Nitrate Immobilization

Allocation of biomass and nutrient elements including Nitrogen to above and belowground compartments of beech seedlings (Fagus sylvatica L.) treated by labeled nitrogen fertilizer in the form of 15 NH 4 and 15 NO 3 were investigated at the end of two successive growing seasons. Pot cultured beech seedlings were grown at a green house on intact soil cores sampled from three adjacent stands including beech, Norway spruce and mixed beech-spruce cultures of Solling forest, Germany. Comparing biomass allocation and nutrients concentrations of the seedlings between the control and 15 N-fertilized treatments revealed no significant effect of N fertilization on nutrients uptake by seedlings over the experiment. The form of N input influenced its movement into plant pools. It was demonstrated that beech seedlings take up nitrogen mainly in the form of nitrate, which is then reduced in the leaves, although the differences between the retention of NO 3 ⎯-N and NH 4 +-N in plants were not statistically significant. Percent recoveries of 15 N in trees were typically greater after 15 NO 3 than after 15 NH 4 additions. It was indicated that immobilization of 15 N tracer in fine roots was a slower process comparing other plant compartments such as stem and coarse roots, but a powerful sink for N during the course of study.

The partitioning of nitrogen deposition among forest soil (including forest floor), leachate and above-and belowground biomass of pot cultured beech seedlings in comparison to non-cultured treatments were investigated by adding 1.92 g•m-2 15 N tracer in throughfall for two successive growing seasons at a greenhouse experiment. Ammonium and nitrate depositions were simulated on four treatments (cultured and non-cultured) and each treatment was labeled with either 15 N-NH 4 + or 15 N-NO 3 ⎯. Total recovery rates of the applied 15 N in the whole system accounted for 74.9% to 67.3% after 15 N-NH 4 + and 85.3% to 88.1% after 15 N-NO 3 ⎯ in cultured and non-cultured treatments, respectively. The main sink for both 15 N tracers was the forest soil (including forest floor), where 34.6% to 33.7% of 15 N-NH 4 + and 13.1% to 9.0% of 15 N-NO 3 ⎯ were found in cultured and non-cultured treatments, respectively, suggesting strong immobilization of both N forms by heterotrophic microorganisms. Nitrogen immobilization by microorganisms in the forest soil (including forest floor) was three times higher when 15 N-NH 4 + was applied compared to 15 N-NO 3 ⎯. The preferential heterotrophic use of ammonium resulted in a two times higher retention of deposited 15 N-NH 4 + in the forest soil as compared to plants. In contrast, nitrate immobilization in the forest soil was lower compared to plants, although statistically it was not significantly different. In total the immobilization of ammonium in the plant-soil system was about 60% higher than nitrate, indicating the importance of the N-forms deposition for retention in forest ecosystems.

1. Anthropogenic nitrogen (N) deposition may have several impacts on upland moorland ecosystems, including changes in vegetation composition, eutrophication and surface water acidification through nitrate leaching, but few studies linking N deposition to key biogeochemical processes have been published. 2. A stable isotope tracer (15 N) was used to determine the fate of inorganic N inputs to four moorland catchments across gradients of N deposition and leaching, through 2-weekly additions to experimental plots on major soil types over 1 year. 3. An apparent decline in total 15 N recovery from soils and vegetation as the proportion of leached N deposition increased was not significant at the P = 0•05 level, but a significant relationship was found for recovery in mosses and lichens. 4. Vegetation retained 31-68% of 15 N inputs, and 15 N recovery increased significantly (P = 0•01) with biomass for all compartments except woody shrubs. Mosses and lichens showed far greater 15 N recovery per unit biomass than grasses or ericaceous shrubs. There was no significant variation in the proportion of 15 N recovered in higher plants across the N deposition gradient (24-29%). In contrast, the proportion recovered in mosses and lichens declined from 44% to 2% as deposition increased, mirroring a decline in their biomass and showing a highly significant inverse relationship (P = 0•01) with nitrate leaching. 5. The proportion of 15 N recovered in litter plus surface soils (33-39%) was remarkably constant across the deposition gradient for a variety of soil types. However, significantly declining recovery per unit biomass in litter (P < 0•05) suggested progressive N saturation of this sink and increasing importance of retention in underlying surface soils as deposition increased. 6. Synthesis and applications. Past studies have demonstrated a decline in mosses and lichens in response to increasing N deposition, but we show here for the first time that reduced N retention might result together with increased nitrate leaching into surface waters. The conservation of bryophyte and lichen flora on moorlands is therefore critical to prevent excessive nitrate leaching and associated surface water acidification and eutrophication. Ensuring management practices such as grazing or burning are at an intensity that does not further degrade the bryophyte and lichen communities may help minimize the impact of N deposition on freshwaters, but the only effective means to reduce the risk of N leaching is a reduction in N emissions.

floors "in all cases". Atmospheric nitrogen deposition is actually intercepted by the forest canopy, particularly at the nine closedcanopy forests studied. It is known that forest canopies chemically interact with HNO 3 , NO x and NH 3 gases (among others), as well as with the NO 3 ǁ and NH 4 + ions in particulate matter. The Integrated Forest Study 4 , covering 13 sites with a wide range of North American and European forest types and atmospheric nitrogen deposition, varying from 5 to 30 kg per hectare per year, reported that "an average 40% of the incoming inorganic N is retained in or transformed at passage through the canopy; the net canopy exchange is greatest at those sites receiving the highest atmospheric N input." At some sites in this study in the eastern United States, the canopy retention of atmospheric inorganic nitrogen deposition is very high: for example, at Howland Forest, Maine 5 , nitrate canopy retention is about 90% and ammonium canopy retention is more than 80%. Retained nitrogen deposition can also be rapidly assimilated by canopy foliage 6. The 15 N applied by Nadelhoffer et al. 1 to the forest floor would have bypassed the canopy retention and assimilation pathway. The nitrogen deposition allocated to the non-woody and woody biomass ecosystem pools (15 and 5%, respectively; Table 2 of ref. 1) may have been significantly increased if it had been possible to apply the labelled nitrogen to the forest canopy. Doubling or tripling the percentage of nitrogen deposition allocated to woody biomass (10 or 15%, rather than the 5% used) would increase forest carbon uptake by 50-100% over that estimated by Nadelhoffer et al. The woody-biomass nitrogen allocation may be increased several-fold, and could be determined by using labelled nitrogen applied to the forest canopy. Using a figure of 5.1 ǂ10 12 g per year as the global nitrogen deposition to forests may also lead to an underestimate. It is recognized that there is much uncertainty in the estimates of forest nitrogen deposition 2,3 , particularly in the case of NH x-N deposition, which may be substantially greater 7,8 than the 2 ǂ10 12 g nitrogen per year considered in ref. 1. Total anthropogenic nitrogen deposition has been estimated to be as much as 18 ǂ10 12 g per year to temperate and boreal forests 9 , which is three or more times the value reported in ref. 1. These two points can provide an upper bound for estimates of forest carbon sequestration of 1-2 ǂ10 15 g carbon per year, with the range dependent on the still poorly known nitrogen deposition allocated to woody biomass.

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15 N tracer application), total ecosystem 15 N retention was negatively correlated with foliar natural-abundance 15 N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance 15 N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15 N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15 N

15N abundances of soils and a grass species (Oeschampsia flexuosa (L.) Trin.) were analysed in a forest fertilization experiment 10 years after the last fertilization. Nitrogen had been given as urea, at seven doses, ranging from 0 to 2400 kg N ha-1. Previously, we have shown that plants in systems experiencing large losses of N become enriched with ~SN. This was explained by the fact that processes leading to loss of N, e.g. ammonia volatilization, nitrification followed by leaching or denitrification and denitrification itself, tend to fractionate against ~SN. In this experiment, aSN abundance increased with dose of N applied in both grass and soil total-N, but more so in the grass. This was interpreted to be due to the grass sampling small but active pools of N subject to losses. In contrast, soil total-N largely consists of inactive N that does not immediately exchange with pools of N from which fractionating losses occur. Hence, soil total-N shows a large pretreatment ~5N memory effect, and is, therefore, an integrator of the long-term N balance. When short-term changes (years, decades) in N balances are monitored using variations in 15N abundance, plants are more suitable indicators of such change than is soil total-N.

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15 N tracer application), total ecosystem 15 N retention was negatively correlated with foliar natural-abundance 15 N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance 15 N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15 N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15 N

Stable isotopes and changing paradigms on soil nitrogen and carbon biogeochemistry. Ecosistemas 19(3):14-23. Many perceptions on ecosystem biogeochemistry are based on flow diagrams in which element pools (boxes) are interconnected by abiotic and biotic mechanisms controlling transformations of chemical species and flows among the pools (arrows). Because of the ability of stable isotopes to integrate such processes over time and space, they have played a central role in our current understanding of nutrient cycling, particularly in the cases of N and C. Most fluxes and transformations involved in terrestrial nutrient cycling cross over or take place in soil compartments. We here review the development of new paradigms in soil nitrogen (and carbon) cycling research, to which stables isotopes contributed through three main approaches: (i) as integrators of nutrient input/output budgets from broad ecosystems compartments or "black-boxes", (ii) as tracers to unravel specific processes and end-member pools operating within these black-boxes, and (iii) as markers or indicators of nutrient use, availability and deficiency to plants. New challenges and future perspective to that respect are also discussed.

The partitioning of nitrogen deposition among forest soil (including forest floor), leachate and above-and belowground biomass of pot cultured beech seedlings in comparison to non-cultured treatments were investigated by adding 1.92 g•m-2 15 N tracer in throughfall for two successive growing seasons at a greenhouse experiment. Ammonium and nitrate depositions were simulated on four treatments (cultured and non-cultured) and each treatment was labeled with either 15 N-NH 4 + or 15 N-NO 3 ⎯. Total recovery rates of the applied 15 N in the whole system accounted for 74.9% to 67.3% after 15 N-NH 4 + and 85.3% to 88.1% after 15 N-NO 3 ⎯ in cultured and non-cultured treatments, respectively. The main sink for both 15 N tracers was the forest soil (including forest floor), where 34.6% to 33.7% of 15 N-NH 4 + and 13.1% to 9.0% of 15 N-NO 3 ⎯ were found in cultured and non-cultured treatments, respectively, suggesting strong immobilization of both N forms by heterotrophic microorganisms. Nitrogen immobilization by microorganisms in the forest soil (including forest floor) was three times higher when 15 N-NH 4 + was applied compared to 15 N-NO 3 ⎯. The preferential heterotrophic use of ammonium resulted in a two times higher retention of deposited 15 N-NH 4 + in the forest soil as compared to plants. In contrast, nitrate immobilization in the forest soil was lower compared to plants, although statistically it was not significantly different. In total the immobilization of ammonium in the plant-soil system was about 60% higher than nitrate, indicating the importance of the N-forms deposition for retention in forest ecosystems.

Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However, the mechanisms determining impacts on the N cycle are not fully understood. To explore the mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent progress in ecosystem N research through empirical studies, conceptual analysis and model simulations. Experimental and observational studies have revealed that the stimulation of plant N uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous losses of N occur at low N availability but increase exponentially and become the dominant fate of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation from plant uptake to soil retention before N losses occur. However, biogeochemical models that simulate simultaneous competition for soil N substrates by multiple processes match the observed patterns of N losses better than models...

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15 N tracer application), total ecosystem 15 N retention was negatively correlated with foliar natural-abundance 15 N but was positively correlated with mineral soil C and N concentration and C : N, showing that plant and soil natural-abundance 15 N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15 N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15 N Manuscript

1. Anthropogenic nitrogen (N) deposition may have several impacts on upland moorland ecosystems, including changes in vegetation composition, eutrophication and surface water acidification through nitrate leaching, but few studies linking N deposition to key biogeochemical processes have been published. 2. A stable isotope tracer (15 N) was used to determine the fate of inorganic N inputs to four moorland catchments across gradients of N deposition and leaching, through 2-weekly additions to experimental plots on major soil types over 1 year. 3. An apparent decline in total 15 N recovery from soils and vegetation as the proportion of leached N deposition increased was not significant at the P = 0•05 level, but a significant relationship was found for recovery in mosses and lichens. 4. Vegetation retained 31-68% of 15 N inputs, and 15 N recovery increased significantly (P = 0•01) with biomass for all compartments except woody shrubs. Mosses and lichens showed far greater 15 N recovery per unit biomass than grasses or ericaceous shrubs. There was no significant variation in the proportion of 15 N recovered in higher plants across the N deposition gradient (24-29%). In contrast, the proportion recovered in mosses and lichens declined from 44% to 2% as deposition increased, mirroring a decline in their biomass and showing a highly significant inverse relationship (P = 0•01) with nitrate leaching. 5. The proportion of 15 N recovered in litter plus surface soils (33-39%) was remarkably constant across the deposition gradient for a variety of soil types. However, significantly declining recovery per unit biomass in litter (P < 0•05) suggested progressive N saturation of this sink and increasing importance of retention in underlying surface soils as deposition increased. 6. Synthesis and applications. Past studies have demonstrated a decline in mosses and lichens in response to increasing N deposition, but we show here for the first time that reduced N retention might result together with increased nitrate leaching into surface waters. The conservation of bryophyte and lichen flora on moorlands is therefore critical to prevent excessive nitrate leaching and associated surface water acidification and eutrophication. Ensuring management practices such as grazing or burning are at an intensity that does not further degrade the bryophyte and lichen communities may help minimize the impact of N deposition on freshwaters, but the only effective means to reduce the risk of N leaching is a reduction in N emissions.

Anthropogenic N-deposition represents a significant input of N into semi-arid chaparral and coastal sage scrub (CSS) shrublands of southern California. High levels of atmospheric N deposition have the potential to increase soil C and N mineralization, and we hypothesize that semiarid shrubland soil exposed to long-term (decades) high N deposition will have significantly higher C and N mineralization potentials. This hypothesis was tested in a laboratory incubation where the inorganic N (NH 4 +NO 3) and CO 2 production of soils maintained at a constant temperature of 25-C and a soil moisture of 0.25 g H 2 O/g (65% water-filled pore space) were sampled sequentially over a 50-week period. The temporal trend in cumulative C and N mineralization was well described by a first-and zero-order model, respectively. Long-term atmospheric N deposition significantly increased potential N mineralization but not C mineralization, and both the rate and total N mineralization were significantly positively correlated with the surface (0-10 cm) soil % 15 N natural abundance and negatively correlated with the surface soil C:N ratio. While the incubation techniques used here do not provide realistic estimates of in situ C or N mineralization, these assays indicate that atmospheric N deposition has significantly altered ecosystem N storage and cycling.

Research Interests Ecosystem ecology, including carbon balance, nutrient cycling, and energy flows and exchanges, primarily in forests, wetlands, and human-dominated landscapes; ecosystem modeling and simulation; integrative modeling of social-ecological systems to explore scenarios, dynamics, and sustainability including biofuels and agricultural systems. Inter-and transdisciplinary research and teaching.

Nitrate immobilization into organic matter is thought to require catalysis by the enzymes of soil microorganisms. However, recent studies suggest that nitrate added to soil is immobilized rapidly and this process may include abiotic pathways. We amended living and sterilized soil with 15 N-labeled nitrate and nitrite to investigate biotic and abiotic immobilization. We report rapid transformation of nitrate in incubations of the O layer of forest soils that have been sterilized to prevent microbial activity and to denature microbial enzymes. Approximately 30, 40, and 60% of the 15 N-labeled nitrate added to live, irradiated, or autoclaved organic horizon soil disappeared from the extractable inorganic-N pool in less than 15 minutes. About 5% or less of the nitrate was recovered as insoluble organic N in live and sterilized soil, and the remainder was determined to be soluble organic N. Added 15 Nnitrite, however, was either lost to gaseous N or incorporated into an insoluble organic N form in both live and sterile organic soils. Hence, the fate and pathway of apparent abiotic nitrate immobilization differs from the better-known mechanisms of nitrite reactions with soil organic matter. Nitrate and nitrite added to live A-horizon soil was largely recovered in the form added, suggesting that rapid conversion of nitrate to soluble organic-N may be limited to C-rich organic horizons. The processes by which this temperate forest soil transforms added nitrate to soluble organic-N cannot be explained by established mechanisms, but appears to be due to abiotic processes in the organic horizon.

The effects of elevated CO 2 (ambient, +175, and +350 ll l À1) and nitrogen fertilization (0, 100, and 200 kg N ha À1 yr À1 as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO 2 caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO 2 decreased in the +175 ll l À1 and increased in the +350 ll l À1 after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO 2 in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO 2. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO 2 , was likely the soil organic N pool. There were no effects of elevated CO 2 on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO 2 and N treatments, indicating that N fertilization had no effect on responses to CO 2 and that CO 2 treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO 2 .

e tivity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

The use of sterilized reinoculated soils appears to be valuable in the study of relationships between numbers and activity of microorganisms, especially in the case of denitrifiers. In this context. residual enzymatic activities and physico-chemical changes after y-irradiation have been studied. The dynamics of disappearance of the residual activities of denitrifying enzymes, CO, release and /I-glucosidase activity were followed for 8 weeks. Evolution of some physico-chemical properties after p-irradiation were also tested: pH. N and C status. concentrations of exchangeable cations, soil porosity. The soil treatments were wet or dry y-sterilized and wet or dry stored after irradiation. Complete sterilization was achieved by a 25 kGy dose in all treatments. No important changes were noticed concerning the pore volume distribution. Very small variations in pH and exchangeable cation concentrations were considered negligible. On the contrary. an increase in soluble organic C. especially in the wet treatment, was attributed both to the lysis of killed cells and to the release of organic acids, particularly through the action of peroxides in wet conditions. The measurement of the enzymatic activities showed the resistance of enzymes to irradiation. leading for example to CO: production. to lysis of /?-glucosidic bonds or to variations in the N status through deamination of nitrogenous organic compounds or progressive proteolysis of microbial cells. Nevertheless. the location of enzymes was considered as a factor controlling resistance to y-rays and to its subsequent activity. We suggested that denitrifying enzymes were remaining active in non-proliferating cells and that the loss of potential denitrifying activity, observed for the 8 weeks period. rcsultcd from cell lysis. It was concluded that y-irradiation of dry soil is less damaging than that of wet soil. Ncverthelcss, the soil must stabilize before use and a control (uninocul;lIed irradiated soil) must be compared in order not to attribute residual activities of the irradiated soil to the introduced microbial populalions. lNTWODUCTlON

Many studies have shown the importance of ectomycorrhizal fungi (EM) in forests both for nutrient availability and for carbon (C) and nutrient cycling in the soil. Yet so far they are not incorporated in forest ecosystem growth and yield models. Recent research suggests phosphorus (P) shortage could be a major constraints to forest productivity in the future. For a realistic simulation of future forest ecosystem functioning, inclusion of detailed soil P cycling and the trees-EM interaction is necessary. We developed a full ecosystem P model that simulates P uptake by roots and EM, allocation within trees, physiological deficiency effects on C assimilation and allocation, release through litter decomposition, coupled with water, C and nitrogen (N) fluxes accounted for in the mechanistic forest stand model ANAFORE. Our results confirm the importance of incorporating EM in forest ecosystem models and suggest that the lack of incorporation of P in models may result in an under- or overe...

) at each of the four study sites, ranging from north (lowest deposition, site A) to south (highest deposition, site D) from 1988-2007. Time trends for leaf N with P ≤ 0.05 shown as dashed lines. Senescent leaf N (mg g -1 ) Leaf litter mass (kg ha -1 ) Supplemental information AF Talhelm et al. www.frontiersinecology.org WebFigure 3. Spring and fall soil solution total (inorganic and organic) dissolved N (circles, dashed trend lines) and growing season (April-October) soil temperature (triangles, solid trend lines). Trend lines shown for time trends with P ≤ 0.05. Total dissolved N measured from 1994-2006 and soil temperature measured from 1988-2005. Soil temperature was measured at 15-cm depth with thermistors (Model ES-060-SW, Data Loggers Inc, Logan, Utah) read every 30 minutes by Omnidata EasyLoggers (Models 824 and 925, Data Loggers Inc, Logan, Utah) and averaged every 3 hours. Total dissolved N (kg N ha -1 ) Soil temperature (˚C) AF Talhelm et al. Supplemental information WebFigure 4. Mean annual Ca wet deposition (circles) and the proportion of wet inorganic N deposition that is in the form of NH 4 + (triangles) averaged from the three atmospheric deposition monitoring sites nearest to the four study sites. The linear trends through time for Ca were not significant overall (P > 0.366) or at any individual site (P > 0.25). The NH 4 + proportion of N deposition increased consistently overall (P < 0.001) and at each site individually (P < 0.002).

In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition–C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobiliza-tion by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha -1 year -1 as dissolved NH 4 NO 3 directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as 15 NH 4 ? or 15 NO 3 -(1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the 15 N added as 15 NH 4 ? and 15 NO 3 -, respectively. Of 15 N recoverable in plant biomass, only 3–6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO 3 -and NH 4 ? , together accounting for 25–50% of 15 N recovery for these ...

A major uncertainty in predicting long-term ecosystem C balance is whether stimulation of net primary production will be sustained in future atmospheric CO 2 scenarios. Immobilization of nutrients (N in particular) in plant biomass and soil organic matter (SOM) provides negative feedbacks to plant growth and may lead to progressive N limitation (PNL) of plant response to CO 2 enrichment. Soil microbes mediate N availability to plants by controlling litter decomposition and N transformations as well as dominating biological N fixation. CO 2 -induced changes in C inputs, plant nutrient demand and water use efficiency often have interactive and contrasting effects on microbes and microbially mediated N processes. One critical question is whether CO 2 -induced N accumulation in plant biomass and SOM will result in N limitation of microbes and subsequently cause them to obtain N from alternative sources or to alter the ecosystem N balance. We reviewed the experimental results that examined elevated CO 2 effects on microbial parameters, focusing on those published since 2000. These results in general show that increased C inputs dominate the CO 2 impact on microbes, microbial activities and their subsequent controls over ecosystem N dynamics, potentially enhancing microbial N acquisition and ecosystem N retention. We reason that microbial mediation of N availability for plants under future CO 2 scenarios will strongly depend on the initial ecosystem N status, and the nature and magnitude of external N inputs. Consequently, microbial processes that exert critical controls over long-term N availability for plants would be ecosystem-specific. The challenge remains to quantify CO 2 -induced changes in these processes, and to extrapolate the results from short-term studies with step-up CO 2 increases to native ecosystems that are already experiencing gradual changes in the CO 2 concentration.

Soil microbial immobilization and plant uptake of N were evaluated for three forest types in Kochi, Shikoku district. During 196-d laboratory incubation, soil NO,-N production in the Hinoki cypress forest was negligible for the initid 40 d and then rapidly increased, whereas NO,-N production was rapid from the beginning in Japanese cedar and deciduous hardwood forests. Microbial immobilization of the labeled I5N decreased in the order of NH4-N > glycine-N>NO,-N. The 16N immobilization was higher for soil in the Hinoki cypress forest than other two soils. The delayed NO,-N production in the Hinoki cypress forest was likely related with low availability of NH4-N due to NH4-N immobilization and substantial NO,-N immobilization. In the field experiment, l6N uptake by roots decreased in the order of NH4-N>N0,-N>glycine-N. The absorption of the labeled "C suggested direct uptake of organic N. The preference of N forms by root uptake was not different among forest types. 'bees in three forest types can absorb inorganic and organic forms of N, suggesting trees absorb the N form that is the most abundant in the soil.

Microbial nitrification and denitrification both can emit nitrous oxide (N 2 O), a major greenhouse gas, and the relative contribution of each pathway depends strongly on soil moisture conditions. We conducted a stable isotope tracer experiment to determine the contribution of nitrification and denitrification to N 2 O and dinitrogen (N 2 ) fluxes in coastal plain wetlands, and to determine the response of these processes to changing soil moisture. We added 15 N-labeled nitrate (NO À 3 ) or ammonium (NH þ 4 ) to intact soil cores collected from an agricultural field, a restored wetland, and a preserved forested wetland, and subjected the cores to a drying or wetting hydrologic manipulation. Across all soils and treatments, the combined fluxes of N 2 O and N 2 ranged widely, between 0.23 and 2900 mg N m À2 h À1 , and N 2 O accounted for as little as 0% to as much as 43% of the total gaseous nitrogen (N) fluxes. Fluxes of both gases increased with increasing soil moisture in all soils and tracer treatments, but the relative enhancement of the two gases varied by soil type and N source. The N 2 O yields [N 2 O/(N 2 O þ N 2 )] derived from both nitrification and denitrification were low (1e3%) in five of eight soils in each tracer experiment. Surprisingly, nitrification-derived N 2 O yields were highest (13e31%) in soils with the highest organic matter and soil moisture (restored wetland under simulated rain and forested wetland under drained and simulated rain), while denitrification-derived N 2 O yields (12e36%) were highest under simulated rain in the two mineral soils (agricultural field and mineral soils of the restored wetland), and under drained conditions in the forested wetland. These results are consistent with field-measured N 2 O fluxes in our previous work in these sites. We suggest that nitrification plays an important and underappreciated role in contributing to N 2 O fluxes from freshwater wetlands with often-saturated, acid-organic soils, while incomplete denitrification is the likely source of N 2 O following rain events in agricultural soils in southeastern U.S. coastal plain wetlands.

Presently, there is uncertainty regarding the degree to which anthropogenic N deposition will foster C storage in the N-limited forests of the Northern Hemisphere, ecosystems which are globally important sinks for anthropogenic CO 2 . We constructed organic matter and N budgets for replicate northern hardwood stands (n ¼ 4) that have received ambient (0.7-1.2 g NÁm À2 Áyr À1 ) and experimental NO 3 À deposition (ambient plus 3 g NO 3 À -NÁm À2 Áyr À1 ) for a decade; we also traced the flow of a 15 NO 3 À pulse over a six-year period. Experimental NO 3 À deposition had no effect on organic matter or N stored in the standing forest overstory, but it did significantly increase the N concentration (þ19%) and N content (þ24%) of canopy leaves. In contrast, a decade of experimental NO 3 À deposition significantly increased amounts of organic matter (þ12%) and N (þ9%) in forest floor and mineral soil, despite no increase in detritus production. A greater forest floor (Oe/a) mass under experimental NO 3 À deposition resulted from slower decomposition, which is consistent with previously reported declines in lignolytic activity by microbial communities exposed to experimental NO 3 À deposition. Tracing 15 NO 3 À revealed that N accumulated in soil organic matter by first flowing through soil microorganisms and plants, and that the shedding of 15 N-labeled leaf litter enriched soil organic matter over a six-year duration. Our results demonstrate that atmospheric NO 3 À deposition exerts a direct and negative effect on microbial activity in this forest ecosystem, slowing the decomposition of aboveground litter and leading to the accumulation of forest floor and soil organic matter. To the best of our knowledge, this mechanism is not represented in the majority of simulation models predicting the influence of anthropogenic N deposition on ecosystem C storage in northern forests.

Anthropogenic nitrogen (N) deposition may have several impacts on upland moorland ecosystems, including changes in vegetation composition, eutrophication and surface water acidification through nitrate leaching, but few studies linking N deposition to key biogeochemical processes have been published.

In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition–C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobiliza-tion by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha -1 year -1 as dissolved NH 4 NO 3 directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as 15 NH 4 ? or 15 NO 3 -(1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the 15 N added as 15 NH 4 ? and 15 NO 3 -, respectively. Of 15 N recoverable in plant biomass, only 3–6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO 3 -and NH 4 ? , together accounting for 25–50% of 15 N recovery for these ...

Sterilized soils are frequently used in experiments related to soil biology. Soil sterilization is known to alter physicochemical characteristics of soil, plant growth and community structure of the newly developed bacterial population. However, little information exists regarding soil sterilization effects on belowground processes mediated through root-microbe-soil interactions, e.g. development of rhizosheaths which significantly promote the plant growth under stress environments. The present study was conducted to elucidate effects of soil sterilization on wheat root growth and formation of rhizosheaths in relation to chemical changes caused by soil sterilization and the proportion of expolysaccharide(EPS)-producers in bacterial population recolonizing the sterilized soils. Wheat plants were grown for two weeks under greenhouse conditions either in the unsterilized soil or in soils sterilized by autoclaving (121°C, one hour) or by gamma(γ)-irradiation (50 kGy). While soil sterilization had no effect on the release of macronutrients, both sterilization procedures significantly increased the electrical conductivity, watersoluble carbon and DTPA-extractable Mn. Seedlings grown in sterilized soils produced higher root biomass and the rhizosheath soil (RS) mass as compared to those grown in the unsterilized soil. Soil sterilization also increased the root length, surface area, volume and number of tips. In bulk soil, RS and on roots, the proportion of EPS-producers in the total bacterial population was higher in sterilized treatments than in the unsterilized. Amending the unsterilized soil with glucose-C increased the root biomass, whereas adding Mn II increased the RS mass. The results showed that soil sterilization by autoclaving or γ-irradiation increases the root growth and RS mass of wheat seedlings. The water-soluble C and DTPA-extractable Mn released upon sterilization, and the increased proportion of EPS-producers in the bacterial population recolonizing the sterilized soils were involved in the observed effects. The results may have implications in studies using autoclaved or γ-irradiated soils to investigate soil-plant-microbe interactions and signify the need to account for intrinsic stimulatory effects of soil sterilization.

S#oil nitrogen transformations were measured the year following nitrogen fertilization of alpine Kobresia myosuroides meadows to determine the influence of greater plant production and N content on net N mineralization and the microbial N pool. Previously fertilized soils contained substantially greater amounts of organic N than control soils. The average increase in soil organic N accounted for 75% of total added :V and, although variable, this quantity suggests a large capacity for retention of added N in these soils. Nitrogen transformations and more active pools also responded to fertilization. Net N mineralization, nitrification and soil inorganic N concentrations clearly were higher in fertilized than in control plots throughout the snow-free season. The most pronounced increase in mineralization in fertilized relative to control soils occurred during the second half of the snow-free period (mid-July-October), primarily after the short alpine growing season (Junemid-August). Although the soil microbial N pool was not affected by fertilization during the growing season, microbial N did increase in the fall in fertilized compared to control soils, coinciding with the time of greatest N mineralization. The late season net uptake of N into the microbial pool exceeded by several times present rates of anthropogenic N inputs to these soils, and the microbial biomass may act as an increasingly important short-term sink for available N if N deposition to these areas continues to increase. Copyright 0 1996

Disturbance affects most terrestrial ecosystems and has the potential to shape their responses to chronic environmental change. Scrub-oak vegetation regenerating from fire disturbance in subtropical Florida was exposed to experimentally elevated carbon dioxide (CO₂) concentration (+350 μl l(-1)) using open-top chambers for 11 yr, punctuated by hurricane disturbance in year 8. Here, we report the effects of elevated CO₂ on aboveground and belowground net primary productivity (NPP) and nitrogen (N) cycling during this experiment. The stimulation of NPP and N uptake by elevated CO₂ peaked within 2 yr after disturbance by fire and hurricane, when soil nutrient availability was high. The stimulation subsequently declined and disappeared, coincident with low soil nutrient availability and with a CO₂ -induced reduction in the N concentration of oak stems. These findings show that strong growth responses to elevated CO₂ can be transient, are consistent with a progressively limited response ...

Ecosystem ecology, including carbon balance, nutrient cycling, and energy flows and exchanges, primarily in forests, wetlands, and human-dominated landscapes; ecosystem modeling and simulation; integrative modeling of social-ecological systems to explore scenarios, dynamics, and sustainability including biofuels and agricultural systems. Inter-and transdisciplinary research and teaching.

The expansion of tropical forest into savanna may potentially be a large carbon sink, but little is known about the patterns of carbon sequestration during transitional forest formation. Moreover, it is unclear how nutrient limitation, due to extended exposure to firedriven nutrient losses, may constrain carbon accumulation. Here, we sampled plots that spanned a woody biomass gradient from savanna to transitional forest in response to differential fire protection in central Brazil. These plots were used to investigate how the process of transitional forest formation affects the size and distribution of carbon (C) and nitrogen (N) pools. This was paired with a detailed analysis of the nitrogen cycle to explore possible connections between carbon accumulation and nitrogen limitation. An analysis of carbon pools in the vegetation, upper soil, and litter shows that the transition from savanna to transitional forest can result in a fourfold increase in total carbon (from 43 to 179 Mg C/ha) with a doubling of carbon stocks in the litter and soil layers. Total nitrogen in the litter and soil layers increased with forest development in both the bulk (þ68%) and plant-available (þ150%) pools, with the most pronounced changes occurring in the upper layers. However, the analyses of nitrate concentrations, nitrate : ammonium ratios, plant stoichiometry of carbon and nitrogen, and soil and foliar nitrogen isotope ratios suggest that a conservative nitrogen cycle persists throughout forest development, indicating that nitrogen remains in low supply relative to demand. Furthermore, the lack of variation in underlying soil type (.20 cm depth) suggests that the biogeochemical trends across the gradient are driven by vegetation. Our results provide evidence for high carbon sequestration potential with forest encroachment on savanna, but nitrogen limitation may play a large and persistent role in governing carbon sequestration in savannas or other equally fire-disturbed tropical landscapes. In turn, the link between forest development and nitrogen pool recovery creates a framework for evaluating potential positive feedbacks on savanna-forest boundaries.

The expansion of tropical forest into savanna may potentially be a large carbon sink, but little is known about the patterns of carbon sequestration during transitional forest formation. Moreover, it is unclear how nutrient limitation, due to extended exposure to firedriven nutrient losses, may constrain carbon accumulation. Here, we sampled plots that spanned a woody biomass gradient from savanna to transitional forest in response to differential fire protection in central Brazil. These plots were used to investigate how the process of transitional forest formation affects the size and distribution of carbon (C) and nitrogen (N) pools. This was paired with a detailed analysis of the nitrogen cycle to explore possible connections between carbon accumulation and nitrogen limitation. An analysis of carbon pools in the vegetation, upper soil, and litter shows that the transition from savanna to transitional forest can result in a fourfold increase in total carbon (from 43 to 179 Mg C/ha) with a doubling of carbon stocks in the litter and soil layers. Total nitrogen in the litter and soil layers increased with forest development in both the bulk (þ68%) and plant-available (þ150%) pools, with the most pronounced changes occurring in the upper layers. However, the analyses of nitrate concentrations, nitrate : ammonium ratios, plant stoichiometry of carbon and nitrogen, and soil and foliar nitrogen isotope ratios suggest that a conservative nitrogen cycle persists throughout forest development, indicating that nitrogen remains in low supply relative to demand. Furthermore, the lack of variation in underlying soil type (.20 cm depth) suggests that the biogeochemical trends across the gradient are driven by vegetation. Our results provide evidence for high carbon sequestration potential with forest encroachment on savanna, but nitrogen limitation may play a large and persistent role in governing carbon sequestration in savannas or other equally fire-disturbed tropical landscapes. In turn, the link between forest development and nitrogen pool recovery creates a framework for evaluating potential positive feedbacks on savanna-forest boundaries.

• Anthropogenic nitrogen (N) addition may substantially alter the terrestrial N cycle. However, a comprehensive understanding of how the ecosystem N cycle responds to external N input remains elusive. • Here, we evaluated the central tendencies of the responses of 15 variables associated with the ecosystem N cycle to N addition, using data extracted from 206 peer-reviewed papers. • Our results showed that the largest changes in the ecosystem N cycle caused by N addition were increases in soil inorganic N leaching (461%), soil NO₃⁻ concentration (429%), nitrification (154%), nitrous oxide emission (134%), and denitrification (84%). N addition also substantially increased soil NH₄+ concentration (47%), and the N content in belowground (53%) and aboveground (44%) plant pools, leaves (24%), litter (24%) and dissolved organic N (21%). Total N content in the organic horizon (6.1%) and mineral soil (6.2%) slightly increased in response to N addition. However, N addition induced a decrease in microbial biomass N by 5.8%. • The increases in N effluxes caused by N addition were much greater than those in plant and soil pools except soil NO₃⁻, suggesting a leaky terrestrial N system.

In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition–C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobiliza-tion by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha -1 year -1 as dissolved NH 4 NO 3 directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as 15 NH 4 ? or 15 NO 3 -(1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the 15 N added as 15 NH 4 ? and 15 NO 3 -, respectively. Of 15 N recoverable in plant biomass, only 3–6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO 3 -and NH 4 ? , together accounting for 25–50% of 15 N recovery for these ...

Human activities have greatly accelerated emissions of both carbon dioxide and biologically reactive nitrogen to the atmosphere 1,2 . As nitrogen availability often limits forest productivity 3 , it has long been expected that anthropogenic nitrogen deposition could stimulate carbon sequestration in forests 4 . However, spatially extensive evidence for depositioninduced stimulation of forest growth has been lacking, and quantitative estimates from models and plot-level studies are controversial 5-10 . Here, we use forest inventory data to examine the impact of nitrogen deposition on tree growth, survival and carbon storage across the northeastern and north-central USA during the 1980s and 1990s. We show a range of growth and mortality responses to nitrogen deposition among the region's 24 most common tree species. Nitrogen deposition (which ranged from 3 to 11 kg ha −1 yr −1 ) enhanced the growth of 11 species and decreased the growth of 3 species. Nitrogen deposition enhanced growth of all tree species with arbuscular mycorrhizal fungi associations. In the absence of disturbances that reduced carbon stocks by more than 50%, above-ground biomass increment increased by 61 kg of carbon per kg of nitrogen deposited, amounting to a 40% enhancement over pre-industrial conditions. Extrapolating to the globe, we estimate that nitrogen deposition could increase tree carbon storage by 0.31 Pg carbon yr −1 .

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar d 15 N may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root d 15 N and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar d 15 N than the regional driver of N deposition. Foliar d 15 N increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar d 15 N was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root d 15 N was more tightly coupled to forest floor properties than was foliar d 15 N. We observed a pattern of decreasing foliar d 15 N values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar d 15 N included species composition and climate. Relationships between foliar d 15 N and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar d 15 N, due to the importance of ammonium deposition in this region. Our results suggest that examining d 15 N values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.

Concern is resurfacing in the United States over the long-term effects of excess nitrogen (N) deposition and mobility in the environment. We present here a new synthesis of existing data sets for the northeastern United States, intended to answer a single question: Is N deposition altering the N status of forest ecosystems in this region? Surface water data suggest a significant increase in nitrate losses with N deposition. Soil data show an increase in nitrification with decreasing ratio of soil carbon to nitrogen (C:N) but weaker relationships between N deposition and soil C:N ratio or nitrification. Relationships between foliar chemistry and N deposition are no stronger than with gradients of climate and elevation. The differences in patterns for these three groups of indicators are explained by the degree of spatial and temporal integration represented by each sample type. The surface water data integrate more effectively over space than the foliar or soil data and therefore allow a more comprehensive view of N saturation. We conclude from these data that N deposition is altering N status in northeastern forests.