Impacts of Soil Nitrogen and Carbon Additions on Forest Understory Communities with a Long Nitrogen Deposition History

Rates of nitrogen (N) deposition have been historically high throughout much of the northeastern United States; thus, understanding the legacy of these high N loads is important for maintaining forest productivity and resilience. Though many studies have documented plant invasions due to N deposition and associated impacts on ecosystems, less is known about whether invasive plants will continue to increase in dominance with further shifting nutrient regimes. Using soil N and carbon additions, we examined the impact of both increasing and decreasing soil N on native and invasive understory plant dynamics over 4 years in a northeastern deciduous forest with a long history of N deposition. Despite applying large quantities of N, we found no difference in soil nitrate (NO₃) or ammonium (NH₄ ⁺) pools in N addition plots over the course of the study. Indicative of the potential N saturation in these forest soils, resin-available NO₃ ^? and NH₄ ⁺ showed evidence that the added N was rapidly moving out of the soil in N addition plots. Accordingly, we also found that adding N to soil altered neither invasive nor native plant abundance, though adding N temporally increased invasive plant richness. Carbon additions decreased soil N availability seasonally, but did not alter the total percent cover of invasive or native plants. Rather than being suppressed by excess N availability, native plant species in this ecosystem are primarily inhibited by the invasive species, which now dominate this site. In conclusion, understory plant communities in this potentially N-saturated ecosystem may be buffered to future alterations in N availability.     

Impacts of Hemlock Loss on Nitrogen Retention Vary with Soil Nitrogen Availability in the Southern Appalachian Mountains

The impacts of exotic insects and pathogens on forest ecosystems are increasingly recognized, yet the factors influencing the magnitude of effects remain poorly understood. Eastern hemlock (Tsuga canadensis) exerts strong control on nitrogen (N) dynamics, and its loss due to infestation by the hemlock woolly adelgid (Adelges tsugae) is expected to decrease N retention in impacted stands. We evaluated the potential for site variation in N availability to influence the magnitude of effects of hemlock decline on N dynamics in mixed hardwood stands. We measured N pools and fluxes at three elevations (low, mid, high) subjected to increasing atmospheric N deposition where hemlock was declining or absent (as reference), in western North Carolina. Nitrogen pools and fluxes varied substantially with elevation and increasing N availability. Total forest floor and mineral soil N increased (P?<?0.0001, P?=?0.0017, resp.) and forest floor and soil carbon (C) to N ratio decreased with elevation (P?<?0.0001, P?=?0.0123, resp.), suggesting that these high elevation pools are accumulating available N. Contrary to expectations, subsurface leaching of inorganic N was minimal overall (<1?kg?ha^?1 9 months^?1), and was not higher in stands with hemlock mortality. Mean subsurface flux was 0.16?±?0.04 (SE) (kg?N?ha^?1 100?days^?1) in reference and 0.17?±?0.05 (kg?N?ha^?1 100?days^?1) in declining hemlock stands. Moreover, although subsurface N flux increased with N availability in reference stands, there was no relationship between N availability and flux in stands experiencing hemlock decline. Higher foliar N and observed increases in the growth of hardwood species in high elevation stands suggest that hemlock decline has stimulated N uptake and growth by healthy vegetation within this mixed forest, and may contribute to decoupling the relationship between N deposition and ecosystem N flux.     

Theory of a Crop Enclosure System for Measuring Nitrogen Fixation, Photosynthesis, Respiration and Biological Processes in the Soil

This paper contains a theory for the analysis of gas exchangemeasurements obtained using a combined shoot and soil enclosure.Particular attention is given to the non-steady state ratesof nitrogen fixation and initial patterns of carbon dioxideevolution. Under steady state conditions virtually all of themathematical complexity disappears leaving equations very similarin form to those traditionally used by physiologists to estimaterates of gas exchange. A theory for measuring hydrogen evolutionfrom legume crops in the field is also presented. Nitrogen fixation, hydrogen, respiration, photosynthesis, soil     

Climate Variation and Soil Carbon and Nitrogen Cycling Processes in a Northern Hardwood Forest

We exploited the natural climate gradient in the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF) to evaluate the effects of climate variation similar to what is predicted to occur with global warming over the next 50–100 years for northeastern North America on soil carbon (C) and nitrogen (N) cycle processes. Our objectives were to (1) characterize differences in soil temperature, moisture and frost associated with elevation at the HBEF and (2) evaluate variation in total soil (TSR) and microbial respiration, N mineralization, nitrification, denitrification, nitrous oxide (N₂O) flux, and methane (CH₄) uptake along this gradient. Low elevation sites were consistently warmer (1.5–2.5°C) and drier than high elevation sites. Despite higher temperatures, low elevation plots had less snow and more soil frost than high elevation plots. Net N mineralization and nitrification were slower in warmer, low elevation plots, in both summer and winter. In summer, this pattern was driven by lower soil moisture in warmer soils and in winter the pattern was linked to less snow and more soil freezing in warmer soils. These data suggest that N cycling and supply to plants in northern hardwood ecosystems will be reduced in a warmer climate due to changes in both winter and summer conditions. TSR was consistently faster in the warmer, low elevation plots. N cycling processes appeared to be more sensitive to variation in soil moisture induced by climate variation, whereas C cycling processes appeared to be more strongly influenced by temperature.     

旱地小麦深施磷肥对土壤水分及植株氮素吸收、利用的影响

为探明磷肥在旱地小麦生产上的作用,寻求旱地小麦最佳施磷方式,在山西省闻喜县进行了低磷(75kg·hm~(-2))、中磷(112.5 kg·hm~(-2))、高磷(150 kg·hm~(-2))3个施磷量条件下20 cm、40 cm 2个深度施磷的田间试验,研究其对旱地麦田土壤水分及植株氮素吸收、利用的影响。结果表明:增加施磷量,越冬期-孕穗期0~100 cm土层土壤蓄水量提高,且深层施磷效果较好,尤其有利于返青期土壤蓄水量提高。增加施磷量,各生育时期植株含氮率提高,各生育时期植株氮素积累量显著提高,且深层施磷效果较好,尤其开花期含氮率。增加施磷量,花前各器官氮素运转量显著提高,深层施磷叶片氮素运转量对籽粒的贡献率提高,成熟期叶片氮素积累量及其所占比例显著降低。40 cm深度施磷150 kg·hm~(-2)花后氮素积累量最高。此外,越冬-孕穗期0~100 cm土层土壤蓄水量与花前氮素运转量关系密切,尤其与叶片氮素运转量关系密切,开花期土壤水分与花后氮素积累量关系系数最大。总之,增加施磷量,有利于提高花前1 m内土壤水分,有利于促进植株氮素积累、运转,且深层施磷效果显著,尤其可促进叶片氮素转移到籽粒,有利于开花期含氮率提高,有利于花后氮素积累。最终,40 cm深度施磷150 kg·hm~(-2)可显著提高旱地小麦氮肥吸收效率、氮肥生产效率、氮素收获指数。     

The Effects of Manure and Nitrogen Fertilizer Applications on Soil Organic Carbon and Nitrogen in a High-Input Cropping System

With the goal of improving N fertilizer management to maximize soil organic carbon (SOC) storage and minimize N losses in high-intensity cropping system, a 6-years greenhouse vegetable experiment was conducted from 2004 to 2010 in Shouguang, northern China. Treatment tested the effects of organic manure and N fertilizer on SOC, total N (TN) pool and annual apparent N losses. The results demonstrated that SOC and TN concentrations in the 0-10cm soil layer decreased significantly without organic manure and mineral N applications, primarily because of the decomposition of stable C. Increasing C inputs through wheat straw and chicken manure incorporation couldn''t increase SOC pools over the 4 year duration of the experiment. In contrast to the organic manure treatment, the SOC and TN pools were not increased with the combination of organic manure and N fertilizer. However, the soil labile carbon fractions increased significantly when both chicken manure and N fertilizer were applied together. Additionally, lower optimized N fertilizer inputs did not decrease SOC and TN accumulation compared with conventional N applications. Despite the annual apparent N losses for the optimized N treatment were significantly lower than that for the conventional N treatment, the unchanged SOC over the past 6 years might limit N storage in the soil and more surplus N were lost to the environment. Consequently, optimized N fertilizer inputs according to root-zone N management did not influence the accumulation of SOC and TN in soil; but beneficial in reducing apparent N losses. N fertilizer management in a greenhouse cropping system should not only identify how to reduce N fertilizer input but should also be more attentive to improving soil fertility with better management of organic manure.     

Estimating Annual Soil Carbon Loss in Agricultural Peatland Soils Using a Nitrogen Budget Approach

Around the world, peatland degradation and soil subsidence is occurring where these soils have been converted to agriculture. Since initial drainage in the mid-1800s, continuous farming of such soils in the California Sacramento-San Joaquin Delta (the Delta) has led to subsidence of up to 8 meters in places, primarily due to soil organic matter (SOM) oxidation and physical compaction. Rice (Oryza sativa) production has been proposed as an alternative cropping system to limit SOM oxidation. Preliminary research on these soils revealed high N uptake by rice in N fertilizer omission plots, which we hypothesized was the result of SOM oxidation releasing N. Testing this hypothesis, we developed a novel N budgeting approach to assess annual soil C and N loss based on plant N uptake and fallow season N mineralization. Through field experiments examining N dynamics during growing season and winter fallow periods, a complete annual N budget was developed. Soil C loss was calculated from SOM-N mineralization using the soil C:N ratio. Surface water and crop residue were negligible in the total N uptake budget (3 – 4 % combined). Shallow groundwater contributed 24 – 33 %, likely representing subsurface SOM-N mineralization. Assuming 6 and 25 kg N ha-1 from atmospheric deposition and biological N2 fixation, respectively, our results suggest 77 – 81 % of plant N uptake (129 – 149 kg N ha^-1) was supplied by SOM mineralization. Considering a range of N uptake efficiency from 50 – 70 %, estimated net C loss ranged from 1149 – 2473 kg C ha^-1. These findings suggest that rice systems, as currently managed, reduce the rate of C loss from organic delta soils relative to other agricultural practices.     

The Nature and Longevity of Agricultural Impacts on Soil Carbon and Nutrients: A Review

Since the domestication of plant and animal species around 10,000 years ago, cultivation and animal husbandry have been major components of global change. Agricultural activities such as tillage, fertilization, and biomass alteration lead to fundamental changes in the pools and fluxes of carbon (C), nitrogen (N), and phosphorus (P) that originally existed in native ecosystems. Land is often taken out of agricultural production for economic, social, or biological reasons, and the ability to predict the biogeochemical trajectory of this land is important to our understanding of ecosystem development and our projections of food security for the future. Tillage generally decreases soil organic matter (SOM) due to erosion and disruption of the physical, biochemical, and chemical mechanisms of SOM stabilization, but SOM can generally reaccumulate after the cessation of cultivation. The use of organic amendments causes increases in SOM on agricultural fields that can last for centuries to millennia after the termination of applications, although the locations that provide the organic amendments are concurrently depleted. The legacy of agriculture is therefore highly variable on decadal to millennial time scales and depends on the specific management practices that are followed during the agricultural period. State factors such as climate and parent material (particularly clay content and mineralogy) modify ecosystem processes such that they may be useful predictors of rates of postagricultural biogeochemical change. In addition to accurate biogeochemical budgets of postagricultural systems, ecosystem models that more explicitly incorporate mechanisms of SOM loss and formation with agricultural practices will be helpful. Developing this predictive capacity will aid in ecological restoration efforts and improve the management of modern agroecosystems as demands on agriculture become more pressing.     

Throughfall Nitrogen Deposition Has Different Impacts on Soil Solution Nitrate Concentration in European Coniferous and Deciduous Forests

Increases in the deposition of atmospheric nitrogen (N) influence N cycling in forest ecosystems and can result in negative consequences due to the leaching of nitrate into groundwaters. From December 1995 to February 1998, the Pan-European Programme for the Intensive and Continuous Monitoring of Forest Ecosystems measured forest conditions at a plot scale for conifer and broadleaf forests, including the performance of time series of soil solution chemistry. The influence of various ecosystem conditions on soil solution nitrate concentrations at these forest plots (n = 104) was then analyzed with a statistical model. Soil solution nitrate concentrations varied by season, and summer concentrations were approximately 25% higher than winter ones. Soil solution nitrate concentrations increased dramatically with throughfall (and bulk precipitation) N input for both broadleaf and conifer forests. However, at elevated levels of throughfall N input (more than 10 kg N ha^–1 y^–1), nitrate concentrations were higher in broadleaf than coniferous stands. This tree-specific difference was not observed in response to increased bulk precipitation N input. In coniferous stands, throughfall N input, foliage N concentration, organic layer carbon–nitrogen (C:N) ratio, and nitrate concentrations covaried. Soil solution nitrate concentrations in conifer plots were best explained by a model with throughfall N and organic layer C:N as main factors, where C:N ratio could be replaced by foliage N. The organic layer C:N ratio classes of more than 30, 25–30, and less than 25, as well as the foliage N (mg N g^–1) classes of less than 13, 13–17, and more than 17, indicated low, intermediate, and high risks of nitrate leaching, respectively. In broadleaf forests, correlations between N characteristics were less pronounced, and soil solution nitrate concentrations were best explained by throughfall N and soil pH (0–10-cm depth). These results indicate that the responses of soil solution nitrate concentration to changes in N input are more pronounced in broadleaf than in coniferous forests, because in European forests broadleaf species grow on the more fertile soils.     

Elevated Atmospheric CO2 Impacts Abundance and Diversity of Nitrogen Cycling Functional Genes in Soil

The concentration of CO₂ in the Earth's atmosphere has increased over the last century. Although this increase is unlikely to have direct effects on soil microbial communities, increased atmospheric CO₂ may impact soil ecosystems indirectly through plant responses. This study tested the hypothesis that exposure of plants to elevated CO₂ would impact soil microorganisms responsible for key nitrogen cycling processes, specifically denitrification and nitrification. We grew trembling aspen (Populus tremuloides) trees in outdoor chambers under ambient (360 ppm) or elevated (720 ppm) levels of CO₂ for 5 years and analyzed the microbial communities in the soils below the trees using quantitative polymerase chain reaction and clone library sequencing targeting the nitrite reductase (nirK) and ammonia monooxygenase (amoA) genes. We observed a more than twofold increase in copy numbers of nirK and a decrease in nirK diversity with CO₂ enrichment, with an increased predominance of Bradyrhizobia-like nirK sequences. We suggest that this dramatic increase in nirK-containing bacteria may have contributed to the significant loss of soil N in the CO₂-treated chambers. Elevated CO₂ also resulted in a significant decrease in copy numbers of bacterial amoA, but no change in archaeal amoA copy numbers. The decrease in abundance of bacterial amoA was likely a result of the loss of soil N in the CO₂-treated chambers, while the lack of response for archaeal amoA supports the hypothesis that physiological differences in these two groups of ammonia oxidizers may enable them to occupy distinct ecological niches and respond differently to environmental change.     

Impacts of Agricultural Management and Climate Change on Future Soil Organic Carbon Dynamics in North China Plain

Dynamics of cropland soil organic carbon (SOC) in response to different management practices and environmental conditions across North China Plain (NCP) were studied using a modeling approach. We identified the key variables driving SOC changes at a high spatial resolution (10 km×10 km) and long time scale (90 years). The model used future climatic data from the FGOALS model based on four future greenhouse gas (GHG) concentration scenarios. Agricultural practices included different rates of nitrogen (N) fertilization, manure application, and stubble retention. We found that SOC change was significantly influenced by the management practices of stubble retention (linearly positive), manure application (linearly positive) and nitrogen fertilization (nonlinearly positive) – and the edaphic variable of initial SOC content (linearly negative). Temperature had weakly positive effects, while precipitation had negligible impacts on SOC dynamics under current irrigation management. The effects of increased N fertilization on SOC changes were most significant between the rates of 0 and 300 kg ha⁻¹ yr⁻¹. With a moderate rate of manure application (i.e., 2000 kg ha⁻¹ yr⁻¹), stubble retention (i.e., 50%), and an optimal rate of nitrogen fertilization (i.e., 300 kg ha⁻¹ yr⁻¹), more than 60% of the study area showed an increase in SOC, and the average SOC density across NCP was relatively steady during the study period. If the rates of manure application and stubble retention doubled (i.e., manure application rate of 4000 kg ha⁻¹ yr⁻¹ and stubble retention rate of 100%), soils across more than 90% of the study area would act as a net C sink, and the average SOC density kept increasing from 40 Mg ha⁻¹ during 2010s to the current worldwide average of ∼55 Mg ha⁻¹ during 2060s. The results can help target agricultural management practices for effectively mitigating climate change through soil C sequestration.     

土壤—作物系统的养分转化及其对土壤养分平衡的影响

土壤作为一个开放系统,与作物间通过物质转化与能量流动组成相互依存和影响的体系。近几年来我们对我国太湖地区稻田土壤生态系统中氮、磷、钾养分的转化及其可能产生的影响作过为期3年的田间定位试验研究,本文谨将其中的部分结果整理成文,以供共同探讨。     

上海农耕区鸟类群落特征及与几种生境因子的关系

2000-2002年冬季和夏季,采用固定样带法在上海郊区所有区县选择了17条样带,对农耕区鸟类做了4次抽样调查。记录了鸟类的组成、数量、出现频率及生境因子特征。根据调查数据,计算鸟类密度、多样性、均匀度、优势度、生物量、相对重要值等群落特征参数。调查共记录到鸟类76种,隶属13目26科。平均密度为5．19只／hm^2,多样性指数为1．8742。优势种为麻雀、家燕、白头鹎、棕背伯劳和白鹊鸰。鸟类组成中水鸟(夜鹭、白鹭等)较多,占了总数的1／3;与上海其他区域相比,鸟类密度较高,但种类相对较少,数量集中在少数几个物种(麻雀、家燕、白头鹎、棕背伯劳等)。鸟类物种丰富度与荒地面积呈显著正相关,与环境污染程度呈显著负相关,与林地面积、人口密度、水体面积等相关不显著。提示荒地对鸟类多样性非常重要。     

Soil Denitrification,the Missing Piece in the Puzzle of Nitrogen Budget in Lowland Agricultural Basins

Ecosystems - Denitrification is a key process buffering the environmental impacts of agricultural nitrate loads but, at present, remains the least understood and poorly quantified sink in nitrogen...     

Activation of Nitrite Reductase in a Cell-Free System from the Denitrifier Alcaligenes sp. by Freeze-Thawing

Nitrite reductase (NiR) activity of the cell-free extract orthe soluble fraction prepared from cells of Alcaligenes sp.NC1B 11015 grown anaerobically in the presence of nitrate wasexamined by measuring the rate of nitrite disappearance withdithionitemethyl viologen (MV) as an electron donor. Freezingat — 20?C and subsequent thawing of the fraction resultedin 5-40 times increase of the specific activity of NiR. Fromthe experiments on the effect of freezing conditions on theactivation, the phase change of solvent water due to freezingis considered to play an important role in the activation. Thisactivation occurred with the preparation in the exponentialgrowth phase, but not that in the stationary growth phase. Clearly,the low-molecular-weight (< 12,000) component which was obtainedfrom the soluble fraction through a collodion bag participatedin the activation. The activated enzyme proved to be the dissimilatory NiR, becauseNO production from nitrite, one of the typical characteristicsof the dissimilatory NiR, was also activated when assayed withascorbate-tetramethyl-p-phenylene diamine (TMPD) as an electrondonor. Nevertheless, the reaction products of nitrite reductionwere identified as hydroxylamine and ammonia with dithionite-MV.The possible pathway of nitrite reduction with this electrondonor is discussed. (Received May 26, 1983; Accepted February 2, 1984)     

Soil Nitrogen Pools Associated with Revegetation of Disturbed Sites in the Lake Tahoe Area

Thin, poorly developed soils in the high elevation, summer‐dry environment near Lake Tahoe, California are easily disturbed by anthropogenic impacts. Subsoils and parent materials that are exposed by vegetation removal and topsoil erosion or by burial during construction activities are difficult to revegetate and may continue to erode for decades after disturbance. The resulting sediment loads contribute to decreased water quality in local watersheds and to the loss of clarity in Lake Tahoe. Field observations suggest that soil disturbance often results in depletion of soil nitrogen (N) reserves and that the remaining substrates may be unable to provide adequate N for revegetation. To quantify the levels of soil N that are associated with higher levels of percent plant cover on previously disturbed soils in the Lake Tahoe area, a basin‐wide survey and a second paired site study were conducted. Results indicate that extractable ammonium and nitrate levels correlate poorly with percent vegetative cover, whereas the correlations of anaerobically mineralizable N and total N are stronger and account for nearly 50% of the variability in plant cover data. Sites with plant cover measuring greater than 40% are associated with total soil N levels of about 1,200 kg N/ha and anaerobic mineralizable N levels of about 26 kg N/ha. Despite high concentrations of N in the surface soils, a large fraction of the N in the 0‐ to 50‐cm profile occurs below 30 cm, when measured on a landscape basis.     

Nitrogen Cycling and Mass Balance for a Forested Catchment in the Italian Alps. Assessment of Nitrogen Status

During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations of SO₄ in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia, by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention in the forest soil, corresponding to 12 kg ha⁻¹ yr⁻¹, despite a relatively high N deposition loading (15 kg ha⁻¹ yr⁻¹). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong N retention in the forested part of the catchment, the stream water N–NO₃ levels were consistently above 10 μg l⁻¹ suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic of an alpine catchment.     

Subsurface Soil Carbon and Nitrogen Losses Offset Surface Carbon Accumulation in Abandoned Agricultural Fields

Ecosystems - Abandoned agricultural fields (old fields) are thought to accumulate soil organic matter (SOM) after cultivation cessation. However, most research on old fields soil carbon (C) and...     

Soil and Hydrological Drivers of Typha latifolia Encroachment in a Marl Wetland

Aggressive species competition by Typha latifolia in wetland systems on marl-derived soils may threaten the unique vegetation in these areas. We examined historic water and land use, soil chemistry, soil genesis, and topography in a wetland (Harewood Marsh) that is under encroachment by T. latifolia. An earthen road that bisects the wetland and active pastures in and around the wetland were also considered in the study due to their potential influence on wetland hydrology and nutrient inputs. Historic land uses and trends in surface water patterns were determined via aerial photography. In addition local landowners and city officials were contacted for information about historic water use in the area around the marsh. Soils were augered and sampled in seven locations in the wetland, and at each auger site, vegetation was described. Six wells were installed near the earthen road and weekly water depth measurements were taken from January 2002 to January 2003. In February 2002, 10 A-horizon soil samples (per transect) were taken from three 150-m transects that spanned areas with and without T. latifolia. Results indicate that T. latifolia encroachment is facilitated by a rising water table (the result of the termination of a local municipal water supply source) and N and P inputs, most likely from cattle grazing on the wetland. An increase in the numbers of rare plant species associated with marl wetland habitats appears to have also occurred and is believed partly due to current wetter conditions. Our study provides insight into the dynamics of T. latifolia encroachment in a unique marl wetland habitat and demonstrates how local factors controlling nutrient and hydrologic dynamics can have significant effects on changes in plant community composition.     

Variability in Soil Nitrogen Retention Across Forest,Urban, and Agricultural Land Uses

In regions of mixed land use, some ecosystems are sinks for N pollution and others are sources. Yet, beyond this gross characterization, we have little understanding of how adjacent land-use types vary in mechanisms of N cycling and retention. This study assessed the rate and magnitude of soil N retention pathways in forest, urban, and agricultural ecosystems. Soil plots in each land use were labeled with inorganic ¹⁵N and cored at 15 min, 2 days, and 20 days following injection. Subsamples were biologically fractionated to differentiate labile and stable pools, while gross N transformations were assessed via the ¹⁵N isotope dilution method. Stable soil organic ¹⁵N formed rapidly (within 15 min) in all land uses when added as ¹⁵NH₄ ⁺, and became a proportionally larger sink for inorganic ¹⁵N over time. Forests had the lowest gross immobilization rates, but the greatest amount of stable N formation. Rapid retention of NH₄ ⁺ in forests may be driven by abiotic processes, with root uptake becoming a more important mechanism of retention over time. Urban sites, on the other hand, had the highest gross microbial immobilization rates and highest root N uptake, suggesting that high short-term N retention may be due to rapid biological processing. Agricultural systems, with low root uptake and the lowest stable N formation, had little capacity for retention of added N. These apparently distinct land-use cases can be understood by synthesizing several emerging aspects of N retention theory that (1) distinguish kinetic and capacity N saturation, (2) recognize links between soil C saturation on minerals and N retention, and (3) account for rapid transfers of NH₄ ⁺ to stable organic pools.