Very small amounts of 13 C or 15 N (<3% of applied) moved to the horizon above or below the placement depth (i.e., O to A or A to O). Our results suggest that plant allocation belowground to fine roots results in more C retained and less N mineralized compared with allocation aboveground to needles, primarily due to litter quality differences.

Soil moisture strongly controls the uptake of atmospheric methane by limiting the diffusion of methane into the soil, resulting in a negative correlation between soil moisture and methane uptake rates under most non-drought conditions. However, little is known about the effect of water stress on methane uptake in temperate forests during severe droughts. We simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA. The treatment significantly increased CH 4 uptake during the first weeks of throughfall exclusion in 2001 and during most of the 2002 treatment period. Low summertime CH 4 uptake rates were found only briefly in both control and exclusion plots during a natural late summer drought, when water contents below 0.15 g cm K3 may have caused water stress of methanotrophs in the A horizon. Because these soils are well drained, the exclusion treatment had little effect on A horizon water content between wetting events, and the effect of water stress was smaller and more brief than was the overall treatment effect on methane diffusion. Methane consumption rates were highest in the A horizon and showed a parabolic relationship between gravimetric water content and CH 4 consumption, with maximum rate at 0.23 g H 2 O g K1 soil. On average, about 74% of atmospheric CH 4 was consumed in the top 4-5 cm of the mineral soil. By contrast, little or no CH 4 consumption occurred in the O horizon. Snow cover significantly reduced the uptake rate from December to March. Removal of snow enhanced CH 4 uptake by about 700-1000%, resulting in uptake rates similar to those measured during the growing season. Soil temperatures had little effect on CH 4 uptake as long as the mineral soil was not frozen, indicating strong substrate limitation of methanotrophs throughout the year. Our results suggest that the extension of snow periods may affect the annual rate of CH 4 oxidation and that summer droughts may increase the soil CH 4 sink of temperate forest soils. q

Improvement of N fertilizer recovery effi ciency (NRE) is necessary to reduce excess N entering the environment and to increase economic returns. A 2-yr fi eld trial was established in 2005 in Northeast Missouri to determine the eff ects of conventional and slow-release N fertilizer sources and landscape position (summit, sideslope, and low-lying) on crop growth and NRE in corn (Zea mays L.) in a claypan soil. Nitrogen fertilizer treatments at 168 kg N ha-1 consisted of pre-plant applied and incorporated polymer-coated urea (PCU), urea, 50% PCU/50% urea mix by weight, anhydrous ammonia (AA), and a nontreated control. Gravimetric soil water content data in 2005 and 2006 indicated that the low-lying position was oft en wetter than the summit and sideslope positions. Anhydrous ammonia and PCU treatments increased grain yield 1470 to 1810 kg ha-1 over urea in the lowlying position in 2005 and 2006. Corresponding increases in plant N uptake and NRE were observed in the low-lying position in 2005 with use of PCU and AA compared to urea, but not in 2006. Based on the grain yield results in this research and diff erent fertilizer cost and crop prices, gross profi t diff erences for use of PCU and preplant-applied AA compared with urea in the lowlying position could range from $50 to $642 ha-1. Th ese results demonstrate that targeted use of diff erent N fertilizer sources in claypan landscapes may increase grain yields and profi tability.

In this paper we compare linear regression with tree regression for analysis of the influence of soil properties on the sorption and retention of added Cd, Cu and Pb by 20 soil horizons typical of cropped soils in Galicia (N.W. Spain); our measure of sorption/retention capacity was K r , a recently introduced adimensional parameter. Sorption and retention of Cd was depressed by the presence of Cu and Pb. The soil parameters that were most associated, overall, with differences in Cd, Cu and Pb sorption and retention were cation exchange capacity (CEC), pH and Mn oxides' content. Tree regression, which can take into account variation on both global and local scales, afforded better-fitting models than linear regression, which only reflects global tendencies; but for coherent interpretability of tree regression results it is just as important to avoid overfitting as in the case of linear regression.

Current wet chemical methods for the speciation of sulphur (S) in soils are inaccurate and do not allow one to assess the S speciation of individual soil particles and colloids. X-ray microscopy and Near Edge X-ray Absorption Fine structure Spectroscopy (NEXAFS) can be used to study individual species of S at the K-adsorption edge. We have used these techniques to identify and quantify S species in bulk soil, soil particles and colloids from Oh and Bh horizons of two forested Podzols. The partitioning of soil sulphur as determined on bulk samples of the Oh horizons by X-ray spectromicroscopy agreed fairly well with the results of a conventional S speciation for the soil at Schluchsee, and reasonably well for that at Rotherdbach. The NEXAFS analyses on individual soil particles revealed that they are richer in reduced organic sulphur than the bulk soil for the Schluchsee Oh and richer in sulphate for Rotherdbach Oh. The techniques can be used reliably to separate and quantify sulphur species with different oxidation states in the soil. The combination of X-ray transmission and sulphur fluorescence images with unfocused and focused NEXAFS spectra at the K-adsorption edge of sulphur at specific microsites allowed us to compare the distribution of S species in bulk soil with that of distinct soil particles and soil colloids. Moreover, we can use it to assess the spatial distribution of different S species on soil particles on a scale of a few hundred nanometres.

The effects of prescribed burning, herbicide application, and thinning treatments were examined in upland Quercus stands with a replicate block field study. Two site types were analyzed: (1) nonsuccessional with a Quercus overstory and Quercus understory, and (2) successional with a Quercus overstory and an Acer saccharum understory. Treatments were examined for their effects on vegetation structure and composition, especially with respect to altering the natural succession from Quercus communities to mesophytic species. In addition, impacts of burning on soil nutrients were examined prior to and immediately following burn treatments. Results from the study indicate that fire had little effect on A horizon soil nutrients following burns. The data suggest fire plays a more important role as a decomposer for nutrient release in February burns than in November burns. Natural precipitation and leaching from the litter layer in the fall rendered similar nutrient dynamics as burning in November. The structural changes to vegetation following fire were more noticeable than compositional changes. Thinning of the canopy was generally necessary for the understory to respond to burning treatments. Burning did not affect stems >3.8 cm dbh (diameter at breast height). Density of most woody species <3.8 cm dbh decreased with burning, but increased with burning and thinning, indicating the stress of shaded conditions. Thus, burning prior to thinning may be an effective method for controlling an A. saccharum invasion into Quercus stands. Herbaceous, seedling, sapling, and overstory strata diversity were unaffected by the treatments. Data indicated that the community responds with species extant at the time of treatment. Thus, the initial conditions prior to disturbance were important factors in the response of the stand to disturbance. Although fire has played a major role in upland forest dynamics prior to settlement, forest structure and composition can no longer be maintained by fire. Succession without fire has resulted in a different structure and composition to oak forests, altering both fuels and fuel moisture levels. Species normally considered intolerant of fire have reached size classes large enough that they tolerate cool to moderate burns. More severe disturbance techniques, or combinations of techniques, are necessary for sustainable oak forest management in the central hardwood region.

A pedo-geochemical survey was carried out in the Nord-Pas de Calais region (France) on soils developed in sedimentary materials to estimate their baseline geochemistry and the effect of selected factors on it. Total concentrations of Al, Fe and 18 trace elements, as well as common soil characteristics, were determined in samples from 271 surface and 487 deep horizons developed in 22 sedimentary rocks and superficial deposits. Compared with the upper continental crust, the parent materials are enriched with most of the trace elements, particularly with As, Cd, Sb and Se, and are impoverished in Cu. Calcareous rocks from the Carboniferous, Cretaceous and Jurassic eras are often enriched in Cd. Correlation analysis shows that the least mobile (i.e. ionic potential (Z/r) ranges from 3 to 7) geogenic elements (Bi, Co, Cr, Cu, In, Ni, Pb, Sn, Tl, V, Zn) are associated with the b 2 μm fraction (which we define as "lutum"). More mobile elements (As, Cd, Hg, Mn, Mo, Sb, Se) are less strongly associated with this fraction. Cadmium is particularly associated with Mn. Topsoils are enriched with all the trace elements examined, except Co, Cr and Ni, probably because of anthropogenic contamination. The amount of Pb introduced into the soils of the entire region (12 450 km 2 ) by human diffuse contamination is estimated to be 45 000 metric tonnes. Organic matter seems to act as a sink for numerous exogenous trace elements. The distribution of [trace element]/([Al] or [Fe]) in the deep soil horizons from Nord-Pas de Calais gives the background contents for soils developed from most sedimentary materials from the Brussels-London and the Parisian basins.

Because soils are both a source and a sink for atmospheric CO 2 , there is an increasing need to characterize the spatial distribution of soil C pools. Large amounts of organic carbon (OC) accumulate in hydric bottom-lands soils. In the Armorican Massif (Western France) where these soils represent 20% of the total surface area, the spatial characterization of OC pools is difficult to assess due to methodological problems such as high spatial variability. Soil color indexes, which combine various characteristics of soil horizons or profiles, are an alternative approach for quantifying the differences in OC storage. In addition, terrain attributes derived from Digital Elevation Models (DEM) may be useful in characterizing the distribution of soil color indexes over large areas. Thus, the overall goal of this work was the development and application of a model for use in predicting the organic carbon (OC) content of soil areas. To accomplish this, extensive examination of soil morphology combined with selected terrain attributes measured in the field and calculated from a digital elevation model (DEM) were used. Soil samples were collected in Western France from a 2-ha agricultural parcel that forms the major part of a hillslope. The results indicate that OC stocks of the entire profile were correlated highly to a soil hydromorphic index (HI) (r 2 ‫؍‬ 0.80). HI is a function of the percent of the total soil profile depth constituted by horizons with some degree of hydromorphic feature development and the moist color of the surface A horizon. Using a stepwise regression technique, we constructed a prediction model of HI distribution by using the relations between HI and (i) the elevation above the stream bank (ES) (r 2 ‫؍‬ 0.80); (ii) the downslope gradient (DG) (r 2 ‫؍‬ 0.55); and (iii) the upslope contributing area (AMU) (r 2 ‫؍‬ 0.60). Validation of this model on a second site showed that topographical attributes explained up to 75% of the profile OC stock variability. These results confirmed that the integration of a soil index and topographical information is a useful tool for prediction of OC distribution. In addition, the use of soil morphologic indexes could significantly improved the construction and the validation of soil-landscape models because it would minimize laboratory measurements of OC reservoirs. (Soil Science 2001;166:48-60)

1] Fine root (<2 mm) cycling rates are important for understanding plant ecology and carbon fluxes in forests, but they are difficult to determine and remain uncertain. This paper synthesizes minirhizotron and isotopic data and a root model and concludes that (1) fine roots have a spectrum of turnover times ranging from months to many years and (2) the mean age of live root biomass (A) and the mean age of roots when they die (i.e., their turnover time (t)) are not equal. We estimated A and t of fine roots in three forests using the root model Radix. For short-lived roots, we constrained t with existing minirhizotron data; for long-lived roots, we used new radiocarbon measurements of roots sampled by diameter size class and root branch order. Long-lived root pools had site mean t of 8-13 y and 5-9 y when sampled by diameter and branch order, respectively. Mean turnover times across sites were in general not significantly different as a function of branch-order, size class, or depth. Our modeling results indicate that ∼20% of fine root biomass has turnover times of about a year, and ∼80% has decadal turnover times. This partitioning is reflected in our predicted mean ages of ∼9 y and turnover times of ∼3 y. We estimate that fine root mortality contributes between 38 and 104 g C m −2 y −1 to soil in these forests. These estimates are 20 to 80% of previous estimates in these and similar forests, in part because we explicitly account for the large portion of fine-root biomass with decadal cycling rates. Our work shows that both fast and slow cycling roots must be modeled jointly to account for the heterogeneous nature of fine-root dynamics. (2010), Measuring and modeling the spectrum of fine-root turnover times in three forests using isotopes, minirhizotrons, and the Radix model, Global Biogeochem. Cycles, 24, GB3029,

Distribution of 137Cs and 239,240Pu in the forest soils horizons of the Opole Anomaly was established. Gamma and alpha spectrometry was used for determination of these isotopes. It was found that the 137Cs activity was approx. 1,000 times higher than that of 239,240Pu. The highest activities of both radioisotopes were found close to the boundary region in soil profile where the organic horizon turns into the inorganic one. Cluster analysis did not clearly indicate the group’s existence in data in respect to 137Cs and 239,240Pu activities and organic matter content. Distributions of 137Cs and 239,240Pu in soil horizons were non-normal but similar to each other. These distributions were substantially different from that one for organic matter content. The data were separated into two groups, for organic and inorganic soil horizons, respectively. Data transformation using Box–Cox formula was performed following by standardization. Mutual relationships between variables were investigated using ordinary and robust regression methods. Good correlation between 137Cs and 239,240Pu was found. No significant relationship between organic matter content and radioisotopes activity was asserted.

Ecosystem sustainability and resilience after a disturbance may be regulated by processes occurring at smaller spatial scales. The matrix of different spatial environments are created by (1) individual plants that accumulate higher concentrations of specific nutrients, trace elements or defensive plant secondary chemicals and thereby modify the chemistry of their ecological space and/or rates of processes, (2) the presence of structures (e.g., coarse woody debris) that may buffer some micro-environments from disturbances by functioning as a hospitable environment or as a reservoir for mycorrhizal fungi to sustain them into the next phase of stand development, and (3) chemical changes in soils during soil development which may result in distinct soil chemical environments. The response of the plants or change in the sustainability of carbon and nutrient cycles may be expressed more strongly at this smaller ecological space of an individual plant and furthermore must be frequently examined separately by the above- and belowground space of that individual. This paper will present three case studies from temperate and tropical forest ecosystems which suggest the importance of studying plant growth and nutrient and trace element cycling by stratifying sampling to encompass the mosaic patterns of existing spatial variability within the ecosystem. The examples show how individual plant species are able to create ecologically distinct spatial environments because of their distribution patterns within the landscape, how nutrient transfers in roots respond to the chemical variations in the soil, and how roots and mycorrhizal fungi are able to maintain themselves in the mosaic of coarse woody debris remaining on a site after the elimination of aboveground tree biomass.

Forest humus, which has a large cation binding capacity (up to 550 meq/100 g), and provides relatively very stable bindings with elements and their compounds, can effectively adsorb elements from atmospheric deposition and retain them overa long period. In order to test the monitoring potential of usingforest humus to retain the recent and long-term accumulated loadsof 14 elements (Al, As,

We have investigated the water retention properties of clayey subsoils horizons according to the variation of clay characteristics. The horizons studied developed on a large range of age and facies of calcareous or calcium saturated clayey sediments. The water retention properties have been studied from −10 hPa to −15 000 hPa water potential using small clods collected in winter when swelling is at a maximum and water content close to field capacity. The specific water content and volume of the clods at field conditions, their specific water content at-15 000 hPa water potential, the clay content, the organic carbon content, the cation exchange capacity, the N 2-BET surface area and calcareous content were measured. The clay fabric, which is the spatial distribution of the elementary clay particles, was quantified when the soil was close to field capacity and we could attribute the whole pore volume to the porosity of the clay fabric. Our results show that the water retention properties of the clay varies greatly

Hydrofluoric acid (HF) is able to dissolve most minerals, so it is often used to enrich organic matter (OM) in soils and sediments, though significant OM losses sometimes occur. The objective of this study was to examine these carbon losses and the hypothesis that HF dissolves the mineral-associated OM along with its mineral carrier. We compared composition, radiocarbon activity and particle morphology of untreated and HF-treated soil samples, using bulk samples, density fractions > 2 g cm À3 and fine particle size fractions < 6.3 lm from a Haplic Podzol and a Dystric Cambisol. A positive correlation between HF-soluble organic carbon and organic carbon in the dense fraction confirms a linkage between the dissolved fraction and the mineral-associated OM fraction. However, treatment of heavy soil fractions shows that removal of the mineral-associated OM fraction is not complete. In the topsoil carbon losses amount to only 20-30% of the mineral-associated C, compared to 40-55% for the sub-soil of the Dystric Cambisol and 70-85% for the sub-soil of the Haplic Podzol. Compositional OM change as a consequence of HF treatment was not detected in upper soil horizons, but was substantial for the lower horizons. With depth or age of soil horizon, the HF-soluble OM becomes enriched in O-alkyl C, aryl C, and carbonyl C compared to the HF-resistant OM. The HF-soluble material in all horizons of the Dystric Cambisol is dominated by high alkyl C content, whereas O-alkyl C dominates in deeper horizons of the Haplic Podzol. In A horizons, radiocarbon ages of the HF-resistant OM fraction were similar, but slightly younger than for bulk soil radiocarbon ages. In contrast, in the subsoil horizons the HF-resistant fraction was up to 2000 yr older than the bulk soil age.

Changes in land use over time impact on soil organic carbon content and turnover. In the present study a pedological survey was carried out in order to determine the evolution of epipedon of forest soils by studying soil organic matter turnover in relation to certain pedogenesis factors. The study area was the "Bosco Frattona" natural reserve, harbouring a relict of the mixed broadleaves woodland that once covered the Apennine hills of the Po Valley in north Italy, and a significant diversification of pedogenetic factors could be found therein despite its limited extent. The aim of the survey was to identify potentially vulnerable sites and to investigate how some parameters used to characterise soil organic matter in the epipedonsuch as the living fraction, humification and thermal stability indiceswere correlated to pedogenetic factors such as lithology, geomorphology and plant cover, which have contributed most to the differentiation of soils belonging to the Ustepts suborder. Total organic carbon (C org ), soil biomass carbon (C mic ), extractable and humified carbon together with corresponding humification parameters were determined to quantify carbon distribution in different organic matter pools. Stable carbon isotope measurements (δ 13 C) have been used to assess previous vegetation, land use changes and the organic matter turnover rate. Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) analyses were applied on soil humified substances in order to highlight differences in their thermal stability and composition. The results showed a correspondence between the diversification of the pedogenetic characteristics of soil and quali-quantitative characteristics of the organic matter of the epipedons. The abundance estimation of the different organic carbon fractions consistently indicated two pedons (both Typic Dyistrustepts) as being potentially vulnerable due to their very low content of C org , the considerable presence of labile fractions versus humified ones and a high C biom /C org ratio, which suggested that these soils had a poor humification capacity and an energetically non-conservative system insofar as the organic resource is concerned. Statistical treatment of data through Principal Component Analysis (PCA) highlighted the role of pedogenetic substrate and type of plant cover as discriminant pedological factors. Among the TG data relating to the extracted humic substances, weight loss at low temperature, generally attributed to adsorbed water and thermo-labile functional groups, proved to be discriminant vis-à-vis the type of vegetation. This result seemed to confirm the influence of plant cover on the turnover of organic carbon in soil and on the process of humification. In the group of chemical parameters, PCA revealed a cluster made up of the soil samples that had formed upon the stony silty sand alluvial deposits and also referred to the same pedotype (Typic Calciustepts). They showed higher microbial and humified carbon contents and higher pH values, whereas no differentiation was found in connection with vegetation changes arising from past land use (δ 13 C). From a methodological viewpoint, the study confirmed that the data set of parameters used was able to identify potentially vulnerable sites and thus to act as soil quality indicators.

Reliable estimates of saturated hydraulic conductivity are a prerequisite for accurate estimations of water flow and chemical transport through soil profiles. While informative, mean estimates of flow and transport have been shown to be inadequate for modeling purposes. Distributions and variances along with mean predictions provide a better form of model input. The spatial variability of field-measured saturated hydraulic conductivity (A' r J was investigated on a transect across a 1-ha field in west-central Georgia. The field includes two soil types, a Pine Flat loamy sand (coarse-loamy, siliceous, thermic Typic Paleudult) and a Troup sand (loamy, siliceous, thermic Grossarenic Kandiudult). Estimates of K t , were calculated using data collected with a constant-head permeameter. Mean, variance, and distribution data were determined for each soil and four selected horizons. Calculated values in the top 2 m of the soil profiles ranged from 0.07 to 117 mm h ' for the Pine Flat loamy sand and from 23 to 406 mm h ' for the Troup sand. Statistical differences between A' ts data calculated for the lower soil profiles coincide with soil textural differences observed with depth and across the plot. Semivariances for In A fs data were calculated and models fit to the semivariograms. Random behavior, with little spatial correlation, dominated the In A" fs data for the upper 0.5 m of the Pine Flat soil and the 2.0-m measurement for the Troup soil. Observed correlation ranges varied from 2 to 166 m. Large statistical differences in K,, were observed for the adjacent soils.

Ž Leaves of 9 different plant species terrestrial moss represented by: Hylocomium splendens and Pleurozium schreberi; and 7 species of vascular plants: blueberry, Vaccinium myrtillus; cowberry, Vaccinium¨itis-idaea; crowberry, . Empetrum nigrum; birch, Betula pubescens; willow, Salix spp.; pine, Pinus syl¨estris and spruce, Picea abies have been Ž 2 . 0048-9697r01r$ -see front matter ᮊ 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 8 -9 6 9 7 0 0 0 0 8 9 0 -1 ( ) C. Reimann et al. r The Science of the Total En¨ironment 278 2001 87᎐112 88 Ž

Organic matter has long been recognized as the main sorbent phase in soils for hydrophobic organic compounds (HOCs). In recent times, there has been an increasing realization that not only the amount, but also the chemical composition, of organic matter can influence the sorption properties of a soil. Here, we show that the organic carbon-normalized sorption coefficient (K OC ) for diuron is 27-81% higher in 10 A11 horizons than in 10 matching A12 horizons for soils collected from a small (2 ha) field. K OC was generally greater for the deeper (B) horizons, although these values may be inflated by sorption of diuron to clays. Organic matter chemistry of the A11 and A12 horizons was determined using solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy. K OC was positively correlated with aryl C (r 2 = 0.59, significance level 0.001) and negatively correlated with O-alkyl C (r 2 = 0.84, significance level <0.001). This is only the second report of correlations between whole soil K OC and NMR-derived measures of organic matter chemistry. We suggest that this success may be a consequence of limiting this study to a very small area (a single field). There is growing evidence that interactions between organic matter and clay minerals strongly affect K OC . However, because the soil mineralogy varies little across the field, the influence of these interactions is greatly diminished, allowing the effect of organic matter chemistry on K OC to be seen clearly. This study in some way reconciles studies that show strong correlations between K OC and the chemistry of purified organic materials and the general lack of such correlations for whole soils.

Forty years after conversion from chaparral to perennial veldt grass in the San Dimas Experimental Forest, we compared land surface and soil properties between areas of the two vegetation types. Our objective was to evaluate the impact of this vegetation conversion on the soil physical properties likely to impact zero-order watershed hydrology. In three watersheds of each vegetation type, surface cover and soils were described within five watershed elements. Surface cover is approximately 90% in both vegetation types, but the frequency of individual plants is significantly higher in converted watersheds, leading to significantly lower variability in surface cover between grass watersheds. In chaparral watersheds, very fine to very coarse roots extend laterally and downwards in all directions. Only very fine roots emanate from grass, forming a dense fibrous mass that is concentrated below individual plants. In areas converted to grass vegetation, A-horizon bulk density is significantly higher due to the development of transitional AB-horizons that are absent from chaparral areas. Vertical changes between the surface soil and subsoil are more gradual under grass than under chaparral. The significantly higher frequency of grass plants caused concurrent transformation of much of the converted areas, removing the layered effect that is observed in the chaparral soils and allowing spatially homogeneous infiltration, distribution and storage of soil water.

Talus flatiron sequences are ubiquitous landforms in arid and semiarid regions characterized by horizontal erodible rocks capped by more resistant rocks. Alternating phases of deposition and erosion lead to the formation of generations of talus flatirons in which ancient ones are located farther from the source cliff. The existing conceptual model of this systematic spatial distribution of talus flatirons is related to glacial–interglacial climatic cycles and to relatively high rates of cliff retreat. Three groups of talus flatirons were analyzed in the northeastern hyperarid Negev desert. The analyses include mapping, age determination using optically stimulated luminescence and cosmogenic nuclide exposure dating, electrical resistivity tomography and sedimentological and soil analyses. All talus flatirons contain gypsic-salic soil catena typical to hyperarid climate (< 80 mm yr− 1). No pedogenic indicators of past wetter environments such as buried calcic soil horizons, evidence of intense biogenic activity, or buried organic material were observed. 10Be exposure ages of the talus flatirons suggest that they were deposited during the middle Pleistocene; ~ 610 ka and ~ 170 ka for the oldest and intermediate-aged talus flatiron groups, respectively. These ages, combined with the present location of the talus flatirons relative to the source cliff yield retreat rates of 6–12 and ~ 200 m Ma− 1 for the bedrock cliff and talus flatiron apex, respectively, and in opposite directions. Our results show that (a) climatic changes at glacial–interglacial time scales are not the main controls over the formation of talus flatiron generation, and (b) that significant cliff retreat is not the main cause for the systematic spatial distribution of talus flatiron generations relative to the cliff. Our results indicate that the process of talus flatiron formation in the Negev desert must be associated with the balance between production, deposition, and removal of clasts under hyperarid conditions with only a minor cliff retreat.

Emissions from primary and secondary lead (Pb) smelters are responsible for high concentrations of metallic contaminants (Pb, Cd and Zn) in soils. However, less attention has been paid to volatile metalloids, such as antimony (Sb), which accompanies Pb smelting activities. This paper is focused on geochemistry of Sb in the forest and agricultural soils in the vicinity of the Příbram Pb smelter (Czech Republic), with emphasis on Sb distribution, chemical fractionation and comparison with the behaviour of another toxic metalloid, arsenic (As). Forest soils were enriched in Sb (4.86-2058 mg kg − 1 , mean: 379 mg kg − 1 , median: 155 mg kg − 1 ) with the highest concentrations in the organic horizons. Arsenic was found in significantly lower concentrations in forest soils (9.16-447 mg kg − 1 , mean: 127 mg kg − 1 , median: 82.9 mg kg − 1 ). High concentrations of metalloids in forest soils are related to higher interception of smelter emissions by tree canopies. The Sb/As concentrations ratios ranged between 4.6 and 16.4 in the organic soil horizons, which is in agreement with (i) higher Sb deposition between ∼ 1890 and the 1970s as recorded by 210-Pb dated peat cores from the vicinity of the smelter and (ii) rare historical analytical data on processed concentrates and dust from the smelter flue-gas cleaning system. In contrast to organic horizons of forest soils, the concentrations of the two metalloids are rather similar in agricultural soils and in mineral horizons of the forest soils. Antimony concentrations in agricultural soils were in the range 3.12-131 mg kg − 1 (mean: 39.3 mg kg − 1 , median: 23.8 mg kg − 1 ), whereas As concentrations were in the range 4.33-154 mg kg − 1 (mean: 60.3 mg kg − 1 , median: 42.5 mg kg − 1 ). Factor analysis showed that Sb and As are statistically correlated to oxalateextractable Fe, corresponding to amorphous or less crystalline Fe-oxides. The 5-step sequential extraction procedure (SEP), which was originally designed for As, and revised BCR SEP were applied to chemically fractionate Sb and As in the most polluted soil profiles. In forest soils, Sb was mostly bound in residual fraction with "mobile" concentrations in the range of 0.6-4% of the total Sb content. In contrast, As was significantly more mobile (1.2-22% of the total content in the "mobile" fractions) with dominant binding to Fe-oxides (up to 57% of total As content). In agricultural soils, Sb was again less "mobile" than As (1.4-5.9% and 0.34-12.1%, respectively of the total concentrations). Whereas Sb was mainly bound to the residual fraction and partly also in the reducible fraction (Fe-oxides), As was predominantly bound to Fe-oxides (up to 71% of the total As content). Good agreement was observed between the two SEP methods in determining the "mobile" concentrations of the metalloids (R 2 Sb = 0.9918 and R 2 As = 0.9104, p b 0.001). Despite probable similarities in the geochemical behaviour of these two metalloids in polluted soil systems, this study indicates that As is chemically more mobile than Sb.

Dissolved organic carbon Carbon isotopes Soil organic carbon Storm event Agricultural catchment Water pathway s u m m a r y Stable carbon isotopes (d 13 C) are assessed in further detail for their potential to (i) trace the relationship between spatial variations in the source of dissolved organic carbon (DOC) in soils and temporal variability of both DOC concentration and composition in streams, and (ii) elucidate water pathway changes during storm events in headwater catchments. For this purpose, we investigated d 13 C DOC values in a wetland soil (0-50 cm), in deep groundwater (until 6 m) and during a storm flow event with high-resolution monitoring (6hourly basis) in a small, lowland catchment in western France (Kervidy-Naizin catchment). The results show a combined increase of stream DOC concentration (from 4 to 14 mg L À1 ) and decrease of stream d 13 C DOC (from À27 to À29‰) with increasing discharge, suggesting a change in DOC sources between base flow and storm flow periods. Such an interpretation is consistent with the d 13 C DOC values in soils that show a 6‰ vertical variation, with d 13 C DOC values of the uppermost soil horizons (0-10 cm) of the wetland domains being close to those measured in the stream channel during the ascending limb of the hydrograph. Overall, the results presented in this study are consistent with a model in which the water-table rise and wetland runoff caused by rainfall lead to a flushing of the DOC stored in the uppermost soil horizons of the wetland domains near the channel network. Subsequently, these wetland soils become the dominant DOC source during storm events (ca. 70% of the total DOC flux). In this way, the stream DOC isotopic composition reflects the combined effects of the vertical variation of soil organic matter composition as well as the changes in water routing through time. This study demonstrates the ability of the stable isotopes of carbon to serve not only as a tool for the location of stream DOC sources in landscapes but also the reconstruction of water pathways in headwater catchments.

Smectite formation in alkaline-saline environments has been attributed to direct precipitation from solution and/or transformation from precursor minerals, but these mechanisms are not universally agreed upon in the literature. The objective of this work was to investigate the mineralogy of smectites in the soils surrounding a representative alkaline-saline lake of Nhecolândia, a sub-region of the Pantanal wetland, Brazil, and then to identify the mechanisms of their formation. Soils were sampled along a toposequence and analyzed by X-ray diffraction, transmission electron microscopy-energy dispersive X-ray analysis, and inductively coupled plasma-mass spectrometry. Water was collected along a transect involving the studied toposequence and equilibrium diagrams were calculated using the databases PHREEQC and AQUA. The fine-clay fraction is dominated by smectite, mica, and kaolinite. Smectites are concentrated at two places in the toposequence: an upper zone, which includes the soil horizons rarely reached by the lake-level variation; and a lower zone, which includes the surface horizon within the area of seasonal lake-level variation. Within the upper zone, the smectite is dioctahedral, rich in Al and Fe, and is classified as ferribeidellite. This phase is interstratified with mica and vermiculite and has an Fe content similar to that of the mica identified. These characteristics suggest that the ferribeidellite originates from transformation of micas and that vermiculite is an intermediate phase in this transformation. Within the lower zone, smectites are dominantly trioctahedral, Mg-rich, and are saponitic and stevensitic minerals. In addition, samples enriched in these minerals have much smaller rare-earth element (REE) contents than other soil samples. The water chemistry shows a geochemical control of Mg and saturation with respect to Mgsmectites in the more saline waters. The REE contents, water chemistry, and the presence of Mg-smectite where maximum evaporation is expected, suggest that saponitic and stevensitic minerals originate by chemical precipitation from the water column of the alkaline-saline lake.

Forest soil disturbance intervals are usually too long to measure using plot-based studies, and thus they are poorly understood. The mean soil disturbance interval (MSDI) in an old-growth forest on the west coast of Vancouver Island was estimated from radiocarbon dates of charcoal from organic and mineral soil horizons. Two assumptions are required to estimate the MSDI: (1) charcoal from forest fires is deposited within the organic horizon and eventually mixed into deeper mineral horizons by soil disturbances, and (2) the probability of soil disturbance is spatially homogeneous and affected only by the time since the last fire or the last soil disturbance. The MSDI is then estimated by the rate at which the proportion of undisturbed sample sites (determined by the proportion of sites with charcoal from the most recent fire in the organic horizon) decreases with increasing time since the last fire. Soil charcoal evidence of time since fire was determined at 83 sites using 141 radiocarbon dates. The estimated MSDI was greater on slopes (ca. 2010 years) than on terraces (ca. 920 years). The long periods between soil disturbances, especially on slopes, are consistent with other evidence from the study area that suggests infrequent tree uprooting is the predominant mode of soil disturbance.

Data on soil chemistry and especially on clay mineral changes are presented, which were ) Corresponding author. Fax: q41-1-635-6848. Ž . E-mail address: megli@geo.unizh.ch M. Egli . 0016-7061r01r$ -see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 016-7061 01 00046-5 ( ) M. Egli et al.r Geoderma 103 2001 307-334 308

Solute and runoff fluxes from two adjacent alpine streams (one glacial and one non-glacial) were investigated to determine how the inorganic solute chemistry of runoff responded to seasonal and interannual changes in runoff sources and volume, and to differences in physical catchment properties. Intercatchment differences in solute composition were primarily controlled by differences in catchment geology and the presence of soils, whereas differences in total solute fluxes were largely dependent on specific discharge. The glacial stream catchment had higher chemical denudation rates due to the high rates of flushing (higher specific discharge). The non-glacial Bow River had higher overall concentrations of solutes despite the greater prevalence of more resistant lithologies in this catchment. This is likely the result of both longer average water-rock contact times, and a greater supply of protons from organic soils and/or pyrite oxidation. Increases in snowpack depth/snowmelt runoff reduced the retention of nitrate in the Bow River catchment (i.e. increased nitrate export), probably by reducing net biological uptake, or by reducing the proportion of runoff that had contact with biologically active soil horizons that tend to remove nitrate.

Four-fold variation in leaf-litter Ca concentration among 14 tree species growing in a common garden in central Poland was linked to variation in soil pH, exchangeable Ca, soil base saturation, forest floor turnover rates, and earthworm abundance. Given the potential importance of tissue Ca to biogeochemical

Recent drought has led to unprecedented levels of plant mortality across the Southwestern US. An unaddressed feature of this drought's impact is how soil characteristics and soil hydrological behavior affect desert plant canopy die-back and mortality. Here, we present a multi-year study in the Mojave Desert assessing canopy die-back and wholeplant mortality of white bursage (Ambrosia dumosa) and creosotebush (Larrea tridentata) in soils varying in surface and sub-surface horizon development, and topographic (hillslope vs. channel) positions. Canopy die-back and mortality was more widespread A. dumosa than in L. tridentata, and dead plants tended to be smaller than surviving plants, especially in channel and hillslope locations. This suggests that juveniles were particularly vulnerable where plants depended heavily on augmentation of incident precipitation by runoff. Canopy die-back was greater in young, weakly developed soils that fostered extensive plant growth, while plants growing in older, well-developed soils showed markedly lower branch and plant mortality, especially in A. dumosa. We attributed these differences in plant response in part to variation in distributions of large rocks within soil profiles, which might affect soil hydrological heterogeneity and intensity of plant competition for water.

The northern-facing forelimbs of the active San Emigdio and Wheeler Ridge anticlines in the southern San Joaquin Valley (SSJV) are marked by numerous, sharply defined terracettes. Terracette treads and risers are 3 -5 and 7 -11 m wide, respectively. The terracettes are discontinuous along their length, with the longest being~300 m, although most terminate more abruptly forming lobate mounds. The terracettes form on 10 -25°slopes underlain by loosely consolidated coarse conglomerate beds of the Pleistocene Tulare Formation that dip subparallel to terracette risers. Soil developed on the slopes is comprised of a 1-to 2-m-thick calcic Bk horizon, overlain by a 0.5-m-thick argillic claypan Bt horizon, followed by 1-to 3-m-thick AB and A horizons that have been extensively bioturbated. A number of hypotheses have been proposed for the genesis of similar terracettes. The two most viable tectonic models for our study area are based on growth of active faultbend folds and include (i) sediment onlap with seismic folding and (ii) flexural slip faulting. We produced a very highresolution digital elevation model (DEM) (1.67-m spatial resolution and F5-cm vertical accuracy) using NASA's RASCAL (RAster SCanning Airborne Lidar) instrument and completed numerical modeling that we interpret to disprove both of the active fold growth models.

Copper (Cu), despite being an essential micronutrient, may be toxic whenever it occurs in the soil solution at high concentrations. Chemical reactions that control copper availability in the soil-plant system are complex. Copper can be found in both organic and inorganic soil fractions. This work studied Cu adsorption into surface (0-0.2 m) and subsurface (in the B 2 horizon) samples from three Brazilian soils, namely a heavy

The objective of this study was to quantify the dynamics of ammonium (NH 4 ) and nitrate (NO 3 À ) in the humus and the uppermost mineral layer of a forest soil treated annually since 1990 with one single dose of nitrogen (N) (0, 30 or 90 kg N ha À1 per year applied as NH 4 NO 3 ). Net N mineralisation, including net ammoni®cation and net nitri®cation was determined in four in situ incubation periods over 1 year in a Scots pine (Pinus sylvestris L.) stand at A Ê mli, southern Norway. Measurements were done using soil cores (14 cm deep) with a resin bag ®lled with combined anion and cation exchange resins placed at the bottom to collect N leaching from the soil. The rate of N accumulation was corrected for deposition and fertiliser N inputs. At the highest N additions, the net mineralisation rate increased relative to the other treatments. In all treatments, the content and accumulation of mineral N, both dominated by NH 4 , were highest in the organic horizons and decreased with depth in the mineral soil. However, at the highest N addition, the contribution from the upper mineral soil to total N accumulation was considerable (34%). The net N mineralisation rate was highest in September± October (4.1 kg N ha À1 per month), but even in winter (November±April) the net mineralisation was considerable (2.2 kg N ha À1 per month). The NO 3 À content of the soil was very small. A net NO 3 À immobilisation generally occurred in the soil. The only NO 3 À accumulation observed was a direct accumulation of fertiliser-NO 3 À during the ®rst month after fertilisation. #

The characterization of soil attributes using hyperspectral sensors has revealed patterns in soil spectra that are known to respond to mineral composition, organic matter, soil moisture and particle size distribution. Soil samples from different soil horizons of replicated soil series from sites located within Washington and Oregon were analyzed with the FieldSpec Spectroradiometer to measure their spectral signatures across the electromagnetic range of 400 to 1,000 nm. Similarity rankings of individual soil samples reveal differences between replicate series as well as samples within the same replicate series. Using classification and regression tree statistical methods, regression trees were fitted to each spectral response using concentrations of nitrogen, carbon, carbonate and organic matter as the response variables. Statistics resulting from fitted trees were: nitrogen R 2 0.91 (p < 0.01) at 403, 470, 687, and 846 nm spectral band widths, carbonate R 2 0.95 (p < 0.01) at 531 and 898 nm band widths, total carbon R 2 0.93 (p < 0.01) at 400, 409, 441 and 907 nm band widths, and organic matter R 2 0.98 (p < 0.01) at 300, 400, 441, 832 and 907 nm band widths. Use of the 400 to 1,000 nm electromagnetic range utilizing regression trees provided a powerful, rapid and inexpensive method for assessing nitrogen, carbon, carbonate and organic matter for upper soil horizons in a nondestructive method.

Many Vertisols in Tigray, Ethiopia, typically carry a discontinuous rock fragment (RF, size 0.5–> 40 · 10− 2 m) cover with 10 to 100 RFs m− 2. Such RF mulches are of agricultural and environmental significance because they influence the water balance in the underlying soils and the crop yield. Natural RF concentrations are mostly considered as eolian or hydraulic lag deposits, or as the result of lateral transport over the soil surface from a rock outcrop, upslope. In cultivated areas RF mulches can develop by tillage.This paper presents the case of a natural RF mulch whose lithology indicates that the RFs are up-squeezed by the local Vertisol. The study site is located in the pass of Enda Maryam, Tigray, Northern Ethiopia (39°8′ E and 13°36′ N). A circular area of 10 m diameter, about 200 m away from the water divide in the valley has been cleared annually between 01/1999 and 05/2003. During this period, 625 RFs, 17 being > 7.5 · 10− 2 m in size, totalling a mass of nearly 62 kg, have been collected. After correction for measurement procedures, the rate of RF up-warping by the Vertisol at Enda Maryam is assessed at 5 RFs m− 2 in 3 years. At this rate of appearance, the formation of current RF concentrations on top of active valley Vertisols is only a matter of 101–2 years, provided the availability of RFs below the soil surface.Although important underground displacements were measured in the Vertisol between 01/1999 and 05/2002, the supposed link between up-squeezing of RFs and plastic deformations of ‘chimney’, ‘diapir’ or ‘intrusion’-like type in the Vertisol could not be evidenced. Instead, RFs are clearly concentrated on the soil surface as well as in depth, along the existing vertical desiccation cracks, often > 1 m deep which display polygonal configurations at the soil surface. Further, bundles of slickensides containing some RFs, have been mapped at the base of the Vertisol. The slickenside configuration suggests that the RF-bearing substrate is being scraped off.While the underground displacement of RFs along active slickensides seems normal, the process of RFs ascending in ‘upright’ position in the edge of desiccation cracks needs explanation. The closure of a desiccation crack is a peristaltic-like movement, following ascent or descent of the capillary fringe. It is hypothesized that this movement gradually pushes the RF to the surface or to another place or level in the soil profile where the crack closes in last instance.The apparent young age of the valley Vertisol mulches in Ethiopia might indicate the very recent formation of yearly recurrent desiccation cracks of Vertisols in the area. Available information confirms that most valleys in the study area used to be perennially marshy. Under these conditions no movements of RFs in the soil profile are expected to occur. Gullying, leading to pronounced seasonal desiccation of the Vertisols, started in several cases not more than 50 years ago.