Gross Nitrogen Turnover of Natural and Managed Tropical Ecosystems at Mt. Kilimanjaro,Tanzania

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH₄ ⁺ immobilization 38.92, NO₃ ^? immobilization 10.74, and DNRA 1.54 µg N g^?1 SDW d^?1) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups.     

Cross-Ecosystem Comparisons of In Situ Plant Uptake of Amino Acid-N and NH4 +

Plant and microbial use of nitrogen (N) can be simultaneously mutualistic and competitive, particularly in ecosystems dominated by mycorrhizal fungi. Our goal was to quantify plant uptake of organic and inorganic N across a broad latitudinal gradient of forest ecosystems that varied with respect to overstory taxon, edaphic characteristics, and dominant mycorrhizal association. Using ¹³C and ¹⁵N, we observed in situ the cycling dynamics of NH₄ ⁺ and glycine through various soil pools and fine roots over 14 days. Recovery of ¹⁵N as soil N varied with respect to N form, forest type, and sampling period; however, there were similarities in the cycling dynamics of glycine and NH₄ ⁺ among all forest types. Microbial immobilization of ¹⁵N was immediately apparent for both treatments and represented the largest sink (~25%) for ¹⁵N among extractable soil N pools during the first 24 h. In contrast, fine roots were a relatively small sink (<10%) for both N forms, but fine root ¹³C enrichment indicated that plants in all forest types absorbed glycine intact, suggesting that plants and microbes effectively target the same labile soil N pools. Relative uptake of amino acid-N versus NH₄ ⁺ varied significantly among sites and approximately half of this variation was explained by mycorrhizal association. Estimates of plant uptake of amino acid-N relative to NH₄ ⁺ were 3× higher in ectomycorrhizal-dominated stands (1.6 ± 0.2) than arbuscular mycorrhizae-dominated stands (0.5 ± 0.1). We conclude that free amino acids are an important component of the N economy in all stands studied; however, in these natural environments plant uptake of organic N relative to inorganic N is explained as much by mycorrhizal association as by the availability of N forms per se.     

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding ¹⁵ N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH₄ ⁺, NO₃ ⁻, ¹⁵NH₄ ⁺ and ¹⁵ NO₃ ⁻. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization increased. In the H layer, however, both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate ¹⁵N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims

Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates.

Methods

A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the ¹⁵N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada.

Results

Gross N mineralization and gross NH ₄ ⁺ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg^-1 soil d^-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH ₄ ⁺ immobilization rates (N/IA) and cumulative N₂O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N₂O emission was lower at 65% WHC.

Conclusions

The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N₂O emissions in both soils.     

The role of gross and net N transformation processes and NH4 + and NO3 − immobilization in controlling the mineral N pool of a temperate mixed deciduous forest soil

In many forests of Europe and north-eastern North America elevated N deposition has opened the forest N cycle, resulting in NO₃ ^? leaching. On the other hand, despite this elevated N deposition, the dominant fate of NO₃ ^? and NH₄ ⁺ in some of these forests is biotic or abiotic immobilization in the soil organic matter pool, preventing N losses. The environmental properties controlling mineral N immobilization and the variation and extent of mineral N immobilization in forest soils are not yet fully understood. In this study we investigated a temperate mixed deciduous forest, which is subjected to an average N deposition of 36.5 kg N ha^?1 yr^?1, but at the same time shows low NO₃ ^? concentrations in the groundwater. The aim of this study was to investigate whether the turnover rate of the mineral N pool could explain these low N leaching losses. A laboratory ¹⁵N pool dilution experiment was conducted to study gross and net N mineralization and nitrification and mineral N immobilization in the organic and uppermost (0–10 cm) mineral layer of the forest soil. Two locations, one at the forest edge (GE) and another one 145 m inside the forest (GF1), were selected. In the organic layers of GE and GF1, the gross N mineralization averaged 10.9 and 11.1 mg N kg^?1 d^?1, the net N mineralization averaged 6.1 and 6.8 mg N kg^?1 d^?1 and NH₄ ⁺ immobilization rates averaged 3.8 and 3.6 mg N kg^?1 d^?1. In the organic layer of GE and GF1, the average gross nitrification was 3.8 and 4.6 mg N kg^?1 d^?1, the average net nitrification was ?25.2 and ?31.3 mg N kg^?1 d^?1 and the NO₃ ^? immobilization rates averaged 29.0 and 35.9 mg N kg^?1 d^?1. For the mineral (0–10 cm) layer the same trend could be observed, but the N transformation rates were much lower for the NH₄ ⁺ pool and not significantly different from zero for the NO₃ ^? pool. Except for the turnover of the NH₄ ⁺ pool in the mineral layer, no significant differences were observed between location GE and GF1. The ratio of NH₄ ⁺ immobilization to gross N mineralization, gross N mineralization to gross nitrification, and NO₃ ^? immobilisation to gross nitrification led to the following observations. The NH₄ ⁺ pool of the forest soil was controlled by N mineralization and NO₃ ^? immobilization was importantly controlling the forest NO₃ ^? pool. Therefore it was concluded that this process is most probably responsible for the limited NO₃ ^? leaching from the forest ecosystem, despite the chronically high N deposition rates.     

Inorganic nitrogen immobilization in live and sterile soil of old-growth conifer and hardwood forests: implications for ecosystem nitrogen retention

Rapid immobilization of inorganic nitrogen (N) in soil contributes to ecosystem N accumulation, even in old-growth and chronically-fertilized forests once thought to have poor N retention capacity. In old-growth conifer and hardwood stands in Pennsylvania, we tested the hypotheses that biotic and abiotic N immobilization are regulated by N form and forest type. We added ¹⁵NH₄ ⁺, ¹⁵NO₂ ^?, and ¹⁵NO₃ ^? to sterile (γ-irradiated) and live organic-horizon soil and define N immobilization as the mass of added ¹⁵N remaining in soil following extractions conducted 15 min, 24 h, and 21 days later. Immobilization of NO₂ ^? (19–25% of added N) occurred in sterile soils within 15 min and was little changed thereafter. Tracer NO₃ ^? immobilization was not observed, although soils had been pretreated (refrigerated) so as to quantify the lower limit of immobilization potential. Immobilization of NH₄ ⁺ (27%) occurred in live conifer soils by 21 days but not in other treatments. In 21-day incubations, tracer N immobilization was greater in NO₃ ^?-poor and humic-rich soils. Immobilization was greater in sterile than in live soil, perhaps owing to artifacts of sterilization. Conifer stands exhibited more massive O-horizons, so NO₂ ^? immobilization per unit area was greater in conifer (1.46 mg N m^?2) than hardwood (0.43 mg N m^?2) stands, possibly accounting for lower N leaching from conifer forests. Areal immobilization rates appear to be fast enough to retain all N transformed to NO₂ ^?, so NO₂ ^? production may be a limiting step in soil N retention in old-growth ecosystems.     

Gross Nitrogen Dynamics in the Mycorrhizosphere of an Organic Forest Soil

The rhizosphere is a hot-spot for biogeochemical cycles, including production of greenhouse gases, as microbial activity is stimulated by rhizodeposits released by roots and mycorrhizae. The biogeochemical cycle of nitrogen (N) in soil is complex, consisting of many simultaneously occurring processes. In situ studies investigating the effects of roots and mycorrhizae on gross N turnover rates are scarce. We conducted a ¹⁵N tracer study under field conditions in a spruce forest on organic soil, which was subjected to exclusion of roots and roots plus ectomycorrhizae (ECM) for 6 years by trenching. The forest soil had, over the 6-year period, an average emission of nitrous oxide (N₂O) of 5.9 ± 2.1 kg N₂O ha^?1 year^?1. Exclusion of roots + ECM nearly tripled N₂O emissions over all years, whereas root exclusion stimulated N₂O emission only in the latest years and to a smaller extent. Gross mineralization–ammonium (NH₄ ⁺) immobilization turnover was enhanced by the presence of roots, probably due to high inputs of labile carbon, stimulating microbial activity. We found contrasting effects of roots and ECM on N₂O emission and mineralization, as the former was decreased but the latter was stimulated by roots and ECM. The N₂O emission was positively related to the ratio of gross NH₄ ⁺ oxidation (that is, autotrophic nitrification) to NH₄ ⁺ immobilization. Ammonium oxidation was only stimulated by the presence of ECM, but not by the presence of roots. Overall, we conclude that plants and their mycorrhizal symbionts actively control soil N cycling, thereby also affecting N₂O emissions from forest soils. Consequently, adapted forest management with permanent tree cover avoiding clearcutting could be a means to reduce N₂O emissions and potential N leaching; despite higher mineralization in the presence of roots and ECM, N₂O emissions are decreased as the relative importance of NH₄ ⁺ oxidation is decreased, mainly due to a stimulated microbial NH₄ ⁺ immobilization in the mycorrhizosphere.     

Stable Nitrogen and Carbon Pools in Grassland Soils of Variable Texture and Carbon Content

Nitrogen (N) inputs to many terrestrial ecosystems are increasing, and most of these inputs are sequestered in soil organic matter within 1–3 years. Rapid (minutes to days) immobilization focused previous N retention research on actively cycling plant, microbial, and inorganic N pools. However, most ecosystem N resides in soil organic matter that is not rapidly cycled. This large, stable soil N pool may be an important sink for elevated N inputs. In this study, we measured the capacity of grassland soils to retain ¹⁵N in a pool that was not mineralized by microorganisms during 1-year laboratory incubations (called “the stable pool”). We added two levels (2.5 and 50 g N m⁻²) of ¹⁵NH₄ ⁺ tracer to 60 field plots on coarse- and fine-textured soils along a soil carbon (C) gradient from Texas to Montana, USA. We hypothesized that stable tracer ¹⁵N retention and stable bulk soil (native + tracer) N pools would be positively correlated with soil clay and C content and stable soil C pools (C not respired during the incubation). Two growing seasons after the ¹⁵N addition, soils (0- to 20-cm depth) contained 71% and 26% of the tracer added to low- and high-N treatments, respectively. In both N treatments, 50% of the tracer retained in soil was stable. Total soil C (r ² = 0.72), stable soil C (r ² = 0.68), and soil clay content (r ² = 0.27) were correlated with stable bulk soil N pools, but not with stable ¹⁵N retention. We conclude that on annual time scales, substantial quantities of N are incorporated into stable organic pools that are not readily susceptible to microbial remineralization or subsequent plant uptake, leaching losses, or gaseous losses. Stable N formation may be an important pathway by which rapid soil N immobilization translates into long-term N retention. Received 2 April 2001; accepted 12 November 2001.     

Changes in nitrogen cycling and retention processes in soils under spruce forests along a nitrogen enrichment gradient in Germany

A network of long-term monitoring sites on nitrogen (N) input and output of forests across Germany showed that a number of Germany's forests are subject to or are experiencing N saturation and that spruce (Picea abies) stands have high risk. Our study was aimed at (1) quantifying the changes in gross rates of microbial N cycling and retention processes in forest soils along an N enrichment gradient and (2) relating the changes in soil N dynamics to N losses. We selected spruce sites representing an N enrichment gradient (indicated by leaching : throughfall N ratios) ranging from 0.04–0.13 (low N),≤0.26 (intermediate N enrichment) to≥0.42 (highly N enriched). To our knowledge, our study is the first to report on mechanistic changes in gross rates of soil N cycling and abiotic NO₃⁻ retention under ambient N enrichment gradient. Gross N mineralization, NH₄⁺ immobilization, gross nitrification, and NO₃⁻ immobilization rates increased up to intermediate N enrichment level and somewhat decreased at highly N-enriched condition. The turnover rates of NH₄⁺ and microbial N pools increased while the turnover rates of the NO₃⁻ pool decreased across the N enrichment gradient. Abiotic immobilization of NH₄⁺ did not differ across sites and was lower than that of NO₃⁻. Abiotic NO₃⁻ immobilization decreased across the N enrichment gradient. Microbial assimilation and turnover appeared to contribute largely to the retention of NH₄⁺. The increasing NO₃⁻ deposition and decreasing turnover rates of the NO₃⁻ pool, combined with decreasing abiotic NO₃⁻ retention, possibly contributed to increasing NO₃⁻ leaching and gaseous emissions across the N enrichment gradient. The empirical relationships of changes in microbial N cycling across the N enrichment gradient may be integrated in models used to predict responses of forest ecosystems (e.g. spruce) to increasing N deposition.     

Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China

The foliar stable N isotope ratio (δ¹⁵N) can provide integrated information on ecosystem N cycling. Here we present the δ¹⁵N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ¹⁵N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ¹⁵N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more ¹⁵N enriched. Our results show that foliar δ¹⁵N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO₃ ^? had low δ¹⁵N (?11.4 to ?3.2 ‰) and plant NO₃ ^? uptake could not explain the negative foliar δ¹⁵N values (NH₄ ⁺ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH₄ ⁺ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH₃/NH₄ ⁺. The variation in foliar δ¹⁵N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ¹⁵N and the enrichment factor (foliar δ¹⁵N minus soil δ¹⁵N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ¹⁵N between primary and secondary forests.     

Microbial immobilization drives nitrogen cycling differences among plant species

In many terrestrial ecosystems nitrogen (N) limits productivity and plant community composition is influenced by N availability. However, vegetation is not only controlled by N; plant species may influence ecosystem N dynamics through positive or negative effects on N cycling. We examined four potential mechanisms of plant species effects on nitrogen (N) cycling. We found no species differences in gross ammonification suggesting there are no changes in the ecosystem N cycling rate between the soil organic matter pool (SOM) and the plant/microbial pool. We also found weak differences among plant species in gross nitrification, thus plant species only marginally change the relative sizes of the NH₄⁺ and NO₃^? pools. Next, more than 90% of mineralized N was microbially immobilized, and microbial N immobilization was positively correlated with root biomass. Finally, while species differed in extractable soil NO3^? concentration, these differences were not related to root biomass suggesting that microbial immobilization drives net N mineralization and soil NO₃^? levels. Our results indicate that plant species do not cause feedbacks on the N cycling rate among the three major ecosystem N pools over nine years. However, plant carbon (C) inputs to the soil control microbial N immobilization and thereby change N partitioning between the plant and microbial N pools. Furthermore our results suggest that the SOM pool can act as a strong bottleneck for N cycling in these systems.     

The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. ¹⁵N-labelled urea, ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (80 kg N ha^-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0–26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop ¹⁵N uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha^-1 day^-1 at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. ¹⁵N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH₄ ⁺ and NO₃ ^- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.     

Dynamics of Aboveground and Soil Carbon and Nitrogen Stocks and Cycling of Available Nitrogen along a Land-use Gradient in Rondônia,Brazil

High rates of deforestation in the Brazilian Amazon have the potential to alter the storage and cycling of carbon (C) and nitrogen (N) across this region. To investigate the impacts of deforestation, we quantified total aboveground biomass (TAGB), aboveground and soil pools of C and N, and soil N availability along a land-use gradient in Rondônia, Brazil, that included standing primary forest, slashed primary and secondary forest, shifting cultivation, and pasture sites. TAGB decreased substantially with increasing land use, ranging from 311 and 399 Mg ha^–1 (primary forests) to 63 Mg ha^–1 (pasture). Aboveground C and N pools declined in patterns and magnitudes similar to those of TAGB. Unlike aboveground pools, soil C and N concentrations and pools did not show consistent declines in response to land use. Instead, C and N concentrations were strongly related to percent clay content of soils. Concentrations of NO₃-N and NH₄-N generally increased in soils following slash-and-burn events along the land-use gradient and decreased with increasing land use. Increasing land use resulted in marked declines in NO₃-N pools relative to NH₄-N pools. Rates of net nitrification and N-mineralization were also generally higher in postfire treatments relative to prefire treatments along the land-use gradient and declined with increasing land use. Results demonstrate the linked responses of aboveground C and N pools and soil N availability to land use in the Brazilian Amazon; steady reductions in aboveground pools along the land-use gradient were accompanied by declines in inorganic soil N pools and transformation rates.     

Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA

Forest fires often result in a series of biogeochemical processes that increase soil nitrate (NO₃ ^?) concentrations for several years; however, the dynamic nature of inorganic nitrogen (N) cycling in the plant–microbe–soil complex makes it challenging to determine the direct causes of increased soil NO₃ ^?. We measured gross inorganic N transformation rates in mineral soils 2 years after wildfires in three central Idaho coniferous forests to determine the causes of the elevated soil NO₃ ^?. We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and carbon (C) availability. We found no significant differences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO₃ ^? uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO₃ ^? caused slightly elevated NO₃ ^? concentrations in the burned soils. C availability was positively correlated with microbial NO₃ ^? uptake rates. Despite reduced microbial NO₃ ^? uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO₃ ^? concentrations 2 years post fire. Soil NH₄ ⁺ concentrations between the treatments were not significantly different (P = 0.673). However, gross NH₄ ⁺ production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively correlated with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region.     

Low sedimentary accumulation of lead caused by weak downward export of organic matter in Hudson Bay,northern Canada

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH₄ ⁺) and nitrate (NO₃ ^?). Previous long-term studies indicate efficient turnover of atmogenic NH₄ ⁺ to NO₃ ^? in weathered, acidic soils of the subtropics, leading to excessive NO₃ ^? leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ ¹⁵N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. ¹⁵NH₄NO₃, NH ₄ ¹⁵ NO₃ and ¹⁵N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (N₂O) were monitored for nine days. Incorporation of ¹⁵NO₃ ^? into soil organic N was negligible and 80% of the applied label was lost from the top soil (0–15 cm) primarily by leaching within 9 days. In contrast, ¹⁵NH₄ ⁺ was largely retained in soil organic N. However, instant production of ¹⁵NO₃ ^? in the ¹⁵NH₄ ⁺ treatment suggested active nitrification. In both the ¹⁵NH₄ ⁺ and ¹⁵N-glutamic acid treatments, the ¹⁵N enrichment in the NO₃ ^? pool exceeded that in the NH₄ ⁺ pool one day after ¹⁵N application, suggesting preferential nitrification of added ¹⁵NH₄ ⁺ with subsequent dilution of the NH₄ ⁺ pool and/or immobilization of ¹⁵NH₄ ⁺ followed by heterotrophic nitrification. The cumulative recovery of ¹⁵N in N₂O after 9 days ranged from 2.5 to 6.0% in the ¹⁵NO₃ ^? treatment, confirming the previously reported significant response of N₂O emission to N deposition. Source partitioning of ¹⁵N₂O demonstrated a measurable contribution of nitrification to N₂O emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH₄ ⁺ in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO₃ ^? and NH₄ ⁺) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO₃ ^? in the soil and (ii) rapid immobilization-mineralization of NH₄ ⁺ followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and N₂O emission.     

Small differences in ombrotrophy control regional-scale variation in methane cycling among <Emphasis Type="Italic">Sphagnum</Emphasis>-dominated peatlands

Across northern Alberta, Canada, bogs experience periodic wildfire and, in the Fort McMurray region, are exposed to increasing atmospheric N deposition related to oil sands development. As the fire return interval shortens and/or growing season temperatures increase, the regional peatland CO₂–C sink across northern Alberta will likely decrease, but the magnitude of the decrease could be diminished if increasing atmospheric N deposition alters N cycling in a way that stimulates post-fire successional development in bogs. We quantified net ammonification, nitrification, and dissolved organic N (DON) production in surface peat along a post-fire chronosequence of five bogs where we also experimentally manipulated N deposition (no water controls plus 0, 10, and 20 kg N ha^?1 yr^?1 simulated deposition, as NH₄NO₃). Initial KCl-extractable NH₄⁺–N, NO₃^?–N and DON averaged 176?±?6, 54?±?0.2, and 3580?±?40 ng N cm^?3, respectively, with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Net ammonification, nitrification, and DON production averaged 3.8?±?0.3, 1.6?±?0.2, and 14.3?±?2.0 ng N cm^?3 d^?1, also with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Our hypothesis that N mineralization would be stimulated after fire because root death would create a pulse of labile soil organic C was not supported, most likely because ericaceous plant roots typically are not killed in boreal bog wildfires. The absence of any N mineralization response to experimental N addition is most likely a result of rapid immobilization of added NH₄⁺–N and NO₃^?–N in peat with a wide C:N ratio. In these boreal bogs, belowground N cycling is likely characterized by large DON pools that turn over relatively slowly and small DIN pools that turn over relatively rapidly. For Alberta bogs that have persisted at historically low N deposition values and begin to receive higher N deposition related to anthropogenic activities, peat N mineralization processes may be largely unaffected until the peat C:N ratio reaches a point that no longer favors immobilization of NH₄⁺–N and NO₃^?–N.     

The fate of 15NH4 + labeled deposition in a Scots pine forest in the Netherlands under high and lowered NH4 + deposition, 8 years after application

To study the long-term fate of deposited ammonium (NH₄ ⁺) in a Scots pine forest stand under high nitrogen (N) deposition in the Netherlands we re-sampled the plots of a ¹⁵N tracer experiment with high (i.e. ambient) and lowered N deposition in this stand 8 years after application of the tracer. The results were compared with results obtained 7 years earlier. In the 7 years between the samplings the ¹⁵N deltas of needles, twigs and upper organic soil layer had converged to similar values still above the natural ¹⁵N abundance, suggesting equilibration as a result of intensive cycling of N among these pools. Bark and wood had lower deltas than needles and twigs, but if the label found was attributed to tissue synthesized since the start of the labeling only, bark values were similar to needles and twigs, whereas wood values were higher indicating retranslocation of N into older wood. Mineral soil lost all ¹⁵N label it had accumulated after 1 year indicating that this label had not been strongly bound. The first year the low N treatment had retained more of the labeled NH₄ ⁺ deposition than the high N treatment, but in the seven subsequent years relatively more label was retained in the latter. This better retention after 7 years was ascribed to a larger fraction of label taken up by the vegetation in the high N treatment. This shows that the vegetation can affect the label dynamics despite the fact that only a relatively small amount of label was present in the aboveground vegetation.     

Effect of soil heterogeneity on gross nitrogen mineralization measured by 15N-pool dilution techniques

Pool dilution techniques, where the soil ammonium pool is labeled with ¹⁵NH₄ ⁺, are commonly used to estimate gross N mineralization rates in soil. To estimate the rates unbiased, it is assumed that the ¹⁵NH₄ ⁺ is distributed homogenously in ambient ¹⁴NH₄ ⁺ pool of the soil. However, completely homogeneous distribution of ¹⁵NH₄ ⁺ may commonly not be feasible. The objective of this paper was to examine the importance of the spatial distribution of ¹⁴NH₄ ⁺ and ¹⁵NH₄ ⁺ on the measured gross N mineralization rate. In a 2-day incubation experiment, gross N mineralization rates were measured in soil, where different distributions of ¹⁴NH₄ ⁺ and ¹⁵NH₄ ⁺ were combined. Generally, distribution of ¹⁵NH₄ ⁺ to 50% of the soil volume did not alter the measured gross mineralization rate however more heterogeneous distribution caused the rate to be underestimated. Certain combinations of ¹⁴NH₄ ⁺ and ¹⁵NH₄ ⁺ distributions caused the rate to be overestimated. Based on the experimental results, we developed a 2-dimensional model array of soil compartments, to estimate the gross N mineralization and gross NH₄ ⁺ consumption rates in local microsites in the soil. If one of the nitrogen-isotopes was more abundant in a compartment with high NH₄ ⁺-concentration, and the other nitrogen-isotope was more abundant in a compartment with low NH₄ ⁺-concentration, the nitrogen-isotopes would have different apparent bioavailability, hence the gross N mineralization rate would be erroneously estimated. On the other hand, in soil where all compartments had low NH₄ ⁺-concentrations, heterogeneous distribution of ¹⁴NH₄ ⁺, ¹⁵NH₄ ⁺ and microbial activity did not influence the measured gross N mineralization rate significantly.     

Nitrogen cycling in forest soils across climate gradients in Eastern China

A ¹⁵N tracing study was carried out to investigate the potential gross nitrogen (N) dynamics in thirteen forest soils in Eastern China ranging from temperate to tropical zones (five coniferous forests, six deciduous broad-leaf forests, one temperate mixed forest, one evergreen broad-leaf forests ecosystems), and to identify the major controlling factors on N cycling in these forest ecosystems. The soil pH ranged from 4.3 to 7.9 and soil organic carbon (SOC) ranged from 6.6 g?kg^?1 to 83.0 g?kg^?1. The potential gross N transformation rates were quantified by ¹⁵N tracing studies where either the ammonium or nitrate pools were ¹⁵N labeled in parallel treatments. Gross mineralization rates ranged from 0.915 μg N g^?1 soil day^?1 to 2.718 μg N g^?1 soil day^?1 in the studied forest soils. The average contribution of labile organic-N (M _Nlab ) to total gross mineralization (M _Nrec +M _Nlab ) was 86% (58% to 99%), indicating that turnover of labile organic N plays a dominant role in the studied forest ecosystems. The gross mineralization rates in coniferous forest soils were significantly lower (ranging between 0.915 and 1.228 μg N g^?1 soil day^?1) compared to broad-leaf forest soils (ranging from 1.621 to 2.718 μg N g^?1 soil day^?1) (p?<?0.01). Thus, the dominant vegetation may play an important role in regulating soil N mineralization. Nitrate production (nitrification) occurred via two pathways, oxidation of NH ₄ ⁺ and organic N the forest soils. Correlations with soil pH indicated that this is a key factor controlling the oxidation of NH ₄ ⁺ and organic N in theses forest ecosystems. NH ₄ ⁺ oxidation decreased with a decline in pH while organic N oxidation increased. The climatic conditions (e.g. moisture status) at the various sites governed the NO ₃ ^? -N consumption processes (dissimilatory NO ₃ ^? reduction to NH ₄ ⁺ (DNRA) or immobilization of NO ₃ ^? ). Total NO ₃ ^? consumption and the proportion of total NO ₃ ^? consumption to total NO ₃ ^? production decreased with an increase in the drought index of ecosystems, showing that strong interactions appear to exist between climatic condition (e.g. the drought index), N mineralization and the rate of DNRA. Interactions between vegetation, climatic conditions govern internal N cycling in these forests soils.     

Biotic and abiotic immobilization of ammonium, nitrite, and nitrate in soils developed under different tree species in the Catskill Mountains, New York, USA

Nitrogen retention in soil organic matter (SOM) is a key process influencing the accumulation and loss of N in forest ecosystems, but the rates and mechanisms of inorganic N retention in soils are not well understood. The primary objectives of this study were to compare ammonium (NH₄⁺), nitrite (NO₂^?), and nitrate (NO₃^?) immobilization among soils developed under different tree species in the Catskill Mountains of New York State, and to determine the relative roles of biotic or abiotic processes in soil N retention. A laboratory experiment was performed, where ¹⁵N was added as NH₄⁺, NO₂^?, or NO₃^? to live and mercury‐treated O horizon soils from three tree species (American beech, northern red oak, sugar maple), and ¹⁵N recoveries were determined in the SOM pool. Mercuric chloride was used to treat soils as this chemical inhibits microbial metabolism without significantly altering the chemistry of SOM. The recovery of ¹⁵N in SOM was almost always greater for NH₄⁺ (mean 20%) and NO₂^? (47%) than for NO₃^? (10%). Ammonium immobilization occurred primarily by biotic processes, with mean recoveries in live soils increasing from 9% at 15 min to 53% after 28 days of incubation. The incorporation of NO₂^? into SOM occurred rapidly (<15 min) via abiotic processes. Abiotic immobilization of NO₂^? (mean recovery 58%) was significantly greater than abiotic immobilization of NH₄⁺ (7%) or NO₃^? (7%). The incorporation of NO₂^? into SOM did not vary significantly among tree species, so this mechanism likely does not contribute to differences in soil NO₃^? dynamics among species. As over 30% of the ¹⁵NO₂^? label was recovered in SOM within 15 min in live soils, and the products of NO₂^? incorporation into SOM remained relatively stable throughout the 28‐day incubation, our results suggest that NO₂^? incorporation into SOM may be an important mechanism of N retention in forest soils. The importance of NO₂^? immobilization for N retention in field soils, however, will depend on the competition between incorporation into SOM and nitrification for transiently available NO₂^?. Further research is required to determine the importance of this process in field environments.