Context Old-growth forests with natural forest development and complex stand structure have become extremely rare in Central Europe. Changes of biogeochemistry and the N cycle across a full forest development cycle are not well understood.61 Aims We tested the hypothesis that net N mineralization and the relative importance of nitrification are increasing with proceeding forest development from regeneration to decay stages.61 Methods In an unmanaged old-growth spruce forest, we measured net ammonification and nitrification rate in the five forest development stages in 202years using the intact soil core incubation method.61 Results Net N mineralization (and ammonification) rates were higher in the closed stands of the optimum and over-mature stages than in the more open decay and regeneration stages. Only a small proportion of NH4+ was oxidized to NO361 in the studied acidic soils.61 Conclusion Lower N mineralization in the more open than the closed patches of this natural forest is unexpected, contrasting with the findings from artificial gaps. Possible reasons are reduced litter supply and lower canopy N interception in gaps in this forest under exposure to high N deposition. Further studies in other old-growth forests are needed to better understand the mechanisms causing long-term change in N cycling with forest development.61 Key messageNitrogen mineralization was higher in the optimum and over-mature stages with closed canopy than in the more open decay and regeneration stages of an unmanaged old-growth forest with high atmospheric nitrogen load, in contrast to published experiments with artificial gaps.

ABSTRACT Sewage sludge amendment of soils leads to an increase in soil fertility, but may induce heterogeneities not initially present in the soil. Spatial variation of soil organic carbon (C) and nitrogen (N) was studied in a heavy clay soil after a sewage sludge application and NPK-treatment (nitrogen, phosphorus, and potassium) using geostatistical approaches. In total, 314 soil samples were taken on two adjacent 40 m 40 m plots (one sludge-amended and one NPK-treated) at three different scales (scale 40 m: 40 m 40 m, scale 10 m: 10 m 10 m, and scale 2.5 m: 2.5 m 2.5 m). The coefficient of variation almost doubled for both C and N after sludge treatment. Because of this, more samples were needed to estimate mean values for the sludge-amended plot compared with the NPK-treated plot. To estimate the population mean at the 95% confidence level with 10% uncertainty (for all scales and all treatments), 5 to 13 samples were required for C and 4 to 7 for N. The C was spatially more structured compared to N. Semivariances of the sludge-amended plot displayed higher values compared with the NPK-treated plot, except for N at the smallest scale. This was probably due mainly to the sludge characteristics and the application technique of the sewage sludge. Constant semivariance (sill) was reached at ranges up to 15 m for C, but it was often unbounded (>40 m) for N.

Global warming may have profound effects on terrestrial ecosystems. However, a comprehensive evaluation of the effects of warming on ecosystem nitrogen (N) pools and dynamics is not available.Here, we compiled data of 528 observations from 51 papers and carried out a meta-analysis of experimental warming effects on 13 variables related to terrestrial N pools and dynamics.We found that, on average, net N mineralization and net nitrification rate were increased by 52.2 and 32.2%, respectively, under experimental warming treatment. N pools were also increased by warming, although the magnitude of this increase was less than that of N fluxes. Soil microbial N and N immobilization were not changed by warming, probably because microbes are limited by carbon sources. Grassland and shrubland/heathland were less responsive to warming than forest, probably because the reduction of soil moisture by warming offset the temperature effect in these areas. Soil heating cable and all-day treatment appeared to be the most effective method on N cycling among all treatment methods.Results of this meta-analysis are useful for better understanding the response of N cycling to global warming and the underlying mechanism of warming effects on plants and ecosystem functions.

Carbon (C) and nitrogen (N) fluxes are largely controlled by the small but highly bio-reactive, labile pools of these elements in terrestrial soils, while long-term C and N storage is determined by the long-lived recalcitrant fractions. Changes in the size of these pools and redistribution among them in response to global warming may considerably affect the long-term terrestrial C and N storage. However, such changes have not been carefully examined in field warming experiments. This study used sulfuric acid hydrolysis to quantify changes in labile and recalcitrant C and N fractions of soil in a tallgrass prairie ecosystem that had been continuously warmed with or without clipping for about 2.5 years. Warming significantly increased labile C and N fractions in the unclipped plots, resulting in increments of 373 mg C kg 1 dry soil and 15 mg N kg 1 dry soil, over this period whilst clipping significantly decreased such concentrations in the warmed plots. Warming also significantly increased soil microbial biomass C and N in the unclipped plots, and increased ratios of soil microbial/labile C and N, indicating an increase in microbial C- and N-use efficiency. Recalcitrant and total C and N contents were not significantly affected by warming. For all measured pools, only labile and microbial biomass C fractions showed significant interactions between warming and clipping, indicating the dependence of the warming effects on clipping. Our results suggest that increased soil labile and microbial C and N fractions likely resulted indirectly from warming increases in plant biomass input, which may be larger than warming-enhanced decomposition of labile organic compounds.

Snow amount is expected to decline in the Northern hemisphere as an effect of climate warming. However, snow amount in alpine regions will probably undergo stronger interannual fluctuations than elsewhere. We set up a short-term (1 year) experiment in which we manipulated snow cover in an alpine bog, with the following protocol: snow removal at the end of winter; snow removal in spring; snow addition in spring; removal of all aboveground plant tissues with no snow manipulation; no manipulation at all. We measured, at different dates from late spring to early autumn: ecosystem respiration (ER), and concentrations of carbon (C), nitrogen (N) and phosphorus (P) in the soil and in microbes. We hypothesized that longer duration of snow cover will lead to: i) higher ER rates associated with increased microbial biomass; and ii) decreased soil nutrient availability. Contrary to our first hypothesis, ER and microbial C content were unaffected by the snow cover manipulations, probably because ER was decoupled from microbial biomass especially in summer, when CO 2 efflux was dominated by autotrophic respiration. Our second hypothesis also was partially contradicted because nutrient content in the soil and in plants did not vary in relation to snow cover. However, we observed unexpected effects of snow cover manipulations on the N : P ratio in the microbial biomass, which declined after increasing snow cover. This probably depended on stimulation of microbial activity, which enhanced absorption of P, rather than N, by microbes. This may eventually reduce P availability for plant uptake.

61N amendments decreased organic soil pH, microbial biomass C and % C.61N amendments altered microbial community structure.61Fertilization decreased arbuscular mycorrhizal and saprotrophic fungal abundance.61Spring, but not fall bacterial abundance was reduced by N amendments.61N degrading enzymes activities were elevated in N amendment plots.

Simulation of decomposition and inorganic nitrogen release in complex biogeochemical models can be based on different principles. A major problem is the link between carbon and nitrogen mineralization and a description of microbial growth dynamics in dependence of a suite of possible substrates. This contribution considers a first order decomposition model with several carbon pools and one nitrogen pool to investigate how the decomposition of plant types and mineralization of nitrogen is related to carbon quality. The model structure assumes that nitrogen is mobilised with the rate at which the lignin compounds decompose. The decomposition module is coupled with microbial dynamics by adjusted Michaelis Menten equations that relate microbial growth to the availability of various substrates. The model was calibrated using Markov Chain Monte Carlo ( MCMC ) applied to measured litter remnants, concentrations of lignin, cellulose and nitrogen from 30 in situ incubations of foliage litters. Additionally, data from a laboratory incubation experiment were used to analyse the formation of microbial biomass, dissolved organic nitrogen, ammonium (NH 4 + ) and microbial respiration. Parameter sensitivity was analysed according to the rate of acceptance of various settings in the MCMC calibration chain. The most important parameters for the decomposition process were the decomposition rate of lignin, and the temperature response parameter Q 10 . The most important parameters for the formation of microbial biomass, dissolved organic nitrogen, ammonium and microbial respiration, were the potential growth rate of the microbial population and the rate of microbial decay. Estimated optimal decomposition rates for field experiments are 0.00365±650.002 d 611 for lignin like compounds, 0.00665±650.004 d 611 for cellulose like compounds and 0.028665±650.052 d 611 for solutes. The temperature response parameter Q 10 is 3.265±650.6 and the optimum decomposition temperature is 28.165±654.3°C. Important model parameters on microbial biomass and nitrification are the maximum microbial growth rate μ MAX 65=650.1365±650.82 gC mic gC mic 611 d 611 or the rate of microbial decay D 65=650.00665±650.014 gC mic gC mic 611 d 611 . The model performance was tested for independent datasets. Generally, correlations between modelled and measured values, expressed in R 2 , were high for the remaining tissue dry weight, or concentrations of lignin, cellulose and solutes or organic nitrogen ( R 2 65>650.84). Due to uncertainties in measurements of DON and NH 4 + concentrations, microbial biomass or basal respiration and significant site variability in these parameters, the model performance for these parameters as expressed as R 2 was somewhat lower, but statistically highly significant, and in the range of 0.1-0.96.

Soil organic matter (SOM) and total nitrogen (STN) play an important role in terrestrial ecosystems. Knowledge of their regional distribution and dynamical change is an important basis for reasonable utilizing and protecting soil resources. However, very little attention has been paid to this in cultivated land of the southern Loess Plateau. In this study, Heyang County, an agricultural county located in the southern Loess Plateau, was chosen as study area. SOM and STN data were collected in 1983 and 2006. Spatial autocorrelation, geostatistics, and fractal methods were used to analyze their spatio-temporal variability. Results showed that mean contents of SOM and STN had significantly increased in the past 23 years. A relatively more increase was found in the southeast and relatively less increase in the northern half. Compared with those in 1983, SOM and STN contents in 2006 showed a higher nugget/sill ratio, shorter spatial autocorrelation distance, and larger fractal dimension, indicating that the spatial dependence of SOM and STN showed a weakening trend and a stronger spatial variability of the 2006 dataset. The main factors affecting SOM and STN changes were topography, soil type, and farm management practices. But, more and more impacts were derived from anthropogenic activities.

Scots pine ( Pinus sylvestris L.) forests of northern Sweden are often considered to be N limited. This limitation may have been exacerbated by the elimination of wildfire as a natural disturbance factor in these boreal forests. Phenolic inhibition of N mineralization and nitrification (due to litter and exudates of ericaceous shrubs) has been proposed as a mechanism for N limitation of these forests, but this hypothesis remains largely untested. N mineralization rates, nitrification rates, and sorption of free phenolic compounds were assessed along a fire-induced chronosequence in northern Sweden. A total of 34 forest stands varying in age since the last fire were identified and characterized. Overstorey and understorey vegetative composition and depth of humus were analysed in replicated plots at all 34 sites. Eight of the forest stands aged 3–35202years since the last fire were selected for intensive investigation in which ten replicate ionic resin capsules (used to assess net N mineralization and nitrification) and non-ionic carbonaceous resin capsules (used to assess free phenolic compounds) were installed at the interface of humus and mineral soil. A highly significant correlation was observed between site age and net sorption of inorganic N to resin capsules. Net accumulation of NH 4 + and NO 3 – on resin capsules followed a linear decrease ( R 2 =0.61, P <0.01) with time perhaps as a result of increased N immobilization with successional C loading. NO 3 – sorption to resin capsules followed a logarithmic decrease ( R 2 =0.80, P <0.01) that may be related to a logarithmic increase in dwarf shrub cover and decreased soil charcoal sorption potential along this chronosequence. A replicated field study was conducted at one of the late successional field sites to assess the influence of charcoal and an added labile N source on N turnover. Three rates of charcoal (0, 100, and 1,00002g M –2 ) and two rates of glycine (0 and 5002g N as glycine M –2 ) were applied in a factorial design to microplots in a randomized complete block pattern. Net ammonification (as assessed by NH 4 + sorption to resins) was readily increased by the addition of a labile N source, but this increase in NH 4 + did not stimulate nitrification. Nitrification was stimulated slightly by the addition of charcoal resulting in similar levels of resin-sorbed NO 3 – as those found in early successional sites. Resin-sorbed polyphenol concentrations were decreased with charcoal amendments, but were actually increased with N amendments (likely due to decomposition of polyphenols). Net N mineralization appears to be limited by rapid NH 4 + immobilization whereas nitrification is limited by the lack of an appropriate environment or by the presence of inhibitory compounds in late successional forests of northern Sweden.

目前我国氮沉降量仅位列欧洲、北美之后,日益增加的氮沉降会改变森林土壤氮素供应,进而影响土壤氮(N)循环及植物、土壤微生物对氮素的吸收利用。本文通过野外模拟氮沉降,运用充分运用野外监测、同位素示踪、室内分析等方法,来研究温带森林土壤微生物、植物对氮素的竞争在增氮情况下的响应,以期为森林系统生态管理及对氮沉降的深入研究提供参考。本研究以温带森林土壤为研究对象,分析不同形态氮(硝态氮、铵态氮和混合态氮)和不同剂量氮添加(对照0 kg·hm-2·a-1、低氮处理50 kg·hm-2·a-1和高氮处理150kg·hm-2·a-1)对温带森林土壤可矿化氮、土壤N20排放通量,及土壤微生物、植物对氮素的竞争的影响,并从土壤酶活性的角度来解释其变化机制。主要研究结果表明：氮添加均促进了土壤氮的矿化速率,且以硝态氮添加氮的平均日净氮矿化速率最高,是对照样地的5.9倍。不同形态、不同剂量的氮添加均显著提高了土壤无机氮含量,其中,铵态氮添加对土壤铵态氮含量影响最大,硝态氮添加对土壤硝态氮含量影响最大,高氮添加比低氮添加对无机氮的增幅更大。施氮显著促进了森林土壤N20的排放。对于不同形态氮添加,混合态氮添加对土壤N20排放的促进作用最为显著,不同剂量氮添加,土壤N20排放随着施氮剂量的增加而增加。土壤脲酶、磷酸酶和过氧化氢酶活性的提高增加了土壤无机氮的含量,即促进了土壤氮的输入；土壤脲酶、多酚氧化酶、磷酸酶和过氧化氢酶活性能显著促进森林土壤N20的排放,即促进了氮的输出。本文通过利用同位素示踪方法对温带森林植物-微生物竞争氮素影响进行研究,结果表明：无论施加NH4+-N还是NO3--N,植物固持的氮均高于土壤微生物,沉降到森林生态系中的氮大部分被植物固持。外加氮源显著提高了植物和微生物对氮素的固持。在氮添加样地和对照样地中,N03--15N的总恢复率分别为50.63%和85.68%,相应NH4+-15N的恢复率则为28.35%和39.59%。土壤微生物和植物对NO3--N的吸收产生了偏好。

目前我国氮沉降量仅位列欧洲、北美之后,日益增加的氮沉降会改变森林土壤氮素供应,进而影响土壤氮(N)循环及植物、土壤微生物对氮素的吸收利用。本文通过野外模拟氮沉降,运用充分运用野外监测、同位素示踪、室内分析等方法,来研究温带森林土壤微生物、植物对氮素的竞争在增氮情况下的响应,以期为森林系统生态管理及对氮沉降的深入研究提供参考。本研究以温带森林土壤为研究对象,分析不同形态氮(硝态氮、铵态氮和混合态氮)和不同剂量氮添加(对照0 kg·hm-2·a-1、低氮处理50 kg·hm-2·a-1和高氮处理150kg·hm-2·a-1)对温带森林土壤可矿化氮、土壤N20排放通量,及土壤微生物、植物对氮素的竞争的影响,并从土壤酶活性的角度来解释其变化机制。主要研究结果表明：氮添加均促进了土壤氮的矿化速率,且以硝态氮添加氮的平均日净氮矿化速率最高,是对照样地的5.9倍。不同形态、不同剂量的氮添加均显著提高了土壤无机氮含量,其中,铵态氮添加对土壤铵态氮含量影响最大,硝态氮添加对土壤硝态氮含量影响最大,高氮添加比低氮添加对无机氮的增幅更大。施氮显著促进了森林土壤N20的排放。对于不同形态氮添加,混合态氮添加对土壤N20排放的促进作用最为显著,不同剂量氮添加,土壤N20排放随着施氮剂量的增加而增加。土壤脲酶、磷酸酶和过氧化氢酶活性的提高增加了土壤无机氮的含量,即促进了土壤氮的输入；土壤脲酶、多酚氧化酶、磷酸酶和过氧化氢酶活性能显著促进森林土壤N20的排放,即促进了氮的输出。本文通过利用同位素示踪方法对温带森林植物-微生物竞争氮素影响进行研究,结果表明：无论施加NH4+-N还是NO3--N,植物固持的氮均高于土壤微生物,沉降到森林生态系中的氮大部分被植物固持。外加氮源显著提高了植物和微生物对氮素的固持。在氮添加样地和对照样地中,N03--15N的总恢复率分别为50.63%和85.68%,相应NH4+-15N的恢复率则为28.35%和39.59%。土壤微生物和植物对NO3--N的吸收产生了偏好。

为探究高寒灌丛生态系统根系分泌物碳（C）输入通量对大气氮（N）沉降的响应规律,该文以青藏高原东缘窄叶鲜卑花（Sibiraea angustata）灌丛为研究对象,采用根系分泌物野外原位收集法,分析了不同施N水平（对照N0=0 g·m~（–2）·a~（–1）;低N处理N5=5 g·m~（–2）·a~（–1）;高N处理N10=10 g·m~（–2）·a~（–1））对根系分泌物C输入速率与通量季节动态变化规律的影响。结果表明：（1）窄叶鲜卑花灌丛单位根生物量、单位根长、单位根表面积根系分泌物C输入速率均表现出明显的季节性动态变化,具体表现为8月〉6月〉10月,并呈现出与5 cm土壤温度相一致的变化趋势。（2）施N降低了窄叶鲜卑花灌丛单位根生物量、单位根长和单位根表面积根系分泌物C输入速率,但仅N10处理与对照（N0处理）间存在显著差异（p〈0.05）。（3）N5和N10处理下,窄叶鲜卑花灌丛细根生物量与N0处理相比分别降低了23.36%和33.84%。（4）由于施N导致根系分泌物C输入速率与细根生物量二者均显著降低,使得施N对窄叶鲜卑花灌丛根系分泌物C输入通量（g·m~（–2）·a~（–1））有显著的抑制作用,并随着施N浓度的增加抑制作用增大。推测其可能的原因是N素富集在一定程度上缓和了植物根系对养分的微生物驱动需求,从而降低了植物根系分泌物C输入通量,即N素富集条件下植物采取了低N收益-低C投入的生理策略。该研究结果对于进一步认知不同环境变化下高寒灌丛生态系统根系分泌物C输入及其介导的土壤生物C-养分循环过程具有重要的理论意义。

为探究高寒灌丛生态系统根系分泌物碳（C）输入通量对大气氮（N）沉降的响应规律,该文以青藏高原东缘窄叶鲜卑花（Sibiraea angustata）灌丛为研究对象,采用根系分泌物野外原位收集法,分析了不同施N水平（对照N0=0 g·m~（–2）·a~（–1）;低N处理N5=5 g·m~（–2）·a~（–1）;高N处理N10=10 g·m~（–2）·a~（–1））对根系分泌物C输入速率与通量季节动态变化规律的影响。结果表明：（1）窄叶鲜卑花灌丛单位根生物量、单位根长、单位根表面积根系分泌物C输入速率均表现出明显的季节性动态变化,具体表现为8月〉6月〉10月,并呈现出与5 cm土壤温度相一致的变化趋势。（2）施N降低了窄叶鲜卑花灌丛单位根生物量、单位根长和单位根表面积根系分泌物C输入速率,但仅N10处理与对照（N0处理）间存在显著差异（p〈0.05）。（3）N5和N10处理下,窄叶鲜卑花灌丛细根生物量与N0处理相比分别降低了23.36%和33.84%。（4）由于施N导致根系分泌物C输入速率与细根生物量二者均显著降低,使得施N对窄叶鲜卑花灌丛根系分泌物C输入通量（g·m~（–2）·a~（–1））有显著的抑制作用,并随着施N浓度的增加抑制作用增大。推测其可能的原因是N素富集在一定程度上缓和了植物根系对养分的微生物驱动需求,从而降低了植物根系分泌物C输入通量,即N素富集条件下植物采取了低N收益-低C投入的生理策略。该研究结果对于进一步认知不同环境变化下高寒灌丛生态系统根系分泌物C输入及其介导的土壤生物C-养分循环过程具有重要的理论意义。

Global warming and nitrogen (N) deposition are important factors impacting soil carbon (C) and N cycling. Termites are important ecosystem engineers that can also strongly affect C and N cycling, potentially in interaction with warming and N deposition. In addition, non-additive effects that magnify or reduce their impacts on soil element cycles may occur when termite nests and adjacent soils are mixed due to their divergent properties but this has not been investigated. Here, we collected termite nests and trails built in wood (“termite nest soils”) and adjacent control soils in forests at Lu Mountain (Jiangxi, China) to investigate effects of termites, warming, and N deposition on C and N processes. We measured CO 2 emissions, N 2 O emissions, net N mineralization, and net nitrification when soils were incubated at different temperatures (1502°C, 2502°C, or 3502°C) and levels of N deposition (control vs. 402g02N02m 61022 ). Termite nest soils were characterized by higher dissolved organic C and CO 2 emissions. CO 2 emissions decreased with N addition and increased with temperature. N 2 O emissions increased with N deposition and increased with temperature, especially in termite nest soils and mixed soils. Net N mineralization rates increased with temperature but increases were smaller and more gradual in control and mixed soils than in termite soils. Mixing termite nest soils and control soils imposed synergistic (N mineralization: up to 57% higher than expected; nitrification: up to 170% higher; N 2 O emissions without N addition: 18% higher) and antagonistic (CO 2 emissions: 7% lower; N 2 O emissions with N addition: 28% lower) mixing effects, indicating termite impacts on soil C and N cycling might be under- and over-estimated, respectively, based on each soil alone. In light of the remarkable abundance of termites, the effects of mixing termite nest soils and the control soils on soil C and N cycling should be considered in the context of global change.

Abstract Soil net nitrogen (N) mineralization (Nmin) is a pivotal process in global N cycle regulating the N availability of plant growth. Understanding the spatial patterns of Nmin its temperature sensitivity to changing temperature (Q10) and regulatory mechanisms is critical to improve the management of soil nutrients. In this study, we collected 379 peer reviewed scientific papers to explore how Nmin and the Q10 of Nmin vary among different ecosystems and regions at global scale. The results showed that Nmin varied significantly among different ecosystems with a global average of 2.41 mg kg 1 soil d 1. Furthermore, Nmin significantly decreased with increasing latitude and altitude. The Q10 varied significantly among different ecosystems with a global average of 2.21, ranging from the highest found in forest soils (2.43) and the lowest found for grassland soils (1.67) and significantly increased with increasing latitude. Path analyses indicated that Nmin was primarily affected by the content of soil organic carbon (C), soil C:N ratio, and clay content, while Q10 was primarily influenced by the soil C:N ratio and soil pH. Furthermore, the activation energy (Ea) of soil N mineralization was significantly and negative correlated with the substrate quality index among all ecosystems, which indicates the applicability of the carbon quality temperature (CQT) hypothesis to soil N mineralization at a global scale. These findings provide empirical evidence that under global warming scenarios soil N availability is expected to increase stronger in colder regions as compared with low latitude regions due to the higher Q10. This may alleviate the restriction of N supply for increased primary productivity at higher latitudes.

Nitrification and denitrification are key microbiological processes in the soil nitrogen cycle and are the main biological sources of N 2 O emissions from soils. In this work, we measured gross nitrification and denitrification rates of northern Tibet alpine grassland ecosystems during the growing season and evaluated the influence of soil environmental factors. The results showed that the soil inorganic nitrogen concentration and gross nitrification and denitrification rates of both alpine meadow and alpine steppe varied obviously across the season. During the growing season mean values of gross nitrification and denitrification rates of the alpine meadow site were 3.0 and 2.3 times greater than those of the alpine steppe site, respectively. Both gross nitrification and denitrification rates were not significantly correlated with the determined soil characteristics which include soil microbial biomass, inorganic nitrogen, and soil temperature, except that gross nitrification seemed associated with the microsite where soil moisture was higher. Our results demonstrate that soil moisture can explain partly the higher soil nitrogen (N) transformation rates in alpine meadow sites, but soil N transformation microorganisms and enzyme activities studies covering prolonged observation periods are still needed to clarify the key soil environmental factors that control gross nitrification and denitrification processes in alpine grassland ecosystems.

Abstract Plant species are important drivers of soil microbial communities. However, how plant functional traits are shaping these communities has received less attention though linking plant and microbial traits is crucial for better understanding plant-microbe interactions. Our objective was to determine how plant–microbe interactions were affected by plant traits. Specifically we analyzed how interactions between plant species and microbes involved in nitrogen cycling were affected by plant traits related to nitrogen nutrition in interaction with soil nitrogen availability. Eleven plant species, selected along an oligotrophic–nitrophilic gradient, were grown individually in a nitrogen-poor soil with two levels of nitrate availability. Plant traits for both carbon and nitrogen nutrition were measured and the genetic structure and abundance of rhizosphere microbial communities, in particular the ammonia oxidizer and nitrate reducer guilds, were analyzed. The structure of the bacterial community in the rhizosphere differed significantly between plant species and these differences depended on nitrogen availability. The results suggest that the rate of nitrogen uptake per unit of root biomass and per day is a key plant trait, explaining why the effect of nitrogen availability on the structure of the bacterial community depends on the plant species. We also showed that the abundance of nitrate reducing bacteria always decreased with increasing nitrogen uptake per unit of root biomass per day, indicating that there was competition for nitrate between plants and nitrate reducing bacteria. This study demonstrates that nitrate-reducing microorganisms may be adversely affected by plants with a high nitrogen uptake rate. Our work puts forward the role of traits related to nitrogen in plant–microbe interactions, whereas carbon is commonly considered as the main driver. It also suggests that plant traits related to ecophysiological processes, such as nitrogen uptake rates, are more relevant for understanding plant–microbe interactions than composite traits, such as nitrophily, which are related to a number of ecophysiological processes.

Urban wetlands potentially play an important role in nitrate (NO 3 61 ) removal from stormwater, but nitrogen loading from the atmosphere and surface water must intersect with soil properties optimal for NO 3 61 removal for this potential to be realized. We examined predictors of NO 3 61 removal via the microbial process of denitrification in an urban wetland system in New Jersey, USA with highly heterogeneous soils. Soil cores representing the wide range of soil textures at the site were collected to examine relationships between intact core denitrification rates, denitrification enzyme activity (DEA), available inorganic nitrogen, and soil water retention characteristics. Water retention curves were characterized for pressure potentials ranging from 611 to 615000cm and used to estimate pore size distribution parameters. The highest intact core denitrification rates occurred in soils located at low elevations, with high macroporosity, and low variability in soil pore radius. High DEA corresponded with high available soil NO 3 61 and high elevation. Soil samples collected at 118 points from the site and analyzed for soil organic matter and texture fractions were used to create interpolated raster layers of properties related to high denitrification rates (“hot spots”). Weighted estimations of whole-site NO 3 61 removal based on denitrification hot spots were higher than site estimations based on average denitrification rates, suggesting that studies using the latter approach may be underestimating NO 3 61 removal at the landscape level. Stormwater channels at the site intersected with denitrification hot spots over 20% of total channel area, indicating that soils may be at least partially reducing total NO 3 61 loads to the adjacent creek. These results show that soil physical properties that are relatively immutable can be used for predicting the location of potential hot spots of microbial activity at the landscape scale.

Forest ecosystems with low soil nitrogen (N) availability are characterized by direct competition for this growth-limiting resource between several players, i.e. various components of vegetation, such as old-growth trees, natural regeneration and understorey species, mycorrhizal fungi, free-living fungi and bacteria. With the increase in frequency and intensity of extreme climate events predicted in current climate change scenarios, also competition for N between plants and/or soil microorganisms will be affected. In this review, we summarize the present understanding of ecosystem N cycling in N-limited forests and its interaction with extreme climate events, such as heat, drought and flooding. More specifically, the impacts of environmental stresses on microbial release and consumption of bioavailable N, N uptake and competition between plants, as well as plant and microbial uptake are presented. Furthermore, the consequences of drying wetting cycles on N cycling are discussed. Additionally, we highlight the current methodological difficulties that limit present understanding of N cycling in forest ecosystems and the need for interdisciplinary studies.

Nitrogen availability is considered limiting for plant growth at the forest-tundra ecotone, and it might modulate ecosystem response to climate warming. The aim of this research was to compare the impact of climate, vegetation cover, and soil organic matter (SOM) chemistry on N mineralization rates at the forest-tundra ecotone. We therefore estimated N mineralization in mountain birch (Betula pubescens Ehrh. ssp. czerepanovii) forest and tundra soil across a broad-scale latitudinal gradient in Fennoscandia, which incorporated 4 research sites (Dovrefjell, Vassijaure, Abisko, and Joatka). During the summer period, ammonium was the dominant form of mineralized nitrogen in forest soils, while nitrate mineralization rates were higher at tundra sites during the winter. A negative regression relationship between an index of climatic continentality and N mineralization was found. Further, summer NH4+ mineralization rates increased with total N content in soils, while NO3- mineralization seemed to be associated with C availability. Our study showed markedly contrasting inorganic N release in forest and tundra soil, and that, although mineralization rates differed between the summer and winter period, the winter activity was relatively high and should not be ignored. We conclude that a shift in the forest-tundra ecotone in response to climate warming will have stronger effects on nitrogen availability at these sites than the direct effects of warming.

Abstract Permafrost represents 26% of terrestrial soil ecosystems; yet its biology, essentially microbiology, remains relatively unexplored. The permafrost environment is considered extreme because indigenous microorganisms must survive prolonged exposure to subzero temperatures and background radiation for geological time scales in a habitat with low water activity and extremely low rates of nutrient and metabolite transfer. Yet considerable numbers and biodiversity of bacteria exist in permafrost, some of which may be among the most ancient viable life on Earth. This review describes the permafrost environment as a microbial habitat and reviews recent studies examining microbial biodiversity found in permafrost as well as microbial growth and activity at ambient in situ subzero temperatures. These investigations suggest that functional microbial ecosystems exist within the permafrost environment and may have important implications on global biogeochemical processes as well as the search for past or extant life in permafrost presumably present on Mars and other bodies in our solar system.

Warming generally accelerates the decomposition of plant litter. However, changes in precipitation could alter the sensitivity of litter decomposition to warming, thereby affecting the formation of litter-derived soil organic matter. As grassland soils store 6520% of Earth's soil carbon, understanding the effect of climatic changes on the decomposition dynamics of grasses is important. However, little is known about how projected changes in climate would affect litter microbial communities and enzyme activities, and the consequences of these changes for the mass loss and compound-specific degradation of grass litter that possess complex lignocellulosic chemistry. Over a period of two years, using litter of the grass Poa trivialis , we studied how mass loss, microbial enzyme activity and fine-level litter chemistry responded to a factorial combination of 4 levels of warming (up to ambient+654°C) and three levels of precipitation [ambient, wet (+50%) and dry (6150%)] at the Boston-Area Climate Experiment (BACE), in Massachusetts, USA. After 393 days of decomposition, supplemental precipitation accelerated mass loss compared to the dry treatment, as a consequence of faster loss of hydroxycinnamates, which protect carbohydrates through cross-linkages with lignins. Only a third as much of the cell wall-bound ferulic and p -coumaric acids remained in litter from the supplemental precipitation treatment compared to the ambient controls. In contrast, the warming treatments did not affect mass loss until later, after 740 days, when the litter in the warmest treatment (+654°C) had lost the most mass. Although warming significantly affected mass loss after 740 days, there was also a trend in the warmest treatments toward greater mass loss in the wet (78% mass loss) and ambient (68%) plots compared to dry plots (61%), possibly due to the higher activity of β-glucosidase. Though mass loss at this final time point varied with both warming and precipitation treatments, the compound-specific degradation of litter captured by diffuse reflectance infra-red Fourier transform (DRIFT) and 13 C Nuclear Magnetic Resonance (NMR) spectroscopy revealed that only the precipitation treatments significantly altered the chemistry of carbon compounds in the decomposed tissue. Litter that decomposed in the dry treatment had a higher proportion of carbohydrates remaining than litter in the wet and ambient treatments. Similarly, although ergosterol content and potential activity of phenol oxidase decreased in the warmer treatments, the consequences of this response were not observed in the degradation of specific compounds in litter, which varied only with precipitation treatments. Our results suggests that mass loss and enzyme activities may not accurately capture the complexity of compound-specific degradation of litter during decomposition. Our results also identified non-linear responses of β-glucosidase and N-acetyl-β-D-glucosaminidase (NAG) activities to warming. These results thus emphasize the complexities of litter decomposition and suggest that similar changes in decomposition across other grass species could alter the carbon budget of grasslands.

Abstract To explain the effects of short-term N addition on plant biomass allocation and on carbon (C) and nitrogen (N) pools in an alpine scrub ecosystem, we carried out a field experiment in Sibiraea angustata scrubland on the eastern margin of the Qinghai-Tibetan Plateau of China. After one and a half years of N addition at four rates (N0, control; N20, 20; N50, 50; N100, 100 kg N ha611 year611), we investigated the amount and allocation of biomass and the C and N pools in several parts of the ecosystem, including shrubs (leaves, shoots and branches, coarse roots and fine roots), grass (above- and below-ground) and litter (wood and leaf debris) components, and seven depth intervals within the soil (0–5, 5–10, 10–20, 20–30, 30–50, 50–70 and 70–100 cm). The results were as follows: (i) total vegetation biomass showed a linear increase with the increase in N (P < 0.05), mainly from the increased root biomass in both shrubs and grasses, (ii) the ecosystem C and N storage were 36 and 3.26 kg m612, respectively, of which the shrub, grass, litter and soil components contributed 11.08, 0.47, 0.25 and 88%, respectively, to the C pool and 3.07, 0.16, 0.08 and 97%, respectively, to the N pool, (iii) the ecosystem N pool did not change in response to the addition of N, whereas the ecosystem C pool responded linearly to increasing N (P < 0.05). These results suggest that the alpine scrub ecosystem functions as a net C sink under increasing atmospheric N deposition, mainly by promoting belowground C sequestration. Effects of short-term N addition on biomass allocation and C and N pools in alpine scrub. Response to N addition in C pool of components of the ecosystem and soil at depth (0–100 cm). Root:shoot ratio of vegetation and ecosystem C pool increased linearly with increasing N. Alpine scrub ecosystem may function as a net C sink under increasing atmospheric N deposition.

After 8 wk of incubation, the amounts of soluble organic carbon leached by 0.001 M CaCl2 solution from flooded soils ranged from 153 to 630 mg C kg-1. In contrast, only 28-107 mg C kg-1 was leached from nonflooded soils. The amounts of soluble organic nitrogen leached from the flooded soils ranged from 10 to 30 mg N kg-1 compared with 5.9-12 mg N kg-1 from nonflooded soils. In the flooded soils, ammonium nitrogen dominated the total inorganic nitrogen leached (99.5-99.9%) whereas in nonflooded soils leachable N was mainly nitrate and nitrite (97.4-99.9%). Methane was emitted from the flooded soils (10-138 mg C kg-1 over 8 wk). The rate of carbon dioxide evolution in flooded soils increased linearly with time and total evolution ranged from 72 to 552 mg C kg-1, whereas CO2 evolution in the nonflooded soils was steady with total evolution ranging from 159 to 1279 mg C kg-1 after 8 wk. -from Authors

在高海拔地区，较短的生长季和较低的温度是限制植物生命活动最重要的因子。高山生态系统中严重匮乏的土壤有效氮限制了植物的生长发育，植物如何更好地适应随着海拔增加的极端气候环境，以及高山草地生态系统中氮的有效利用如何实现亟需研究。高山植物在较宽的温度范围内维持新陈代谢并且积极生长，在较短的周期内完成营养生长和生殖生长，且具有较高的物种丰富度，并在季节明显的高山地带具有适应气候条件的节律性变化，而丰富多变的植物物候期与生活史对策有关。土壤中不同化学形态的氮素为植物生长发育过程中的养分利用提供了多元选择。植物物候格局...

在高海拔地区，较短的生长季和较低的温度是限制植物生命活动最重要的因子。高山生态系统中严重匮乏的土壤有效氮限制了植物的生长发育，植物如何更好地适应随着海拔增加的极端气候环境，以及高山草地生态系统中氮的有效利用如何实现亟需研究。高山植物在较宽的温度范围内维持新陈代谢并且积极生长，在较短的周期内完成营养生长和生殖生长，且具有较高的物种丰富度，并在季节明显的高山地带具有适应气候条件的节律性变化，而丰富多变的植物物候期与生活史对策有关。土壤中不同化学形态的氮素为植物生长发育过程中的养分利用提供了多元选择。植物物候格局...

We studied the uptake of ammonium,nitrate,and a variety of amino acids by alpine plant species in the Kobresia humilis alpine meadow ecosystem in situ.We examined the extent of niche separation in uptake of N source by different plant species in alpine communities,and investigated the contribution of symbiotically fixed N to the total N in alpine meadow.The results are(1) δ 15 N natural abundance values of 13 plant species lie between 2.680‰ and 5.169‰,and the scope is 7.849‰.(2) Leguminous plants,such as Trigonella ruthenica,Gueldenstaedtia diversiffolia,and Oxytyopis ochrocephala,and non-leguminous plant Gentiana straminea uptake low amounts of 15 N labeled ammonium,nitrate,glycine or aspartate in soil.(3) As far as the plant uptake of organic N is concerned,Kobresia humilis,Poa pratensis,and Gentiuna spathutifolta can effectively uptake organic nitrogen,and about 37%-40% of the nitrogen of these species comes from soil organic nitrogen sources(such as glycine and aspartate).Stipa aliena can effectively uptake nitrate,and 60% of its nitrogen comes from soil nitrate.Potentilla anserina,Poa pratensis,and Thalictrum alpinum can effectively absorb ammonium in comparason to other plant species in the meadow,and about 25%-27% of the nitrogen in these plants comes from soil ammonium.(4) The contribution of leguminous fixed N to total N is 7.48%-9.26% in Kobresia humilis alpine meadow.(5) These data show many plant species of alpine meadow may effectively utilize dissolved organic nitrogen such as amino acids,and these plants have diverse ways to uptake soil nitrogen in alpine meadows.Based on the results we can partly explain why there are abundant biodiversities and how plants at alpine habitat utilize the limited soil N sources.

基于海北站野外长期增温和降水改变控制平台,研究高寒草甸生态系统生长季土壤无机氮对增温和降水改变的响应.结果表明,增温使铵态氮降低47.5％(p=0.001),硝态氮降低46.1％(p=0.021).降水的改变对无机氮的影响存在不对称性,增加降水使铵态氮增加74.7％(p=0.046),硝态氮增加154％(p=0.017);减少降水使铵态氮降低,对硝态氮无显著影响.铵态氮、硝态氮随着土壤湿度的增加而增加,与土壤温度无显著关系.这表明增温和降水改变主要通过改变土壤湿度而不是土壤温度影响生长季土壤无机氮.因此预测,未来气候变化背景下,土壤湿度的增加可能导致青藏高原高寒草甸土壤无机氮的可利用性增加.

基于海北站野外长期增温和降水改变控制平台,研究高寒草甸生态系统生长季土壤无机氮对增温和降水改变的响应.结果表明,增温使铵态氮降低47.5％(p=0.001),硝态氮降低46.1％(p=0.021).降水的改变对无机氮的影响存在不对称性,增加降水使铵态氮增加74.7％(p=0.046),硝态氮增加154％(p=0.017);减少降水使铵态氮降低,对硝态氮无显著影响.铵态氮、硝态氮随着土壤湿度的增加而增加,与土壤温度无显著关系.这表明增温和降水改变主要通过改变土壤湿度而不是土壤温度影响生长季土壤无机氮.因此预测,未来气候变化背景下,土壤湿度的增加可能导致青藏高原高寒草甸土壤无机氮的可利用性增加.

窄叶鲜卑花灌丛是青藏高原东缘高山灌丛中特有的、具代表性的类型。选择该群落的典型分布区域，运用数值分类法，将窄叶鲜卑花群系分为４个群丛组、１０个群丛类型。用ＰＣＡ排序技术定量分析了各类型在空间地理上的分布格局，以及与环境因子之间的关系。结果表明：各类型的分布主要取决于环境中土壤条件（主要是土壤水分和土壤有机质含量）和热量条件的垂直梯度变化，而群落的分类等级越低，对环境因子的变化越敏感。生物量的研究表

窄叶鲜卑花灌丛是青藏高原东缘高山灌丛中特有的、具代表性的类型。选择该群落的典型分布区域，运用数值分类法，将窄叶鲜卑花群系分为４个群丛组、１０个群丛类型。用ＰＣＡ排序技术定量分析了各类型在空间地理上的分布格局，以及与环境因子之间的关系。结果表明：各类型的分布主要取决于环境中土壤条件（主要是土壤水分和土壤有机质含量）和热量条件的垂直梯度变化，而群落的分类等级越低，对环境因子的变化越敏感。生物量的研究表

In order to understand the effects of projected global warming on soils in different land-use types, we compared the impacts of warming on soils in two contrasting forest ecosystems (a dragon spruce plantation and a natural forest) using the open-top chamber (OTC) method in the Eastern Tibetan Plateau of China. The OTC on average enhanced daily mean soil temperatures by 0.61 C (plantation) and by 0.55 C (natural forest) throughout the growing season, respectively. Conversely, soil volumetric moisture declined by 4.10% in the plantation and by 2.55% in the natural forest, respectively. Warming did not affect dissolved organic C (DOC) and N (DON) in the plantation but significantly increased them in the natural forest. Elevated temperature significantly increased net N mineralization rates and extractable inorganic N pools in both sites. Warming had no effects on microbial biomass C (MBC) and N (MBN) and their ratios (MBC/MBN) in the plantation and significantly increased MBC and MBN only late in the growing season in the natural forest. Warming did not affect basal respiration in the plantation but significantly increased it in the natural forest. No clear change was observed in metabolic quotient between warming regimes for both forest types. Experimental warming tended to increase invertase and urease in both forest soils. Measured pools related to N turnover generally showed significant interactions in warming, forest type and sampling date. Taken together, our results indicate that responses of soils to experimental warming depend strongly on forest managements and seasons.

采取定性与定量相结合的方法，评价了岷江上游典型植物群落的稳定性。将植被盖度、多样性、复杂性、演替度等反映植被稳定性的数量特征指标结合气候、地形、土壤等外部环境因子建立评价体系，利用层次分析法确定指标权重，利用综合指数法计算群落稳定性。结果表明：源区植物群落中，云杉林、柳灌丛、窄叶鲜卑花灌丛、冷杉林、绣线菊灌丛有较高的稳定性值，沙棘和小果小檗灌丛稳定性值较低；草甸群落中，苔草草甸和白茅草草甸具有较高的稳定性值，蒿草草甸和高山草甸稳定性值较低。干旱河谷植物群落中，绣线菊灌丛、瑞香灌丛、小花滇紫草灌丛、小马鞍羊蹄甲-白刺花灌丛、莸灌丛有较高的稳定性值，西南野丁香灌丛、驼绒藜灌丛、川甘亚菊灌丛的稳定性值较低。

采取定性与定量相结合的方法，评价了岷江上游典型植物群落的稳定性。将植被盖度、多样性、复杂性、演替度等反映植被稳定性的数量特征指标结合气候、地形、土壤等外部环境因子建立评价体系，利用层次分析法确定指标权重，利用综合指数法计算群落稳定性。结果表明：源区植物群落中，云杉林、柳灌丛、窄叶鲜卑花灌丛、冷杉林、绣线菊灌丛有较高的稳定性值，沙棘和小果小檗灌丛稳定性值较低；草甸群落中，苔草草甸和白茅草草甸具有较高的稳定性值，蒿草草甸和高山草甸稳定性值较低。干旱河谷植物群落中，绣线菊灌丛、瑞香灌丛、小花滇紫草灌丛、小马鞍羊蹄甲-白刺花灌丛、莸灌丛有较高的稳定性值，西南野丁香灌丛、驼绒藜灌丛、川甘亚菊灌丛的稳定性值较低。

Despite the perceived importance of exudation to forest ecosystem function, few studies have attempted to examine the effects of elevated temperature and nutrition availability on the rates of root exudation and associated microbial processes. In this study, we performed an experiment in which in situ exudates were collected from Picea asperata seedlings that were transplanted in disturbed soils exposed to two levels of temperature (ambient temperature and infrared heater warming) and two nitrogen levels (unfertilized and 25 g N m612 a611). Here, we show that the trees exposed to an elevated temperature increased their exudation rates I (μg C g611 root biomass h611), II (μg C cm611 root length h611) and III (μg C cm612 root area h611) in the unfertilized plots. The altered morphological and physiological traits of the roots exposed to experimental warming could be responsible for this variation in root exudation. Moreover, these increases in root-derived C were positively correlated with the microbial release of extracellular enzymes involved in the breakdown of organic N (R2 = 0.790; P = 0.038), which was coupled with stimulated microbial activity and accelerated N transformations in the unfertilized soils. In contrast, the trees exposed to both experimental warming and N fertilization did not show increased exudation rates or soil enzyme activity, indicating that the stimulatory effects of experimental warming on root exudation depend on soil fertility. Collectively, our results provide preliminary evidence that an increase in the release of root exudates into the soil may be an important physiological adjustment by which the sustained growth responses of plants to experimental warming may be maintained via enhanced soil microbial activity and soil N transformation. Accordingly, the underlying mechanisms by which plant root-microbe interactions influence soil organic matter decomposition and N cycling should be incorporated into climate-carbon cycle models to determine reliable estimates of long-term C storage in forests.

采用封顶埋管原位培养法研究川西亚高山不同海拔3种森林群落（岷江冷杉原始林A、针阔混交林B和珉江冷杉次生林C）土壤氮转化的季节动态特征。结果表明：3种森林群落的土壤净氨化速率、净硝化速率、净氮矿化速率和净氮固持速率都存在明显的季节动态；在非生长季，3种森林群落的净硝化速率、净氮矿化速率和净氮固持速率都为正值，而净氨化速率极低且为负值；各时期土壤氮矿化过程以土壤硝化作用为主，且土壤净硝化速率与净氮矿化速率季节动态趋势一致；土壤净氮矿化速率在生长季初期最低，在生长季中期达全年最高峰；不同海拔之间土壤净氨化速率差异显著（<em>P</em><0.05）；B海拔的土壤净硝化速率和净氮矿化速率低于其他2个海拔，但差异未达到显著水平；非生长季氮素固持作用最强。

采用封顶埋管原位培养法研究川西亚高山不同海拔3种森林群落（岷江冷杉原始林A、针阔混交林B和珉江冷杉次生林C）土壤氮转化的季节动态特征。结果表明：3种森林群落的土壤净氨化速率、净硝化速率、净氮矿化速率和净氮固持速率都存在明显的季节动态；在非生长季，3种森林群落的净硝化速率、净氮矿化速率和净氮固持速率都为正值，而净氨化速率极低且为负值；各时期土壤氮矿化过程以土壤硝化作用为主，且土壤净硝化速率与净氮矿化速率季节动态趋势一致；土壤净氮矿化速率在生长季初期最低，在生长季中期达全年最高峰；不同海拔之间土壤净氨化速率差异显著（<em>P</em><0.05）；B海拔的土壤净硝化速率和净氮矿化速率低于其他2个海拔，但差异未达到显著水平；非生长季氮素固持作用最强。