Soil microbial biomass carbon and nitrogen in forest ecosystems of Northeast China: a comparison between natural secondary forest and larch plantation

Aims Natural secondary forest (NSF) and larch plantation are two of the predominant forest types in Northeast China. However, how the two types of forests compare in sustaining soil quality is not well understood. This study was conducted to determine how natural secondary forest and larch plantation would differ in soil microbial biomass and soil organic matter quality.Methods Microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), soil organic carbon (SOC) and total nitrogen (TN) in the 0- to 15-cm and 15- to 30-cm soil layers were investigated by making chemical and biological measurements in the montane region of eastern Liaoning Province, Northeast China, during the growing season of 2008 in stands of NSF and Larix olgensis plantation (LOP).Important findings We found that soil MBC and MBN were significantly lower in the LOP than in the NSF. Both MBC and MBN declined significantly with increasing soil depth in the two types of stands. The ratios of MBC to SOC (MBC/SOC) and MBN to TN (MBN/TN) were also significantly lower in the LOP than in the NSF. Moreover, the values of MBC, MBC/SOC, and MBN/TN significantly varied with time and followed a similar pattern during the growing season, all with an apparent peak in summer. Our results indicate that NSF is better in sustaining soil microbial biomass and nutrients than larch plantation in the temperate Northeast China. This calls for cautions in large-scale conversions of the native forests to coniferous plantations as a forest management practice on concerns of sustaining soil productivity.     

Aspects of the nitrogen cycle in peatland and plantation forest ecosystems in western Ireland

Extensive plantation forests cover large areas of blanket peatland in western Ireland. Sites are characterised by the ombrotrophic nature of the peat and the often extreme maritime conditions prevailing. The study area is located close to two coastlines and in consequence, ions of marine origin are dominant in the bulk precipitation. Mean annual nitrogen deposition is 2.26 kg ha^-1. Forestry development in the region dates from the early 1950s. Deficiency of phosphorus is universally encountered, sometimes accompanied by a shortage of nitrogen. A fertilizer experiment in the study area was maintained for 16 years. The principal response was to applied phosphorus and although nitrogen had a positive influence on growth in the early years, it was of little consequence in the longer term.Over 900 kg N ha^-1 was accumulated in the forest floor. In a mineralisation study of peat collected from plots fertilized 14 years previously, differences in total mineral nitrogen production between treatments were small, but in nitrogen-treated plots a higher proportion of the mineral nitrogen was as nitrate than in those which had not received fertilizer nitrogen. Throughfall measurements in pole-stage crops of Sitka spruce and lodgepole pine which had received no fertilizer nitrogen, showed significantly greater quantities of nitrogen than bulk precipitation.     

Symptoms of nitrogen saturation in two central Appalachian hardwood forest ecosystems

By synthesizing more than twenty years of research at the Fernow Experimental Forest, we have documented 7 symptoms of nitrogen saturation in two adjacent watersheds. The symptoms include: 1) high relative rates of net nitrification, 2) long-term increases in stream-water concentrations of nitrate and base cations, 3) relatively high nitrate concentrations in solution losses, 4) little seasonal variability in stream-water nitrate concentrations, 5) a high discharge of nitrate from a young aggrading forest, 6) a rapid increase in nitrate loss following fertilization of a young aggrading forest, and 7) low retention of inorganic nitrogen when compared with other forested sites. These data support current conceptual models of nitrogen saturation and provide a strong, and perhaps the best, example of nitrogen saturation in the United States.     

A decennial control of N-cycle in the Belgian Ardenne forest ecosystems

As a rule, N-supply of mature Ardenne forest ecosystems is satisfactory. Mineralization rates of soil organic matter are generally high and nitrogen is not a frequent nor an important growth limiting factor. Light N-deficiency is likely to depend on unsatisfactory root absorbing power in very acid soils with dysmoder humus. For other major elements, especially for Mg, Ca and P, near optimal nutrition is rarely observed.During the late sixties, fertilizer experiments have shown that nutrition equilibrium of stands growing on acid soils, poor in exchangeable Mg, is very sensitive to artificial NH₄-addition. Mg-deficiency symptoms have been induced. The present continuing atmospheric NH₄ input is believed to produce similar but lasting nutritional stress which might be accentuated by additional acidity generated during nitrification of excess NH₄. Evolving Mg deficiency leads to trees' death and developing forest decline is likely to enhance N-output under NO₃-form.Awaiting adequate air purification, fertilizing should restore nutrition equilibrium in order to save damaged stands, to slow down soil acidification and to incorporate excess atmospheric N-input into improved biomass production.     

Water regime of metal-contaminated soil under juvenile forest vegetation

In a three-year factorial lysimeter study in Open Top Chambers (OTCs), we investigated the effect of topsoil pollution by the heavy metals Zn, Cu, and Cd on the water regime of newly established forest ecosystems. Furthermore, we studied the influence of two types of uncontaminated subsoils (acidic vs. calcareous) and two types of irrigation water acidity (ambient rainfall chemistry vs. acidified chemistry) on the response of the vegetation. Each of the eight treatment combinations was replicated four times. The contamination (2700 mg kg^–1 Zn, 385 mg kg^–1 Cu and 10 mg kg^–1 Cd) was applied by mixing filter dust from a non-ferrous metal smelter into the upper 15 cm of the soil profile, consisting of silty loam (pH 6.5). The same vegetation was established in all 32 lysimeters. The model forest ecosystem consisted of seedlings of Norway spruce (Picea abies), willow (Salix viminalis), poplar (Populus tremula) and birch (Betula pendula) trees and a variety of herbaceous understorey plants. Systematic and significant effects showed up in the second and third growing season after canopies had closed. Evapotranspiration was reduced in metal contaminated treatments, independent of the subsoil type and acidity of the irrigation water. This effect corresponded to an even stronger reduction in root growth in the metal treatments. In the first two growing seasons, evapotranspiration was higher on the calcareous than on the acidic subsoil. In the third year the difference disappeared. Acidification of the irrigation water had no significant effect on water consumption, although a tendency to enhance evapotranspiration became increasingly manifest in the second and third year. Soil water potentials indicated that the increasing water consumption over the years was fed primarily by intensified extraction of water from the topsoil in the lysimeters with acidic subsoil, whereas also lower depths became strongly exploited in the lysimeters with calcareous subsoil. These patterns agreed well with the vertical profiles of fine root density related with the two types of subsoil. Leaf transpiration measurements and biomass samples showed that different plant species in part responded quite differently and occasionally even in opposite ways to the metal treatments and subsoil conditions. They suggest that the year-to-year changes in treatment effects on water consumption and extraction patterns were related to differences in growth dynamics, as well as to shifts in competitiveness of the various species. Results showed that the uncontaminated subsoil offered a possibility to compensate the reduction in root water extraction in the topsoil under drought, as well as metal stress.     

A mechanism of abiotic immobilization of nitrate in forest ecosystems: the ferrous wheel hypothesis

Forest soils, rather than woody biomass, are the dominant long‐term sink for N in forest fertilization studies and, by inference, for N from atmospheric deposition. Recent evidence of significant abiotic immobilization of inorganic‐N in forest humus layers challenges a previously widely held view that microbial processes are the dominant pathways for N immobilization in soil. Understanding the plant, microbial, and abiotic mechanisms of N immobilization in forest soils has important implications for understanding current and future carbon budgets. Abiotic immobilization of nitrate is particularly perplexing because the thermodynamics of nitrate reduction in soils are not generally favorable under oxic conditions. Here we present preliminary evidence for a testable hypothesis that explains abiotic immobilization of nitrate in forest soils. Because iron (and perhaps manganese) plays a key role as a catalyst, with Fe(II) reducing nitrate and reduced forms of carbon then regenerating Fe(II), we call this ‘the ferrous wheel hypothesis’. After nitrate is reduced to nitrite, we hypothesize that nitrite reacts with dissolved organic matter through nitration and nitrosation of aromatic ring structures, thus producing dissolved organic nitrogen (DON). In addition to ignorance about mechanisms of DON production, little is known about DON dynamics in soil and its fate within ecosystems. Evidence from leaching and watershed studies suggests that DON production and consumption may be largely uncoupled from seasonal biological processes, although biological processes ultimately produce the DOC and reducing power that affect DON formation and the entire N cycle. The ferrous wheel hypothesis includes both biological and abiological processes, but the reducing power of plant‐derived organic matter may build up over seasons and years while the abiotic reduction of nitrate and reaction of organic matter with nitrite may occur in a matter of seconds after nitrate enters the soil solution.     

生石灰对林下酸化土壤的调控作用及三七生长的影响

土壤酸化是导致林下三七生长不良和根腐病发生的重要因素。本试验利用不同量(0、500、1000、1500和2000 kg·hm^-2)的生石灰改良林下酸化土壤,评价生石灰对土壤理化性质、酚类物质、根际微生物和三七生长的影响。结果表明: 适量的生石灰(500～1000 kg·hm^-2)可显著提高土壤pH值,降低酚类物质(羟基苯甲酸、香草醛、丁香酸、阿魏酸和香草酸)含量,促进三七的生长,并降低根腐病的发生。适量的生石灰(500～1000 kg·hm^-2)能显著降低土壤的真菌/细菌,提高细菌多样性,提高门水平上子囊菌门、变形菌门和属水平上马赛菌属和鞘脂单胞菌属的相对丰度。但是过量的生石灰(1500～2000 kg·hm^-2)会降低土壤碱解氮和有机质含量,抑制三七的生长。因此,500～1000 kg·hm^-2生石灰可显著改善林下土壤的化学性质和微生物群落,从而促进三七生长,降低根腐病的发生。     

氮沉降对森林土壤磷循环的影响

磷是生物体必需的大量元素之一,也是许多生态系统的主要限制因子。近年来,大气氮沉降日益加剧,已对森林生态系统磷循环产生了不可忽视的影响。关于氮沉降对生态系统磷循环的影响已开展了一系列的研究,然而尚缺少对其整体的认识。因此,通过收集国内外已发表的相关文章,从以下五个方面综述氮沉降对森林生态系统土壤磷循环的影响及其机理:1)阐述了森林生态系统土壤磷循环的概念;2)介绍了氮沉降对森林土壤磷循环影响的研究方法,包括长期定位模拟氮沉降法、自然氮沉降梯度法和同位素示踪法等;3)概述了氮沉降对森林生态系统土壤磷循环的影响。目前的研究结论趋向于认为长期氮沉降使森林土壤磷循环速率加快。长期氮输入易于使土壤中可溶性磷向非活性磷酸盐库迁移而难以被利用。因此,为了满足需求,土壤磷酸酶活性将增加以加速有机磷的矿化,从而加速磷素在土壤-植物-微生物之间的周转。4)探讨了氮沉降影响森林土壤磷循环的机制。氮沉降可通过改变土壤有机质的性质、微生物群落组成、磷酸酶活性以及阳离子的流动性等途径影响森林土壤磷循环;5)指出了当前研究存在的问题及未来的研究方向。     

Carbon : nitrogen stoichiometry in forest ecosystems during stand development

Aim Carbon (C) and nitrogen (N) stoichiometry is a critical indicator of biogeochemical coupling in terrestrial ecosystems. However, our current understanding of C : N stoichiometry is mainly derived from observations across space, and little is known about its dynamics through time. Location Global secondary forests. Methods We examined temporal variations in C : N ratios and scaling relationships between N and C for various ecosystem components (i.e. plant tissue, litter, forest floor and mineral soil) using data extracted from 39 chronosequences in forest ecosystems around the world. Results The C : N ratio in plant tissue, litter, forest floor and mineral soil exhibited large variation across various sequences, with an average of 145.8 ± 9.4 (mean ± SE), 49.9 ± 3.0, 38.2 ± 3.1 and 18.5 ± 0.9, respectively. In most sequences, the plant tissue C : N ratio increased significantly with stand age, while the C : N ratio in litter, forest floor and mineral soil remained relatively constant over the age sequence. N and C scaled isometrically (i.e. the slope of the relationship between log‐transformed N and C is not significantly different from 1.0) in litter, forest floor and mineral soil both within and across sequences, but not in plant tissue either within or across sequences. The C : N ratio was larger in coniferous forests than in broadleaf forests and in temperate forests than in tropical forests. In contrast, the N–C scaling slope did not reveal significant differences either between coniferous and broadleaf forests or between temperate and tropical forests. Main conclusions These results suggest that C and N become decoupled in plants but remain coupled in other ecosystem components during stand development.     

Biodiversity of Dutch forest ecosystems as affected by receding groundwater levels and atmospheric deposition

Forests in the Netherlands are heavily under stress. Recent surveys suggest that about one-third of the forest area in the Netherlands is affected by desiccation. Generally, plant species of moist situations decline, whereas drought tolerant species tend to increase. Besides desiccation, adverse ecological effects of acidification and nitrogen deposition also occur. Their combined action is held responsible for, among others, the decline of oligotrophic vascular plants, lichens and mycorrhizal fungi. At the same time, N-demanding species increase, which is partly caused by nitrogen deposition, and is partly a secondary effect of desiccation through aeration and concomitant mineralization. Nutrient balance of trees is disrupted. Effects on animals also occur: small snails in forest on acid soil decrease, causing Ca deficiency in birds. Measures to reduce these impacts include restoration of the former hydrology, liming, fertilization and removal of N-saturated littler layers.     

Review of root dynamics in forest ecosystems grouped by climate,climatic forest type and species

Patterns of both above- and belowground biomass and production were evaluated using published information from 200 individual data-sets. Data sets were comprised of the following types of information: organic matter storage in living and dead biomass (e.g. surface organic horizons and soil organic matter accumulations), above- and belowground net primary production (NPP) and biomass, litter transfers, climatic data (i.e. precipitation and temperature), and nutrient storage (N, P, Ca, K) in above- and belowground biomass, soil organic matter and litter transfers. Forests were grouped by climate, foliage life-span, species and soil order. Several climatic and nutrient variables were regressed against fine root biomass or net primary production to determine what variables were most useful in predicting their dynamics. There were no significant or consistent patterns for above- and belowground biomass accumulation or NPP change across the different climatic forest types and by soil order. Similarly, there were no consistent patterns of soil organic matter (SOM) accumulation by climatic forest type but SOM varied significantly by soil order—the chemistry of the soil was more important in determining the amount of organic matter accumulation than climate. Soil orders which were high in aluminum, iron, and clay (e.g. Ultisols, Oxisols) had high total living and dead organic matter accumulations-especially in the cold temperate zone and in the tropics. Climatic variables and nutrient storage pools (i.e. in the forest floor) successfully predicted fine root NPP but not fine root biomass which was better predicted by nutrients in litterfall. The importance of grouping information by species based on their adaptive strategies for water and nutrient-use is suggested by the data. Some species groups did not appear to be sensitive to large changes in either climatic or nutrient variables while for others these variables explained a large proportion of the variation in fine root biomass and/or NPP.     

Endurance of larch forest ecosystems in eastern Siberia under warming trends

The larch (Larix spp.) forest in eastern Siberia is the world's largest coniferous forest. Its persistence is considered to depend on near‐surface permafrost, and thus, forecast warming over the 21st century and consequent degradation of near‐surface permafrost is expected to affect the larch forest in Siberia. However, predictions of these effects vary greatly, and many uncertainties remain about land – atmosphere interactions within the ecosystem. We developed an integrated land surface model to analyze how the Siberian larch forest will react to current warming trends. This model analyzed interactions between vegetation dynamics and thermo‐hydrology, although it does not consider many processes those are considered to affect productivity response to a changing climate (e.g., nitrogen limitation, waterlogged soil, heat stress, and change in species composition). The model showed that, under climatic conditions predicted under gradual and rapid warming, the annual net primary production of larch increased about 2 and 3 times, respectively, by the end of the 21st century compared with that in the previous century. Soil water content during the larch‐growing season showed no obvious trend, even when surface permafrost was allowed to decay and result in subsurface runoff. A sensitivity test showed that the forecast temperature and precipitation trends extended larch leafing days and reduced water shortages during the growing season, thereby increasing productivity. The integrated model also satisfactorily reconstructed latitudinal gradients in permafrost presence, soil moisture, tree leaf area index, and biomass over the entire larch‐dominated area in eastern Siberia. Projected changes to ecosystem hydrology and larch productivity at this geographical scale were consistent with those from site‐level simulation. This study reduces the uncertainty surrounding the impact of current climate trends on this globally important carbon reservoir, and it demonstrates the need to consider complex ecological processes to make accurate predictions.     

大气氮沉降对阔叶林红壤淋溶水化学模拟研究

在氮饱和的森林生态系统中,氮沉降的增加将导致NO3-淋溶的增加及土壤酸度的提高,从而影响土壤质量及林业的可持续发展。然而,大气氮沉降对我国南方红壤地区森林生态系统中土壤的影响研究还很少,尤其是氮沉降引起的土壤淋溶液化学组成方面。研究中,以中国科学院红壤生态实验站林草生态试验区阔叶林红壤为对象,在恒温(20℃)条件下,通过土壤淋洗柱(直径10cm、高60cm)进行了8个月间隙性淋溶试验,来模拟研究不同氮输入量(0、7.8、26mg月-1.柱-1和52mg月-1柱-1)对阔叶林红壤NO3-、NH4 、SO42-、H 和土壤盐基离子(Ca2 、Mg2 、K 和Na )的淋溶和土壤酸度的影响。结果表明,随氮输入量增加,淋溶液中NO3-、EC、H 和总盐基离子逐渐增加,但淋溶液中无NH4 。不同氮处理时,土壤有机氮总表观矿化量分别为189.6、263.9、372.8mg月-1.柱-1与554.2mg柱-1,氮输入明显促进了土壤有机氮的矿化,且土壤有机氮的表观矿化量与氮输入量间呈正线性相关(R2=0.997**)。无氮(0mg月-1柱-1)、低氮(7.8mg月-1柱-1)、中氮(26mg月-1柱-1)和高氮(52mg月-1柱-1)输入处理下,土壤交换态盐基淋溶总量分别占土壤交换性盐基总量的13.6、18.4、27.7%和48.1%。不同的盐基离子对氮输入的反应不同,Ca2 和Mg2 淋溶量随氮输入量的增加而增加,对Na 和K 则无明显影响。土壤交换态离子中随淋洗液输出最多的为Ca2 (无氮、低氮、中氮和高氮输入处理的土壤交换态输出量占土壤交换态的比例分别为22.6、31.4、46.7%和82.5%),其次为Na (无氮、低氮、中氮和高氮输入处理的土壤交换态输出量占土壤交换态的比例分别为16.0、10.7、17.6%和26.3%),最少的为Mg2 (无氮、低氮、中氮和高氮输入处理的土壤交换态输出量占土壤交换态的比例分别为5.0、6.9、11.1%和16.9%),几乎没有土壤交换性K 输出。与对照相比,有氮处理后土壤中硫酸根离子的淋失量明显减少(p<0.05)。表层土壤pH值随氮输入量的增加而显著下降,各处理间差异极显著(p<0.01)。可见,大气氮沉降的增加将加速阔叶林红壤的养分淋失和土壤酸化的程度。     

Indirect partitioning of soil respiration in a series of evergreen forest ecosystems

A simple estimation of heterotrophic respiration can be obtained analytically as the y-intercept of the linear regression between soil-surface CO₂ efflux and root biomass. In the present study, a development of this indirect methodology is presented by taking into consideration both the temporal variation and the spatial heterogeneity of heterotrophic respiration. For this purpose, soil CO₂ efflux, soil carbon content and main stand characteristics were estimated in seven evergreen forest ecosystems along an elevation gradient ranging from 250 to 1740 m. For each site and for each sampling date the measured soil CO₂ efflux (R _S) was predicted with the model R _S = a × S _C + b × R _D ± ε, where S _C is soil carbon content per unit area to a depth of 30 cm and R _D is the root density of the 2–5 mm root class. Regressions with statistically significant a and b coefficients allowed the indirect separation of the two components of soil CO₂ efflux. Considering that the different sampling dates were characterized by different soil temperature, it was possible to investigate the temporal and thermal dependency of autotrophic and heterotrophic respiration. It was estimated that annual autotrophic respiration accounts for 16–58% of total soil CO₂ efflux in the seven different evergreen ecosystems. In addition, our observations show a decrease of annual autotrophic respiration at increasing availability of soil nitrogen. Section Editor: A. Hodge     

Entomogenous fungi in tropical forest ecosystems: an appraisal

Abstract. 1. Species of the genus Cordyceps (Ascomycotina; Clavicipitales) are the commonest fungi encountered on arthropods in tropical forests.
2. The asexual states of Cordyceps may occur in conjunction with or separately from the perfect state and are classified in the genera Hirsutella, Hymenostilbe, Nomuraea, Paecilomyces and Verticillium of the imperfect fungi (Deuteromycotina; Moniliales).
3. Most Cordyceps species have a restricted host range and this rigid host specificity, at the generic or tribal level, is especially evident in ant-fungal associations.
4. Pathogenicity was not tested but circumstantial evidence is presented which supports the view that Cordyceps species are primary pathogens of arthropods.
5. Observations of living, infected ants suggest that behavioural patterns are radically altered, possibly altruistically orientated.
6. It is thought that entomogenous fungi are involved in the regulation of arthropod populations and may help to maintain stability in tropical forest ecosystems.
7. The value of these organisms to man may be in the metabolites they produce rather than in their direct usage as biological control agents of insect pests.     

Tropical forest recovery: legacies of human impact and natural disturbances

Land-use history interacts with natural forces to influence the severity of disturbance events and the rate and nature of recovery processes in tropical forests. Although we are far from an integrated view of forest recovery processes, some generalizations can be made. Recovery of forest structure and composition is relatively rapid following disturbances that primarily impact forest canopies, such as hurricanes. Recovery is considerably slower following disturbances that heavily impact soils as well as aboveground vegetation, such as bulldozing, heavy or long-term grazing, and severe fires, often with long-lasting effects on species composition. The landscape matrix plays a critical role in local recovery processes. Proximity of disturbed areas to remnant forest patches promotes more rapid recovery, which depends heavily on seed dispersal. Recovery of aboveground biomass is constrained by soil fertility and texture across regions as well as across soil types within a region. Restoration of soil fertility may be a prerequisite for forest recovery on sites with severely degraded soils. Despite evidence of rapid forest recovery following large-scale deforestation, many degraded areas of today's tropics will require human assistance to recover forest structure, species composition, and species interactions typical of mature tropical forests.     

Successional changes in soil nitrogen availability,non-symbiotic nitrogen fixation and carbon/nitrogen ratios in southern Chilean forest ecosystems

Vast areas of southern Chile are now covered by second-growth forests because of fire and logging. To study successional patterns after moderate-intensity, anthropogenic fire disturbance, we assessed differences in soil properties and N fluxes across a chronosequence of seven successional stands (2–130 years old). We examined current predictions of successional theory concerning changes in the N cycle in forest ecosystems. Seasonal fluctuations of net N mineralization (N_min) in surface soil and N availability (N_a; N_a=NH ₄ ⁺ –N+NO ₃ ^– –N) in upper and deep soil horizons were positively correlated with monthly precipitation. In accordance with theoretical predictions, stand age was positively, but weakly related to both N_a (r ²=0.282, P<0.001) and total N (N_tot; r ²=0.192, P<0.01), and negatively related to soil C/N ratios (r ²=0.187, P<0.01) in surface soils. A weak linear increase in soil N_min (upper plus deep soil horizons) was found across the chronosequence (r ²=0.124, P<0.022). N_min occurred at modest rates in early successional stands, suggesting that soil disturbance did not impair microbial processes. The relationship between N fixation (N_fix) in the litter layer and stand age best fitted a quadratic model (r ²=0.228, P<0.01). In contrast to documented successional trends for most temperate, tropical and Mediterranean forests, non-symbiotic N_fix in the litter layer is a steady N input to unpolluted southern temperate forests during mid and late succession, which may compensate for hydrological losses of organic N from old-growth ecosystems.     

Nitrogen deposition and forest nitrogen cycling along an urban–rural transect in southern China

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ¹⁵N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha^?1 yr^?1, while the throughfall covered a larger range of 11.7–65.1 kg N ha^?1 yr^?1. Foliar N concentration, NO₃^? leaching to stream, and soil NO₃^? concentration were low and NO₃^? production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO₃^? leaching to stream, and negatively with pH values in soil and stream water. Foliar δ¹⁵N was between ?6.6‰ and 0.7‰, and was negatively correlated with soil NO₃^? concentration and NO₃^? leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ¹⁵N values. We proposed several potential mechanism that could contribute to the δ¹⁵N pattern, including (1) increased plant uptake of ¹⁵N‐depleted soil NO₃^?, (2) foliage uptake of ¹⁵N‐depleted NH₄⁺, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.