Soil Aggregation

Since the 1900s, the link between soil biotic activity, soil organic matter (SOM) decomposition and stabilization, and soil aggregate dynamics has been recognized and intensively been studied. By 1950, many studies had, mostly qualitatively, investigated the influence of the five major factors (i.e. soil fauna, microorganisms, roots, inorganics and physical processes) on this link. After 1950, four theoretical mile-stones related to this subject were realized. The first one was when Emerson [Nature 183 (1959) 538] proposed a model of a soil crumb consisting of domains of oriented clay and quartz particles. Next, Edwards and Bremner [J. Soil Sci. 18 (1967) 64] formulated a theory in which the solid-phase reaction between clay minerals, polyvalent cations and SOM is the main process leading to microaggregate formation. Based on this concept, Tisdall and Oades [J. Soil Sci. 62 (1982) 141] coined the aggregate hierarchy concept describing a spatial scale dependence of mechanisms involved in micro-and macroaggregate formation. Oades [Plant Soil 76 (1984) 319] suggested a small, but very important, modification to the aggregate hierarchy concept by theorizing the formation of microaggregates within macroaggregates. Recent research on aggregate formation and SOM stabilization extensively corroborate this modification and use it as the base for furthering the understanding of SOM dynamics. The major outcomes of adopting this modification are: (1) microaggregates, rather than macroaggregates protect SOM in the long term; and (2) macroaggregate turnover is a crucial process influencing the stabilization of SOM. Reviewing the progress made over the last 50 years in this area of research reveals that still very few studies are quantitative and/or consider interactive effects between the five factors. The quantification of these relationships is clearly needed to improve our ability to predict changes in soil ecosystems due to management and global change. This quantification can greatly benefit from viewing aggregates as dynamic rather than static entities and relating aggregate measurements with 2D and 3D quantitative spatial information.

Soil disturbance from tillage is a major cause of organic matter depletion and reduction in the number and stability of soil aggregates when native ecosystems are converted to agriculture. No-till (NT) cropping systems usually exhibit increased aggregation and soil organic matter relative to conventional tillage (CT). However, the extent of soil organic matter changes in response to NT management varies between soils and the mechanisms of organic matter stabilization in NT systems are unclear. We evaluated a conceptual model which links the turnover of aggregates to soil organic matter dynamics in NT and CT systems; we argue that the rate of macroaggregate formation and degradation (i.e. aggregate turnover) is reduced under NT compared to CT and leads to a formation of stable microaggregates in which carbon is stabilized and sequestered in the long term. Therefore, the link between macroaggregate turnover, microaggregate formation, and C stabilization within microaggregates partly determines the observed soil organic matter increases under NT. q

A simple method of estimating changes in biologically active soil carbon (C) could help evaluate soil quality impacts of alternative management practices. Most reports of permanganate for active C determination use highly concentrated solutions (0.333 M) that are dif®cult to work with and tend to react with a large fraction of soil C that is not well distinguished from total organic C. We report on a highly simpli®ed method in which dilute, slightly alkaline KMnO 4 reacts with the most readily oxidizable (active) forms of soil C, converting Mn(VII) to Mn(II), and proportionally lowering absorbance of 550 nm light. The amount of soil C that reacted increased with concentration of KMnO 4 used (0.01 to 0.1 M), degree of soil drying (moist fresh soil to air-dried for 24 hour) and time of shaking (1±15 minutes). Shaking of air-dry soil in a 0.02 M KMnO 4 solution for 2 minutes produced consistent and managementsensitive results, both in the laboratory and with a ®eld kit that used a hand-held colorimeter. Addition of 0.1 M CaCl 2 to the permanganate reagent enhanced settling of the soil after shaking, eliminating the need for centrifugation in the ®eld kit. Results from the laboratory and ®eld-kit protocols were nearly identical (R 2 = 0.98), as were those from an inter-laboratory sample exchange (R 2 = 0.91). The active soil C measured by the new procedure was more sensitive to management effects than total organic C, and more closely related to biologically mediated soil properties, such as respiration, microbial biomass and aggregation, than several other measures of soil organic C.

but also in heavy textured ones . Strongly hydrophobic or-Soil organic matter is thought to increase aggregate stability by ganic coatings can prevent water from entering the aglowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) gregates or the horizon, restrict infiltration, and cause have been intensively cropped for 40 yr, decreasing the soil organic intense surface runoff (Wallis and Horne, 1992). pool and lowering the soil agregate stability. This study assessed (i) Apart from the case of strongly hydrophobic soils, the contribution of organic matter to aggregate stability by decreasing SOM may impart partial repellency to soil aggregates aggregate wettability and (ii) the specific role of clay-associated orand thereby contribute to their stability. Haynes and ganic matter. Soil samples with a C content of 4 to 53 g kg Ϫ1 were reported that dried aggregates from a passampled and soil aggregate stability was measured. Aggregate wettature soil rich in organic matter, were more stable than bility was assessed by measuring water drop penetration times on field moist ones, and that it was the opposite for arable individual 3-to 5-mm aggregates. The Ͻ2-m fractions were extracted soils with low C content. The slower rewetting of pasture without organic matter destruction and their wettability was deteraggregates as compared to arable counterparts was asmined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content (r 2 ϭ cribed to hydrophobic properties of SOM (Sullivan, 0.71 for fast wetting). Water drop penetration time increased with C 1990). Furthermore, Capriel et al. (1990) reported good contents from 1 to 32 s and was very heterogeneous among individual correlations between the aliphatic fraction of a soil exaggregates from a given soil. The contact angle of water on the clay tracted with supercritical hexane, and its aggregate stafraction increased linearly with the C content (r 2 ϭ 0.86). This change bility.

We examined the role of arbuscular mycorrhizal fungi (AMF) in ecosystems using soil aggregate stability and C and N storage as representative ecosystem processes. We utilized a wide gradient in AMF abundance, obtained through long-term (17 and 6 years) large-scale field manipulations. Burning and N-fertilization increased soil AMF hyphae, glomalin-related soil protein (GRSP) pools and water-stable macroaggregates while fungicide applications reduced AMF hyphae, GRSP and water-stable macroaggregates. We found that AMF abundance was a surprisingly dominant factor explaining the vast majority of variability in soil aggregation. This experimental field study, involving longterm diverse management practices of native multispecies prairie communities, invariably showed a close positive correlation between AMF hyphal abundance and soil aggregation, and C and N sequestration. This highly significant linear correlation suggests there are serious consequences to the loss of AMF from ecosystems.

To assess the effects of forest management on soil erosion in Southeast Asia, clear distinctions must be made between surface erosion and landslide processes. Although surface erosion is a natural process, it is exacerbated by surface disturbance and compaction that reduce the soil hydraulic conductivity and break down soil aggregates. Management practices and attributes such as roads and trails, agricultural cultivation, fire, land clearing, and recreation all accelerate surface erosion processes due to their disturbance, compaction, and connectivity along hillslopes. Agroforestry practices in Southeast Asia that incorporate cover crops with trees reduce surface erosion by more than an order of magnitude compared to monoculture plantations with no ground cover. Cleared fields tilled up and down steep slopes are highly erodible; passive conservation practices (e.g., contour tillage, strip cropping, reduced tillage; maintaining adequate ground cover) are effective in reducing surface erosion if properly implemented. Poorly designed and managed terraces are not effective in controlling surface erosion and may actually increase mass wasting if they concentrate water.

Soil quality indicators (SQIs) can be used to evaluate sustainability of land use and soil management practices in agroecosystems. The objective of this study was to identify appropriate SQI from factor analysis (FA) of five treatments: no-till corn (Zee mays) without manure (NT), no-till corn with manure (NTM), no-till corn-soybean (Glycine max) rotation (NTR), conventional tillage corn (CT), and meadow (M) in Coshocton, Ohio. Soil properties were grouped into five factors (eigenvalues > 1) for the 0-10 cm depth as: (Factor 1) water transmission, (Factor 2) soil aeration, (Factor 3) soil pore connection 1, (Factor 4) soil texture and (Factor 5) moisture status. Factor 2 was the most dominant, with soil organic carbon (SOC) the most dominant measured soil attribute contributing to this factor. For the 10-20 cm depth, factors identified were: (Factor 6) soil aggregation, (Factor 7) soil pore connection 2, (Factor 8) soil macropore, and (Factor 9) plant production. At 10-20 cm depth, Factor 6 was most dominant with SOC the most dominant measured soil attribute. Management  sample and slope position  sample interactions were significant among some factors for both depths. Overall, SOC was the most dominant measured soil attribute as a SQI for both depths. Other key soil attributes were field water capacity, air-filled porosity, pH and soil bulk density for the 0-10 cm depth, and total N and mean weight diameter of aggregates for the 10-20 depth. Therefore, SOC could play an important role for monitoring soil quality.

Production of exopolysaccharides (EPS) can be used as a criteria for the isolation of stress tolerant microorganisms. In the present study, EPS-producing fluorescent pseudomonads were isolated from alfisols, vertisols, inseptisols, oxisols, and aridisols of different semiarid millet growing regions of India and were screened in vitro for drought tolerance in trypticase soy broth supplemented with different concentrations of polyethylene glycol (PEG6000). Out of the total 81 isolates, 26 could tolerate maximum level of stress (−0.73 MPa) and were monitored for the amount of EPS produced under maximum level of water stress. The strain GAP-P45, isolated from alfisol of sunflower rhizosphere, showed the highest level of EPS production under water stress conditions, was identified as Pseudomonas putida on the basis of 16S rDNA sequence analysis, and was used as seed treatment to study its effect in alleviating drought stress effects in sunflower seedlings. Inoculation of Pseudomonas sp. strain GAP-P45 increased the survival, plant biomass, and root adhering soil/root tissue ratio of sunflower seedlings subjected to drought stress. The inoculated bacteria could efficiently colonize the root adhering soil and rhizoplane and increase the percentage of stable soil aggregates. Scanning electron microscope studies showed the formation of biofilm of inoculated bacteria on the root surface and this, along with a better soil structure, might have protected the plants from the water stress.

Soil aggregation in relation to other soil properties was studied along a climatological transect in the Southeast of Spain. Three sites were selected along this transect ranging from semiarid to subhumid climatological conditions. The aggregate size distribution, the macro and microaggregate stability of the superficial soil horizon and their relations with other soil properties were Ž. analysed. Large aggregates) 10, 10-5, 5-2 mm were present in highest proportions in the most arid of the studied areas. These large aggregates were associated with high values of water-stable microaggregates; however, they did not improve soil structure and are related to high bulk densities and low water retention. Aggregates 1-0.105 mm were positively correlated to medium, fine, very fine sand and silt fractions and to organic matter. Aggregates-0.105 mm were Ž positively correlated to organic matter and clay content. Small aggregate sizes 1-0.105 and .-0.105 mm had a positive influence on soil water retention and they seem a good indicator of soil degradation. Water stability of microaggregates showed a positive correlation with clay content while the stability of the macroaggregates depended on the organic matter content when the organic matter content was greater than 5 or 6%. Below that threshold the carbonate content was strongly correlated with aggregate stability. A general conceptual scheme of associations between aggregate size distribution, water-stable microaggregation and textural characteristics for the area was developed.

Root-adhering soil (RAS) forms the immediate environment where plants take up water and nutrients for their growth. We report the effect of an exopolysaccharide (EPS)-producing rhizobacterium (strain YAS34) on the physical properties of sunflower (Helianthus annuus L.) RAS, associated with plant growth promotion, under both water stress and normal water supply conditions. Strain YAS34 was isolated as a major EPSproducing bacterium from the rhizoplane of sunflowers grown in a French dystric cambisol. Strain YAS34 was assigned to the Rhizobium genus by 16S ribosomal DNA gene sequencing. Inoculation of sunflower seeds and soil with strain YAS34 caused a significant increase in RAS per root dry mass (dm) (up to 100%) and a significant increase in soil macropore volume (12 to 60 m in diameter). The effect of inoculation on sunflower shoot dm (up to ؉50%) and root dm (up to ؉70%) was significant under both normal and water stress conditions. Inoculation with strain YAS34 modified soil structure around the root system, counteracting the negative effect of water deficit on growth. Using [ 15 N]nitrate, we showed that inoculation made the use of fertilizer more effective by increasing nitrogen uptake by sunflower plantlets.

Soil hydrophobicity is known to enhance runoff responses to rainstorms and to increase soil aggregate stability (AS). It has been widely reported for acidic soils particularly under burnt, but also unburnt pine forests following dry periods. Few studies have reported hydrophobicity from alkaline soils, but they have not established whether hydrophobicity also occurs in burnt or unburnt pine forests on alkaline soil. This study examines the wettability and stability of air-dry aggregates and their size fractions ( < 0.25, 0.25 -0.5, 0.5 -1 and 1 -2 mm) taken from surface layers (0 -2.5 and 2.5 -5 cm depth) of alkaline, calcareous loamy soils. Four sites in southeastern Spain were sampled with comparable vegetation (>30-year-old Aleppo pine {Pinus halepensis} and associated shrub community), geology (limestone), soil type (Lithic Xerorthents), slope angle and aspect (5 -8jSW). Included were three sites (A, B, C) burned, respectively, in 1998, 1999 and 2000, and one unburnt for >30 years (D). Hydrophobicity was detected in samples from all sites. Both spatial frequency and persistence of hydrophobicity (Water Drop Penetration Times (WDPT) ranged from 10 to 600 s), however, was lower than reported from studies of acidic soils under pine. This might be associated with a lower susceptibility of alkaline soils to hydrophobicity development and/or the comparatively low biomass production in the region. Probably because it had been most recently affected by severe fire, spatial frequency of hydrophobicity was higher at site B (53% of samples), compared to A, C and D (6%, 33% and 10% of samples, respectively). In contrast to some previous studies, the finest size fraction of the samples consistently had the highest degree of hydrophobicity. Degree of hydrophobicity was positively correlated with organic matter (OM) content (r = 0.714). It is speculated that fine, interstitial hydrophobic organic matter accumulating in the finest sieve fraction contributes to this enhanced hydrophobicity. As shown in previous studies on acidic soils, aggregate stability increased with hydrophobicity (r = 0.897 in the fraction 0.25 -2 mm) for the samples investigated here. This elevated stability occurs despite an already relatively high level of aggregate stability amongst all samples investigated. Hydrophobicity observed at the study sites was not spatially contiguous and it may therefore enhance overland flow and slope wash over only short distances for most, except the very high intensity rainstorms that occur in the region. The increased stability of hydrophobic soil aggregates against slaking, however, may counter an otherwise enhanced susceptibility to erosion. D

In recent years conventional production technologies in the rice-wheat (RW) system have been leading to deterioration of soil health and declining farm profitability due to high inputs of water and labour. Conservation agriculture (CA)-based resource-conserving technologies (RCTs) vis-à -vis zero-till (ZT), raised-bed planting and direct-seeded rice (DSR) have shown promise as alternatives to conventional production technologies to overcome these problems. The integration of CA-based RCTs with precision agriculture (PA)-based technologies in a systems perspective could provide a better option for sustainable RW production systems. In this study we attempted to evaluate conservation and precision agriculture (CPA)-based RCTs as a double-ZT system integrated with laser-assisted precision land leveling (PLL) in the RW system. A field experiment was conducted in the western IGP for 2 years to evaluate various tillage and crop establishment methods under PLL and traditional land leveling (TLL) practices to improve water productivity, economic profitability and soil physical quality. Irrespective of tillage and crop establishment methods (TCE), PLL improved RW system productivity by 7.4% in year 2 as compared to traditional land leveling. Total irrigation water savings under PLL versus TLL were 12-14% in rice and 10-13% in wheat. PLL improved RW system profitability by US$113 ha À1 (year 1) to $175 ha À1 (year 2). Yields were higher in conventionally transplanted rice followed by direct-drill-seeded rice after ZT. In wheat, yields were higher in ZT when followed by DSR than in the conventional-till (CT) system. RW system productivity under double ZT was equivalent to that of the conventional method. Among different TCE, conventional puddled-transplanted rice-CT wheat required 12-33% more water than other TCE techniques. Compared with CT systems, double ZT consumed 12-20% less water with almost equal system productivity and demonstrated higher water productivity. The CT system had higher bulk density and penetration resistance in 10-15 and 15-20 cm soil layers due to compaction caused by the repeated wet tillage in rice. The steady-state infiltration rate and soil aggregation (>0.25 mm) were higher under permanent beds and double ZT and lower in the CT system. Under CT, soil aggregation was static across seasons, whereas it improved under double no-till and permanent beds. Similarly, mean weight diameter of aggregates was higher under double ZT and permanent beds and increased over time. The study reveals that to sustain the RW system, CPA-based RCTs could be more viable options: however, the long-term effects of these alternative technologies need to be studied under varying agro-ecologies. ß

One reason why the total Fe (hydr)oxide content of a soil does not always correlate with degree of aggregation Differences in crystallinity may explain why total Fe (hydr)oxide content has a variable effect on aggregate stability. Therefore, surface may be the crystallinity of the Fe (hydr)oxides. Poorly soil samples with a range of poorly crystalline Fe (hydr)oxide contents crystalline Fe (hydr)oxides have a much larger and more were characterized for water-stable aggregates Ͼ0.25 mm (WSA), reactive surface area than crystalline Fe (hydr)oxides, and mean-weighted diameter (MWD), soil organic C (OC), particle-size may increase aggregation more than crystalline Fe (hydr) distribution, pH, exchangeable cations, citrate/bicarbonate/dithionite oxides. This suggestion was supported in a lab study with (subscript d), and acid ammonium oxalate (subscript o) extractable synthetic Fe (hydr)oxides (Schahabi and Schwertmann, Fe, Al, and Si. The WSA and MWD range from 23 to 95%, and 0.3 to 1970) and in a study of B horizons of Italian soils (Ardu-5.1 mm, respectively. The effects of Fe o (1.1-6.8 g kg Ϫ1), Fe d (3.2-19.6 g ino et al., 1989; Barberis et al., 1991). According to Rhokg Ϫ1), OC (2.4-24.0 g kg Ϫ1) and clay (141-467 g kg Ϫ1) contents on ton et al. (1998), Fe (hydr)oxide crystallinity explained WSA and MWD of both A and B horizons of these soils was studied using linear regression. The poorly crystalline Fe (hydr)oxide (Fe o) differences in erodibility (a process highly correlated and OC contents are significantly correlated with WSA in the A toWSA) of loess-derived soils in Mississippi. horizons (r 2 ϭ 0.95, n ϭ 6, p ϭ 0.001, and r 2 ϭ 0.93, n ϭ 6, p ϭ 0.002, Factors other than crystallinity that may influence the respectively) and in the B horizons (r 2 ϭ 0.73, n ϭ 6, p ϭ 0.029, and effect of Fe (hydr)oxides on aggregation are: pH, path r 2 ϭ 0.76, n ϭ 6, p ϭ 0.024, respectively). When regressed against of formation of the Fe (hydr)oxides, the size of Fe (hydr) MWD, Fe o has an r 2 of 0.89 (n ϭ 6, p ϭ 0.004) in the A, and 0.97 oxide crystals, the ionic composition of the soil solution, (n ϭ 6, p ϭ 0.000) in the B horizons. The coefficient of determination and the presence of certain organic molecules. It has been of MWD vs. OC contents is 0.98 (n ϭ 6, p ϭ 0.000) in the A and suggested that the Fe (hydr)oxides form aggregates with 0.79 (n ϭ 6, p ϭ 0.018) in the B horizons. Clay and Fe d contents are clay particles only when the pH is below their zero point not significantly correlated to WSA or MWD. Apparently, the Fe o component (poorly crystalline) is more effective than Fe d at stabilizing of charge (Goldberg, 1989). Although this may be true in soil aggregates, even though it is present in lower concentrations. The many cases, some studies report opposite results (Des-Fe o component appears more important than OC in terms of WSA

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha À1 2 year À1) combined with P, K and N fertiliser application on total organic C (C T), labile C (C L), non-labile C (C NL), total N (N T), mean weight diameter (MWD) and unsaturated hydraulic conductivity (K unsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha À1 year À1) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly C L , where application of 30 t ha À1 2 year À1 increased C L by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while N T increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased K unsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha À1 year À1 of FYM increased concentrations of C L by 173% and of C NL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, N T and MWD compared with the cropped site, while K unsat (10 mm tension) was increased on the 0 and 50 t ha À1 year À1 treatments as a result of a recent tillage. There was no difference in K unsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha À1 year À1. There were similar significant positive correlations of all C fractions and N T with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and N T with K unsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and K unsat was with C L. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the www.elsevier.com/locate/still Soil & Tillage Research 91 (2006) 39-47 DOIs of related articles

It is generally accepted that particulate organic matter derives from plants. In contrast, the enriched labile fraction is thought by many to derive from microbes, especially fungi. However, no detailed chemical characterization of these fractions has been done. In this study, we wanted to assess the sources (plants or microbes; fungi or bacteria) and degree of microbial alteration of (i) three particulate organic matter fractions ± namely the free light fraction (1.85 g cm ±3 ), the coarse (250±2000 "m) and the ®ne (53± 250 "m) intra-aggregate particulate organic matter fractions ± and of (ii) three density fractions of ®nesilt associated carbon ± namely < 2.0, 2.0±2.2 (i.e. enriched labile fraction) and > 2.2 g cm ±3 ± by analysing the amino sugars, by CuO oxidation analyses, and by 13 C-, 1 H-and 31 P-NMR analyses. Macroaggregates (250±2000 "m) were separated by wet-sieving from a former grassland soil now under a no-tillage arable regime. The three particulate organic matter fractions and the three density fractions were isolated from the macroaggregates by a combination of density¯otation, sonication and sieving techniques. Proton NMR spectroscopy on alkaline extracts showed that the enriched labile fraction is not of microbial origin but is strongly degraded plant material that is enriched in aliphatic moieties partly bound to aromatics. In addition, the enriched labile fraction had a glucosamine content less than the whole soil, indicating that it is not enriched in carbon derived from fungi. Decreasing yields of phenolic CuO oxidation products and increasing side-chain oxidation in the order coarse intra-aggregate particulate organic matter < ®ne inter-aggregate particulate organic matter < ®ne-silt fractions indicate progressive alteration of lignin as particle size decreases. The light fraction was more decomposed than the coarse inter-aggregate particulate organic matter, as indicated by (i) its larger ratio of acid-toaldehyde of the vanillyl units released by CuO oxidation, (ii) the smaller contribution of H in carbohydrates to total extractable H as estimated by 1 H-NMR spectroscopy, and (iii) a larger contribution of monoester P to total extractable P in the 31 P-NMR spectra. In conclusion, the four fractions are derived predominantly from plants, but microbial alteration increased as follows: coarse inter-aggregate particulate organic matter < light fraction » ®ne inter-aggregate particulate organic matter < enriched labile fraction.

Arbuscular mycorrhizal (AM) fungi are key organisms of the soil/plant system, influencing soil fertility and plant nutrition, and contributing to soil aggregation and soil structure stability by the combined action of extraradical hyphae and of an insoluble, hydrophobic proteinaceous substance named glomalin-related soil protein (GRSP). Since the GRSP extraction procedures have recently revealed problems related to co-extracting substances, the relationship between GRSP and AM fungi still remains to be verified. In this work the hypothesis that GRSP concentration is positively correlated with the occurrence of AM fungi was tested by using Medicago sativa plants inoculated with different isolates of Glomus mosseae and Glomus intraradices in a microcosm experiment. Our results show that (i) mycorrhizal establishment produced an increase in GRSP concentration – compared to initial values – in contrast with non-mycorrhizal plants, which did not produce any change; (ii) aggregate stability, evaluated as mean weight diameter (MWD) of macroaggregates of 1–2 mm diameter, was significantly higher in mycorrhizal soils compared to non-mycorrhizal soil; (iii) GRSP concentration and soil aggregate stability were positively correlated with mycorrhizal root volume and weakly correlated with total root volume; (iv) MWD values of soil aggregates were positively correlated with values of total hyphal length and hyphal density of the AM fungi utilized.The different ability of AM fungal isolates to affect GRSP concentration and to form extensive and dense mycelial networks, which may directly affect soil aggregates stability by hyphal enmeshment of soil particles, suggests the possibility of selecting the most efficient isolates to be utilized for soil quality improvement and land restoration programs.

Glomalin is a soil proteinaceous substance produced by arbuscular mycorrhizal fungi. Most of the information available concerning this protein has been collected in relation to its role in soil aggregation. In this study, we explored the distribution of glomalin across soil horizons, decomposition of glomalin, and relationship with soil C and N in an agricultural field, a native forest, and an afforested system. Glomalin was present in A, B, and C horizons in decreasing concentrations. Land-use type significantly affected glomalin concentrations (mg cm −3 ), with native forest soils having the highest concentrations of the three land-use types in both A and B horizons. In terms of glomalin stocks (Mg ha −1 ), calculated based on corrected horizon weights, the agricultural area was significantly lower than both afforested and native forest areas. As measured after a 413 day laboratory soil incubation, glomalin was least persistent in the A horizon of the afforested area.. In agricultural soils and native soils, ca. 50% of glomalin was still remaining after this incubation, indicating that some glomalin may be in the slow or recalcitrant soil C fraction. Comparison of glomalin decomposition with CO 2 -C respired during incubation indicates that glomalin makes a large contribution to active soil organic C pools. Soil C and N were highly correlated with glomalin across all soils and within each land-use type, indicating that glomalin may be under similar controls as soil C. Our results show that glomalin may be useful as an indicator of land-use change effects on deciduous forest soils.

. Long-term applications of Agricultural management practices that alter the soil organic matanimal manure increase the SOM content in two ways: ter (SOM) content are expected to cause changes in soil stability and (i) by adding OM in manure (Sommerfeldt and Chang, aggregation. Animal manure is a source of organic matter (OM) that 1985) and (ii) by adding OM in crop residues, due to has been demonstrated to increase macroaggregate formation and higher crop yields in soils receiving manure (Jenkinson stability. The objectives of this study were to determine how long term cattle manure applications to a calcareous Haploboroll clay loam and Rayner, 1977; Angers and N'Dayegamiye, 1991). (Lethbridge, AB, Canada) affected aggregate size distribution, the One year after manure application, Sun et al. (1995) total C, N, and P content of aggregate size fractions, and water-stable found a significantly higher proportion of water stable aggregates. Beef cattle manure applied at rates Ͼ30 Mg ha Ϫ1 yr Ϫ1 macroaggregates (1-2 mm) in eroded soils treated with under dryland production and Ͼ60 Mg ha Ϫ1 yr Ϫ1 to soils under irrigacattle, hog, or poultry manure, compared with untion resulted in fewer dry-sieved aggregates Ͼ7.1 mm and more dryamended soils. The proportion of water stable macroagsieved aggregates between 0.47 and 1.2 mm in the 0-to 5-cm depth, gregates (Ͼ0.25 mm) increased in soils that received compared with unamended soils. The dry-sieved aggregate fractions annual cattle manure applications for more than a debetween 0.47 and 1.2 mm include the Ͻ0.84-mm fraction that has cade (Angers and N'Dayegamiye, 1991; Aoyama et al., been associated with increased susceptibility to wind erosion in the 1999). The decomposition rate of animal manure is ex-Canadian semiarid prairies. There was more total C, N, and P in all pected to affect macroaggregate formation. Organic madry-sieved aggregate fractions of soils receiving Ͼ30 Mg manure ha Ϫ1 terials that decompose quickly may produce a rapid yr Ϫ1 than unamended soils, and dry-sieved aggregates between 0.47 increase in aggregation, but the effect may be tempoand 2.0 mm tended to have the highest C, N, and P contents. Water rary; whereas organic materials that decompose slowly aggregate stability was higher in irrigated than dryland soils, but did

In Brazil, no tillage (NT) is a soil conservation practice now widely adopted by farmers, including smallholders. The effect of NT and conventional tillage (disc ploughing followed by two light disc harrowings, CT) was investigated on the aggregation properties of a clayey Rhodic Ferralsol from southern Brazil under different crop rotations. The same soil type under secondary forest was used as reference. Macro-and microaggregate classes were separated by wet sieving using a series of eight sieves (8, 4, 2, 1, 0.5, 0.25, 0.125, 0.053 mm) at four sampling layers (0-5, 5-10, 10-20, 20-30 cm). The soil in general had high structural stability. At 0-5 cm, meanweight diameter (MWD, 11.1 mm) and total organic C in macroaggregates (TOC, 39 g kg −1 soil) were highest for the forest soil. Soil under NT had a more similar distribution of aggregate size classes and TOC to the forest soil than CT. The most pronounced difference between tillage systems was observed in the surface soil layer (0-5 cm). In this layer, NT had higher aggregate stability (AS NT : 96%; AS CT : 89%), had higher values of aggregate size distribution (MWD NT : 7.9 mm, MWD CT : 4.3 mm), and had on average 28% greater TOC in all aggregate size classes than CT. Soil under NT had greater TOC in macroaggregates (NT: 22 g kg −1 ; CT: 13 g kg −1 ). Crop rotation did not have a significant effect on soil aggregate distribution and TOC. By increasing macroaggregation NT increased organic carbon accumulation in soil.

Organic matter controls aggregate stability in loam soils. Intensive farming can lead to a decrease in soil organic matter content. In areas where livestock have disappeared, the recycling of composted urban organic wastes on agricultural soils may represent a valuable source of organic matter fir restoring soil organic matter content. The effects on the aggregate stability of a silt loam soil of three urban composts (a municipal solid waste compost, a co-compost of sewage sludge and green waste, and a biowaste compost) sampled at two different stages of maturity (immature and mature composts) were studied during laboratory incubations. The results were related to (i) compost organic matter biodegradability, biochemical fractions, and humic substance content, (ii) microbial activity evaluated through organic C mineralization and microbial and fungal biomass evolution, (iii) hot-water-extractable polysaccharides, and (iv) aggregate hydrophobicity as revealed by the water drop penetra...

Crop-livestock systems are regaining their importance as an alternative to unsustainable intensive farming systems. Loss of biodiversity, nutrient pollution and habitat fragmentation are a few of many concerns recently reported with modern agriculture. Integrating crops and pastures in no-till systems can result in better environmental services, since conservation agriculture is improved by system diversity, paths of nutrient flux, and other processes common in nature. The presence of large herbivores can positively modify nutrient pathways and soil aggregation, increasing soil quality. Despite the low diversity involved, the integration of crops and pastures enhances nature's biomimicry and allows attainment of a higher system organization level. This paper illustrates these benefits focusing on the use of grazing animals integrated with crops under no-tillage systems characteristic of southern Brazil.

An understanding of the effects of salinity and sodicity on soil carbon (C) stocks and fluxes is critical in environmental management, as the areal extents of salinity and sodicity are predicted to increase. The effects of salinity and sodicity on the soil microbial biomass (SMB) and soil respiration were assessed over 12weeks under controlled conditions by subjecting disturbed soil samples from a vegetated soil profile to leaching with one of six salt solutions; a combination of low-salinity (0.5dSm −1 ), mid-salinity (10dSm −1 ), or high-salinity (30dSm −1 ), with either low-sodicity (sodium adsorption ratio, SAR, 1), or high-sodicity (SAR 30) to give six treatments: control (lowsalinity low-sodicity); low-salinity high-sodicity; mid-salinity low-sodicity; mid-salinity high-sodicity; high-salinity lowsodicity; and high-salinity high-sodicity. Soil respiration rate was highest (56-80mg CO 2 -C kg −1 soil) in the low-salinity treatments and lowest (1-5mg CO 2 -C kg −1 soil) in the midsalinity treatments, while the SMB was highest in the highsalinity treatments (459-565mg kg −1 soil) and lowest in the low-salinity treatments (158-172mg kg −1 soil). This was attributed to increased substrate availability with high salt concentrations through either increased dispersion of soil aggregates or dissolution or hydrolysis of soil organic matter, which may offset some of the stresses placed on the microbial population from high salt concentrations. The apparent disparity in trends in respiration and the SMB may be due to an induced shift in the microbial population, from one dominated by more active microorganisms to one dominated by less active microorganisms.

Through their feeding activities and cast production, earthworms influence both aggregate turnover and soil organic matter (SOM) dynamics. We studied the impact of earthworm activity on soil macro-and microaggregation and SOM incorporation in different farming systems. Dry-sieved aggregates (4-12.5 mm) of a permanent pasture (PP), a conventional arable field (CA) and an organic arable field (OA) (0-10 and 10-20 cm depth) were separated into different aggregate fractions, which were analyzed for organic C and N. The separation was based on macromorphological characteristics that reflect the dominant process of aggregate formation and the degree of degradation. We distinguished: two classes of biogenic macroaggregates (fresh casts and welded casts); one class of physicogenic macroaggregates (angular to subangular blocky macroaggregates); and an intermediate fraction (rounded to subrounded macroaggregates). The structural arrangement of mineral particles and organic matter and the quantitative contribution of particulate organic matter (POM) and microaggregates were studied in thin sections. Total organic C contents tended to be higher in biogenic than in physicogenic macroaggregates of the PP and OA soils, whereas the reverse was found for the CA soil. Comparison of the different macroaggregate types in thin sections revealed that the worm-made macroaggregates of the PP soil were considerably enriched in fine POM and microaggregates, in which large amounts of organic matter were intimately mixed with fine mineral material. By contrast, worm casts of the CA and OA soils were hardly enriched in POM and microaggregates. Our study demonstrated that earthworms can directly initiate the formation of microaggregates, which in turn affects the physical protection of SOM against microbial decay. Farming practices that stimulate earthworm activity may thus constitute an important aspect of sustainable agricultural management. However, the much smaller amounts of POM and microaggregates present in worm casts of CA and OA than PP indicate a different impact of earthworms on C stabilization depending on land use. Further research is needed to elucidate the exact management conditions that favour the

The stability of soil aggregates against water erosion is a largely unstudied factor in research on the spatially-distributed impacts of tropical forestry. Soil strata with particularly weak aggregates are likely to be the focus for erosional activity whether buried or exposed by either natural processes or forestry activities. Understanding the location and likely cause of such intrinsic instability would, therefore, allow better spatially-distributed parameterisation of erosion models. This study has aimed to identify the range in aggregate stability within a region of Bornean Ultisol disturbed by forestry operations, and to identify the bulk soil properties associated with such stability variations. The sites sampled were subject to a range of denudational processes including piping, rilling and landslide-triggered erosion. Soil profiles with rates of erosion in excess of 10 mm a−1 were shown to have less aggregate stability (as characterised by the rainfall simulation survival index (RSSI)) than those with no visible signs of erosion. Further, large differences in aggregate stability between soil horizons of the same profile were observed and seen to be statistically correlated with the bulk soil properties of organic carbon and clay content (i.e. stabilising agents) and the dispersing agent of exchangeable sodium percentage (ESP) at sites undergoing erosion. Organic carbon appeared to be the most important governing factor, accounting for 56% of the variance in the aggregate stability. The presence of strongly and weakly expanding 2 : 1 clays within the soils may have been an additional de-stabilising factor. Reduced porosity was also observed at eroding sites with weak soil aggregates. The properties of EC25 and sodium adsorption ratio (SAR) were not correlated with the RSSI or the presence of soil erosion. Identification of the role of organic carbon, clay and ESP in the stability of these tropical forest aggregates is important in focusing future, more intensive research on the spatial parameterisation of models to simulate forestry impacts on erosion.

Understanding the mechanisms controlling crop effects on soil aggregation is necessary to develop sustainable soil management practices. Changes in soil aggregation, carbohydrates, and fungal vs. bacterial biomass were monitored following a shift from fallow (bare soil) to various cropping systems. Faba bean (Viciafaba L.) and wheat (Triticum aestivum L.) were used as annual cropping systems, while alfalfa (Medicago sativa L.), timothy (Phleumpratense L.), bromegrass (Bromus inermis L.), and reed canarygrass (Phalaris arundinacea L.) were used as perennial cropping systems. Crops were established in spring 1989, on a silty clay loam (fine, mixed, frigid Typic Dystrochrept) and a clay loam (fine-loamy, mixed, frigid Typic Humaquept) near Quebec City (Canada). After three growing seasons, the meanweight diameter of water-stable aggregates (MWD) was higher under reed canarygrass (2.26 mm in the clay loam and 2.45 mm in the silty clay loam) and timothy (2.13 and 2.26 mm) than under faba bean (1.92 and 1.89 mm) or wheat (2.06 and 1.57 mm). Intermediate values were found under alfalfa and bromegrass. Changes in MWD were not correlated with microbial biomass C. Acid-hydrolyzable carbohydrates correlated with MWD in the silty clay loam (r 2 = 0.42**) but the relationship decreased with higher carbohydrate levels in the clay loam (r 2 = 0.05). Close correlations were found between MWD and both fungal glucosamine (r 2 = 0.68***, soils combined) and bacterial muramic acid (r 2 = 0.48***). Changes in MWD were mostly reflected in aggregates >2 mm, and the close relationship with fungal glucosamine suggests that fungi played a dominant role in soil macroaggregation. Measurement of muramic acid and glucosamine is proposed as a potential approach to compare bacterial vs. fungal contributions to soil aggregation.

Fire p w g e rs accompanied by a heaf wave and ash deposifion afjecfing the upper soil layer. Changes in soii properfies are directly related fo heaf intmsity, the amomis of ashes deposifed, and soil type. We subjecfed two soils (gypsiferous soil and caicareous soil) lo artificial heafing and ash incorporation and compared changes in selecf chemical and physical properties. The two soils srudied were selected to provide a wide range of charaeferisfics in soils of the semiarid Ebro Volley ( N E Spain). Samples of both soüs were heated for 30 minutes in a m f j e furnnce at temperatures of 25'. l 5 P . 25P and 5OPC. Ashes were added only on soil samples heated al 25PC. in a quantity related fo plant b w m s growing on each soil (twice the amount in calcareous soil than in gypsiferous soil). Increasing heat iniensity increased organic matter combustion as well as nufrienf availability. Heating soil lo 250°C cawed a decrease m pH and un increase in elecfrolytic conducfivfty (ECe) and soluble Ca. Heating soil lo 5OO"C c m e d an increase in pH and a decrease in ECe and soIuble Ca. Totd N confent decreased at femperafures greater fhan 25PC. with abart one-fhird being volatilized. Changes in chernicai properties were similar for both soils although quanfitalive diiferences befween soíis were found. Catzon exchange capacify (CEC) was reduced for gypsiferous soil heated lo 5OO"C and fo 25PC for caicareous soii. Heafing increcrred sand-sized particles by f i w n of clay, greafest in soil heated to 50PC. Soil aggregate sfabdity (SAS) of bofh soils wcrr reduced by heating fo 250°C wifh greater reductions at 5 W C , likely due to a reducfion in organic matter and clay size particle conteni. A negative correlatwn was observed m o n g SAS and soil erodibiiity (K-USLE). Bulk density and parficle densify increased in bofh soils when heated to 500°C. Water availabilify increased when soils were heated to 5OO"C. likely due lo texture andsfructural modificafions. Addifwn of the &S increased organic maffer contenf, C/N ratio, andpH in bofh soils and increased nutrient availability. These responses were greater in caicareous than in gypsiferous soil. Physical soil properties were not significanfly modifed by ash additian.

Aggregation is important for soil functioning, providing physical protection of organic matter and microbial inhabitants. Tillage disrupts aggregates, increases wind and water erosion of soils and exposes formerly protected organic matter to decomposition and losses. Microbial biomass and community dynamics in dry-sieved aggregate-size classes from long-term no-till (NT) and conventionally tilled (CT) soils were examined using phospholipid fatty acid analysis (PLFA). Bacterial, fungal, and total biomass were up to 32% greater in NT compared to CT aggregates. Aggregate size also affected microbial biomass, which was highest in the 1-2 mm size class. Arbuscular mycorrhizal fungi (AMF) were particularly affected by tillage disturbance with increases of 40-60% among aggregate-size classes in NT vs. CT, but glomalin related soil protein concentration was not different between tillage treatments or among aggregate-size classes. Bacterial stress biomarkers were higher in CT than NT aggregates but were not significantly correlated with total C, total N or C:N ratio, indicating that the physiological status of bacteria within aggregates was not simply governed by the quantity of available resources. Ordination analysis of PLFA profiles demonstrated a shift in microbial community structure between NT and CT aggregates, correlated with AMF abundance in NT aggregates and increased bacterial stress biomarkers in CT aggregates. Our results demonstrated greater microbial biomass and altered microbial community structure in NT vs. CT aggregates. This work demonstrates that tillage management influences microbial community structure within aggregates and may provide a potential explanation for differences in process rates observed in NT vs. CT soils. Further research into the processes that govern community structure in aggregates from NT and tilled soils is needed to better understand how the interaction of microorganisms with their physical environment affects nutrient turnover and availability.

Soil structure and soil aggregation play an important role in an array of processes such as soil erodibility, organic matter protection and soil fertility. Modeling attempts of these processes would benefit substantially from including soil structural parameters such as soil aggregation. However, quantitative data on soil structural dynamics is lacking. Therefore, we conducted short-term (3 weeks) incubations to acquire necessary soil structural parameters for modeling purposes. Prior to incubation, all structures O53 mm were destroyed from three soils with varying texture but under similar management. Five different amounts of wheat residue, ranging from 0 to 3 wt%, were added to each of these soils. After 3 weeks, samples were analyzed for large water-stable macroaggregates (O2000 mm) using a wet sieving method and for fungal growth using epifluorescence microscopy. Aggregate formation increased linearly with increasing amounts of residue at a rate of 12.0G1.24 g aggregates g K1 residue added. We found no differences in aggregate formation among the three soils, even though the equilibrium level of macroaggregates differed in the field. While amounts of water-stable macroaggregates in the sandy loam and the silt loam soil corresponded well with fungal lengths, this was not the case for the silty clay loam soil. This suggests that fungi are less important in aggregate formation in more clayey soils. Cumulative respiration correlated well (rZ0.89-0.91) with water-stable macroaggregates for all three soils. A model assuming an aggregate formation rate proportional to the respiration rate was very successful in fitting the measured aggregate amounts. This model predicted about 65% of the changes in aggregation when different amounts were added, and about 85% of the changes in aggregation over time. This model yielded a macroaggregate turnover time of 40-60 days. The quantitative results presented here can directly be incorporated into models describing and predicting soil aggregate dynamics, as a determining factor for physical protection of organic matter within a soil. q

Three dimensional imaging and image processing has become an important part for investigations of fluid distribution and flow in porous media. We describe two methods of computed tomography with different characteristics, namely X-ray-and neutron-based. We give an overview of image processing sequences and their methods. We investigated image enhancement with a focus on filters using partial differential equations, classification and structure identification that we used to prepare our images for quantitative evaluations. These methods are demonstrated on a partially saturated sand sample. Finally, we show an application with soil aggregates where investigations using synchrotron X-rays and thermal neutrons have led to new insights and refined fluid distribution and flow models.

This study aimed to compare results obtained with winter cover plants, in its first crop cycle, in the resistence to soil physical properties and biomass productivity of the plant. Four species of soil cover plants were used: vetch (Vicia sativa L.), radish (Raphanus sativus L.), ryegrass (Lolium multiflorum Lam) and black oat (Avena strigosa Schreb). It was developed in Guarapuava, PR, in the CEDETEG Campus of the Midwestern State Universisty (UNICENTRO). It was carried out the monitoring of a cycle of these crops in the fall/winter of 2010. The soil resistence data were obtained with the penetrometer of impact in its period of greatest development (flowering). It was observed that the use of cover plants in its first crop cycle did not promote changes in soil penetration resistance. The crops of radish presented the best production of vegetal biomass.

Soil aggregation is of great importance in agriculture due to its positive effect on soil physical properties, plant growth and the environment. A long-term (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) field experiment was performed to investigate the role of mycorrhizal inoculation and organic fertilizers on some of soil properties of Mediterranean soils (Typic Xerofluvent, Menzilat clay-loam soil). We applied a rotation with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) as a second crop during the periods of 1996 and 2008. The study consisted of five experimental treatments; control, mineral fertilizer (300-60-150 kg N-P-K ha −1 ), manure at 25 t ha −1 , compost at 25 t ha −1 and mycorrhiza-inoculated compost at 10 t ha −1 with three replicates. The highest organic matter content both at 0-15 cm and 15-30 cm soil depths were obtained with manure application, whereas mineral fertilizer application had no effect on organic matter accumulation. Manure, compost and mycorrhizal inoculation + compost application had 69%, 32% and 24% higher organic matter contents at 0-30 cm depth as compared to the control application. Organic applications had varying and important effects on aggregation indexes of soils. The greatest mean weight diameters (MWD) at 15-30 cm depth were obtained with manure, mycorrhiza-inoculated compost and compost applications, respectively. The decline in organic matter content of soils in control plots lead disintegration of aggregates demonstrated on significantly lower MWD values. The compost application resulted in occurring the lowest bulk densities at 0-15 and 15-30 cm soil depths, whereas the highest bulk density values were obtained with mineral fertilizer application. Measurements obtained in 2008 indicated that manure and compost applications did not cause any further increase in MWD at manure and compost receiving plots indicated reaching a steady state. However, compost with mycorrhizae application continued to significant increase (P b 0.05) in MWD values of soils. Organic applications significantly lowered the soil bulk density and penetration resistance. The lowest penetration resistance (PR) at 0-50 cm soil depth was obtained with mycorrhizal inoculated compost, and the highest PR was with control and mineral fertilizer applications. The results clearly revealed that mycorrhiza application along with organic fertilizers resulted in decreased bulk density and penetration resistance associated with an increase in organic matter and greater aggregate stability, indicated an improvement in soil structure.

Bulk soil samples (BS) (n = 120) from long-term tillage experiments and areas covered by secondary natural forest were collected in the Brazilian savanna region (Cerrado) at Santo Antônio de Goiás, and southern Atlantic forest region at Londrina. Bulk soils were separated into 8 aggregate size fractions (ASF) by wet sieving (n = 700). Bulk soil samples (BS) and ASF were analyzed for total C, N by dry combustion. BS were also analyzed for sand, silt, and clay by densitometry, and several measures of soil aggregation were determined. All samples were scanned in the mid-and near-infrared (Mid-IR, NIR) by diffuse reflectance of ground, non-KBr diluted samples using a Digilab FTS-7000 FTIR spectrometer equipped with a Pike Auto-Diff auto-sampler. Quantitative calibrations for the various analytes were performed separately for the original set of 120 BS samples and the set of 700 ASF samples using partial least squares regression (PLSR). Results for the set of 120 BS for total C and N were excellent using either spectral range with R 2 > 0.97, except for C by Mid-IR (R 2 = 0.93). Similar results were obtained for determinations of sand and clay (R 2 > 0.94). Silt, determined by difference (100% − %sand − %clay), gave much lower R 2 (0.80 and 0.63 for Mid-IR and NIR, respectively). For aggregation indices, R 2 of ∼ 0.80 were achieved in the Mid-IR, NIR results were not as good (R 2 < 0.67). Efforts with the 700 ASF samples resulted in R 2 of 0.98 and 0.96 for C, and 0.96 and 0.97 for N, for Mid-IR and NIR calibrations, respectively. Calibration efforts to determine the values for the set of 700 ASF samples using set of 120 BS from which they were fractionated gave R 2 = 0.90 and 0.87 in the Mid-IR and NIR, respectively, when 5% of the ASF were added to the set of 120 BS. Spectral discrimination of sample source was easily and very accurately determined for all data sets examined.

Several management systems can improve soil productivity. By studying aggregate stability it is possible to quantify whether or not the management is ameliorating the natural soil properties and the land capability for agriculture. The effect of three tillage systems on the stability of soil aggregates and soil organic carbon was studied in comparison to reference plots with grass and bare soil. Samples were collected at the Pesagro Experimental Research Station in Paty do Alferes, state of Rio de Janeiro, Brazil, from an experiment that has been carried out from 1995 to 2001, on a Dystrophic Red Latosol (Typic Haplorthox). Aggregate size distribution mean weight diameter, geometric mean diameter of the aggregates, and total organic carbon in each aggregate size fraction were determined. The proportion of aggregates with diameter ≥2 mm appeared to be a suitable indicator of the influence of tillage systems on aggregation. At a depth of 0-5 cm, aggregates ≥2 mm were 50% of soil under no-tillage, which was greater than under animal traction (35%) and conventional tillage (30%). Total organic carbon concentration was greater under no-tillage (19 g kg −1 ) than under conventional tillage (11 g kg −1 ) at a depth of 0-5 cm, but not significantly different (average 13 g kg −1 ) at a depth of 5-10 cm. Soil exposure with tillage and lack of residue inputs caused declines in aggregation and organic carbon, both of which make soil susceptible to erosion. Adoption of no-tillage led to a decline in aggregation compared with grass reference, but did significantly alter soil organic concentration, suggesting it was a valuable conservation practice for vegetable production on sloping soils.

It is well known that no-tillage (NT) practices can promote greater stocks of soil organic matter (SOM) in the soil surface layer compared to conventional tillage (CT) by enhancing the physical protection of aggregate-associated C in temperate soils. However, this link between tillage, aggregation and SOM is less well established for tropical soils, such as Oxisols. The objective of this study was to investigate the underlying mechanisms of SOM stabilization in Oxisols as affected by different crop rotations and tillage regimes at two sites in southern Brazil. Soils were sampled from two agricultural experiment sites (Passo Fundo and Londrina) in southern Brazil, with treatments comparing different crop rotations under NT and CT management, and a reference soil under native vegetation (NV). Free light fraction (LF) and intra-aggregate particulate organic matter (iPOM) were isolated from slaking-resistant aggregates. Of the total C associated with aggregates, 79-90% was found in the mineral fraction, but there were no differences between NT and CT. In contrast, tillage drastically decreased LF-C concentrations in the 0-5 cm depth layer at both sites. In the same depth layer of NT systems at Londrina, the concentrations of iPOM-C were greater when a legume cover crop was included in the rotation. At Londrina, the order of total iPOM-C levels was generally NV > NT > CT in the 0-5 cm depth interval, but the difference between NT and CT was much less than in Passo Fundo. At Passo Fundo, the greatest concentrations and differences in concentrations across tillage treatments were found in the fine (53-250 mm) iPOM fractions occluded within microaggregates. In conclusion, even though no aggregate hierarchy exists in these Oxisols, our results corroborate the concept of a stabilization of POM-C within microaggregates in no-tillage systems, especially when green manures are included in the rotation. #

Increased belowground carbon (C) transfer by plant roots at elevated CO 2 may change properties of the microbial community in the rhizosphere. Previous investigations that focused on total soil organic C or total microbial C showed contrasting results: small increase, small decrease or no changes. We evaluated the effect of 5 years of elevated CO 2 (550 ppm) on four extracellular enzymes: b-glucosidase, chitinase, phosphatase, and sulfatase. We expected microorganisms to be differently localized in aggregates of various sizes and, therefore analyzed microbial biomass (C mic by SIR) and enzyme activities in three aggregate-size classes: large macro-(42 mm), small macro-(0.25-2 mm), and microaggregates (o0.25 mm). To estimate the potential enzyme production, we activated microorganisms by substrate (glucose and nutrients) amendment. Although C total and C mic as well as the activities of b-glucosidase, phosphatase, and sulfatase were unaffected in bulk soil and in aggregate-size classes by elevated CO 2 , significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO 2 were 1.2-1.9-fold higher than under ambient CO 2 . This indicates the increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO 2 . Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO 2 revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungal hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO 2 affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools.

The smaller the aggregate, the larger was the aggregate stability according to NMWD. The rankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams Ž. Ž. from sandstone Sc and Sw were the weakest soils while the purple mudstone Pp was the strongest. All the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in SOC content after cultivation.