Seasonal variations in plant species effects on soil N and P dynamics

It is well established that plant species influence ecosystem processes, but we have little ability to predict which vegetation changes will alter ecosystems, or how the effects of a given species might vary seasonally. We established monocultures of eight plant species in a California grassland in order to determine the plant traits that account for species impacts on nitrogen and phosphorus cycling. Plant species differed in their effects on net N mineralization and nitrification rates, and the patterns of species differences varied seasonally. Soil PO4- and microbial P were more strongly affected by slope position than by species. Although most studies focus on litter chemistry as the main determinant of plant species effects on nutrient cycling, this study showed that plant species affected biogeochemical cycling through many traits, including direct traits (litter chemistry and biomass, live-tissue chemistry and biomass) and indirect traits (plant modification of soil bioavailable C and soil microclimate). In fact, species significantly altered N and P cycling even without litter inputs. It became particularly critical to consider the effects of these multiple traits in order to account for seasonal changes in plant species effects on ecosystems. For example, species effects on potential rates of net N mineralization were most strongly influenced by soil bioavailable C in the fall and by litter chemistry in the winter and spring. Under field conditions, species effects on soil microclimate influenced rates of mineralization and nitrification, with species effects on soil temperature being critical in the fall and species effects on soil moisture being important in the dry spring. Overall, this study clearly demonstrated that in order to gain a mechanistic, predictive understanding of plant species effects on ecosystems, it is critical to look beyond plant litter chemistry and to incorporate the effects of multiple plant traits on ecosystems.     

Plants control the seasonal dynamics of microbial N cycling in a beech forest soil by belowground C allocation

Soil microbes in temperate forest ecosystems are able to cycle several hundreds of kilograms of N per hectare per year and are therefore of paramount importance for N retention. Belowground C allocation by trees is an important driver of seasonal microbial dynamics and may thus directly affect N transformation processes over the course of the year. Our study aimed at unraveling plant controls on soil N cycling in a temperate beech forest at a high temporal resolution over a time period of two years, by investigating the effects of tree girdling on microbial N turnover. In both years of the experiment, we discovered (1) a summer N mineralization phase (between July and August) and (2) a winter N immobilization phase (November-February). The summer mineralization phase was characterized by a high N mineralization activity, low microbial N uptake, and a subsequent high N availability in the soil. During the autumn/winter N immobilization phase, gross N mineralization rates were low, and microbial N uptake exceeded microbial N mineralization, which led to high levels of N in the microbial biomass and low N availability in the soil. The observed immobilization phase during the winter may play a crucial role for ecosystem functioning, since it could protect dissolved N that is produced by autumn litter degradation from being lost from the ecosystem during the phase when plants are mostly inactive. The difference between microbial biomass N levels in winter and spring equals 38 kg N/ha and may thus account for almost one-third of the annual plant N demand. Tree girdling strongly affected annual N cycling: the winter N immobilization phase disappeared in girdled plots (microbial N uptake and microbial biomass N were significantly reduced, while the amount of available N in the soil solution was enhanced). This was correlated to a reduced fungal abundance in autumn in girdled plots. By releasing recently fixed photosynthates to the soil, plants may thus actively control the annual microbial N cycle. Tree belowground C allocation increases N accumulation in microorganisms during the winter which may ultimately feed back on plant N availability in the following growing season.     

Interactive effects of plant species diversity and elevated CO2 on soil biota and nutrient cycling

Terrestrial ecosystems consist of mutually dependent producer and decomposer subsystems, but not much is known on how their interactions are modified by plant diversity and elevated atmospheric CO2 concentrations. Factorially manipulating grassland plant species diversity and atmospheric CO2 concentrations for five years, we tested whether high diversity or elevated CO2 sustain larger or more active soil communities, affect soil aggregation, water dynamics, or nutrient cycling, and whether plant diversity and elevated CO2 interact. Nitrogen (N) and phosphorus (P) pools, symbiotic N2 fixation, plant litter quality, soil moisture, soil physical structure, soil nematode, collembola and acari communities, soil microbial biomass and microflora community structure (phospholipid fatty acid [PLFA] profiles), soil enzyme activities, and rates of C fluxes to soils were measured. No increases in soil C fluxes or the biomass, number, or activity of soil organisms were detected at high plant diversity; soil H2O and aggregation remained unaltered. Elevated CO2 affected the ecosystem primarily by improving plant and soil water status by reducing leaf conductance, whereas changes in C cycling appeared to be of subordinate importance. Slowed-down soil drying cycles resulted in lower soil aggregation under elevated CO2. Collembola benefited from extra soil moisture under elevated CO2, whereas other faunal groups did not respond. Diversity effects and interactions with elevated CO2 may have been absent because soil responses were mainly driven by community-level processes such as rates of organic C input and water use; these drivers were not changed by plant diversity manipulations, possibly because our species diversity gradient did not extend below five species and because functional type composition remained unaltered. Our findings demonstrate that global change can affect soil aggregation, and we advocate that soil aggregation should be considered as a dynamic property that may respond to environmental changes and feed back on other ecosystem functions.     

Soil responses to management, increased precipitation, and added nitrogen in ponderosa pine forests.

Forest management, climatic change, and atmospheric N deposition can affect soil biogeochemistry, but their combined effects are not well understood. We examined the effects of water and N amendments and forest thinning and burning on soil N pools and fluxes in ponderosa pine forests near Flagstaff, Arizona (USA). Using a 15N-depleted fertilizer, we also documented the distribution of added N into soil N pools. Because thinning and burning can increase soil water content and N availability, we hypothesized that these changes would alleviate water and N limitation of soil processes, causing smaller responses to added N and water in the restored stand. We found little support for this hypothesis. Responses of fine root biomass, potential net N mineralization, and the soil microbial N to water and N amendments were mostly unaffected by stand management. Most of the soil processes we examined were limited by N and water, and the increased N and soil water availability caused by forest restoration was insufficient to alleviate these limitations. For example, N addition caused a larger increase in potential net nitrification in the restored stand, and at a given level of soil N availability, N addition had a larger effect on soil microbial N in the restored stand. Possibly, forest restoration increased the availability of some other limiting resource, amplifying responses to added N and water. Tracer N recoveries in roots and in the forest floor were lower in the restored stand. Natural abundance delta15N of labile soil N pools were higher in the restored stand, consistent with a more open N cycle. We conclude that thinning and burning open up the N cycle, at least in the short-term, and that these changes are amplified by enhanced precipitation and N additions. Our results suggest that thinning and burning in ponderosa pine forests will not increase their resistance to changes in soil N dynamics resulting from increased atmospheric N deposition or increased precipitation due to climatic change. Restoration plans should consider the potential impact on long-term forest productivity of greater N losses from a more open N cycle, especially during the period immediately after thinning and burning.     

土壤温室气体产生与排放影响因素研究进展

土壤是温室气体(如CO2、CH4和N2O)产生的重要源,土壤温室气体主要来自于微生物呼吸,植物根呼吸和土壤动物呼吸。土壤温室气体排放机制及其影响因素是研究全球碳氮循环的重要组成部分。研究表明,影响土壤呼吸的因素很多,土壤理化性质如温度、含水量、有机质含量、pH值、氧化还原电位(Eh)、土壤质地等因素都可以直接影响土壤微生物量及其生理生化过程,从而影响温室气体排放。其中,土壤温度,湿度、有机质含量是关键性因素。此外,地域气候、土地利用以及土地覆盖变化也可以通过改变土壤理化性质及呼吸底物来影响温室气体排放。文章重点论述了土壤温室气体排放机制,排放影响因素以及排放的日变化和季节变化规律。认为今后的研究方向应该是土壤微环境碳氮循环机制,土壤呼吸模型在尺度上的推延,以及注重中国陆地与近海生态系统碳固定及减少碳排放的对策和应用技术研究,特别在人工林碳固定及农业固碳减排方面加大研究力度等。     

Biogeochemistry of a temperate forest nitrogen gradient

Wide natural gradients of soil nitrogen (N) can be used to examine fundamental relationships between plant-soil-microbial N cycling and hydrologic N loss, and to test N-saturation theory as a general framework for understanding ecosystem N dynamics. We characterized plant production, N uptake and return in litterfall, soil gross and net N mineralization rates, and hydrologic N losses of nine Douglas-fir (Pseudotsuga menziesii) forests across a wide soil N gradient in the Oregon Coast Range (U.S.A.). Surface mineral soil N (0-10 cm) ranged nearly three-fold from 0.29% to 0.78% N, and in contrast to predictions of N-saturation theory, was linearly related to 10-fold variation in net N mineralization, from 8 to 82 kg N.ha(-1) x yr(-1). Net N mineralization was unrelated to soil C:N, soil texture, precipitation, and temperature differences among sites. Net nitrification was negatively related to soil pH, and accounted for <20% of net N mineralization at low-N sites, increasing to 85-100% of net N mineralization at intermediate- and high-N sites. The ratio of net: gross N mineralization and nitrification increased along the gradient, indicating progressive saturation of microbial N demands at high soil N. Aboveground N uptake by plants increased asymptotically with net N mineralization to a peak of approximately 35 kg N.ha(-1) x yr(-1). Aboveground net primary production per unit net N mineralization varied inversely with soil N, suggesting progressive saturation of plant N demands at high soil N. Hydrologic N losses were dominated by dissolved organic N at low-N sites, with increased nitrate loss causing a shift to dominance by nitrate at high-N sites, particularly where net nitrification exceeded plant N demands. With the exception of N mineralization patterns, our results broadly support the application of the N-saturation model developed from studies of anthropogenic N deposition to understand N cycling and saturation of plant and microbial sinks along natural soil N gradients. This convergence of behavior in unpolluted and polluted forest N cycles suggests that where future reductions in deposition to polluted sites do occur, symptoms of N saturation are most likely to persist where soil N content remains elevated.     

Changes in microbial community characteristics and soil organic matter with nitrogen additions in two tropical forests

Microbial communities and their associated enzyme activities affect the amount and chemical quality of carbon (C) in soils. Increasing nitrogen (N) deposition, particularly in N-rich tropical forests, is likely to change the composition and behavior of microbial communities and feed back on ecosystem structure and function. This study presents a novel assessment of mechanistic links between microbial responses to N deposition and shifts in soil organic matter (SOM) quality and quantity. We used phospholipid fatty acid (PLFA) analysis and microbial enzyme assays in soils to assess microbial community responses to long-term N additions in two distinct tropical rain forests. We used soil density fractionation and 13C nuclear magnetic resonance (NMR) spectroscopy to measure related changes in SOM pool sizes and chemical quality. Microbial biomass increased in response to N fertilization in both tropical forests and corresponded to declines in pools of low-density SOM. The chemical quality of this soil C pool reflected ecosystem-specific changes in microbial community composition. In the lower-elevation forest, there was an increase in gram-negative bacteria PLFA biomass, and there were significant losses of labile C chemical groups (O-alkyls). In contrast, the upper-elevation tropical forest had an increase in fungal PLFAs with N additions and declines in C groups associated with increased soil C storage (alkyls). The dynamics of microbial enzymatic activities with N addition provided a functional link between changes in microbial community structure and SOM chemistry. Ecosystem-specific changes in microbial community composition are likely to have far-reaching effects on soil carbon storage and cycling. This study indicates that microbial communities in N-rich tropical forests can be sensitive to added N, but we can expect significant variability in how ecosystem structure and function respond to N deposition among tropical forest types.     

Nutrient regulation of organic matter decomposition in a tropical rain forest

Terrestrial biosphere-atmosphere CO2 exchange is dominated by tropical forests, so understanding how nutrient availability affects carbon (C) decomposition in these ecosystems is central to predicting the global C cycle's response to environmental change. In tropical rain forests, phosphorus (P) limitation of primary production and decomposition is believed to be widespread, but direct evidence is rare. We assessed the effects of nitrogen (N) and P fertilization on litter-layer organic matter decomposition in two neighboring tropical rain forests in southwest Costa Rica that are similar in most ways, but that differ in soil P availability. The sites contain 100-200 tree species per hectare and between species foliar nutrient content is variable. To control for this heterogeneity, we decomposed leaves collected from a widespread neotropical species, Brosimum utile. Mass loss during decomposition was rapid in both forests, with B. utile leaves losing >80% of their initial mass in <300 days. High organic matter solubility throughout decomposition combined with high rainfall support a model of litter-layer decomposition in these rain forests in which rapid mass loss in the litter layer is dominated by leaching of dissolved organic matter (DOM) rather than direct CO2 mineralization. While P fertilization did not significantly affect mass loss in the litter layer, it did stimulate P immobilization in decomposing material, leading to increased P content and a lower C:P ratio in soluble DOM. In turn, increased P content of leached DOM stimulated significant increases in microbial mineralization of DOM in P-fertilized soil. These results show that, while nutrients may not affect mass loss during decomposition in nutrient-poor, wet ecosystems, they may ultimately regulate CO2 losses (and hence C storage) by limiting microbial mineralization of DOM leached from the litter layer to soil.     

Nitrogen dynamics in an Australian semiarid grassland soil

We conducted a four-week laboratory incubation of soil from a Themeda triandra Forsskal grassland to clarify mechanisms of nitrogen (N) cycling processes in relation to carbon (C) and N availability in a hot, semiarid environment. Variation in soil C and N availability was achieved by collecting soil from either under tussocks or the bare soil between tussocks, and by amending soil with Themeda litter. We measured N cycling by monitoring: dissolved organic nitrogen (DON), ammonium (NH4+), and nitrate (NO3-) contents, gross rates of N mineralization and microbial re-mineralization, NH4+ and NO3- immobilization, and autotrophic and heterotrophic nitrification. We monitored C availability by measuring cumulative soil respiration and dissolved organic C (DOC). Litter-amended soil had cumulative respiration that was eightfold greater than non-amended soil (2000 compared with 250 microg C/g soil) and almost twice the DOC content (54 compared with 28 microg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 microg N/g soil) and lower gross N rates (1-4 compared with 13-26 microg N x [g soil](-1) x d(-1)) and NO3- accumulation (0.5 compared with 22 microg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 microg C/g soil), and greater DOC content (33 compared with 24 microg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3-20 compared with 17-31 microg N x [g soil](-1) d(-1)) and NO3- accumulation (18 compared with 25 microg N/g soil) relative to soil from between tussocks. We conclude that N cycling in this grassland is mediated by both C and N limitations that arise from the patchiness of tussocks and seasonal variability in Themeda litterfall. Heterotrophic nitrification rate explained >50% of total nitrification, but this percentage was not affected by proximity to tussocks or litter amendment. A conceptual model that considers DON as central to N cycling processes provided a useful initial framework to explain results of our study. However, to fully explain N cycling in this semiarid grassland soil, the production of NO3- from organic N sources must be included in this model.     

Grassland establishment under varying resource availability: a test of positive and negative feedback

The traditional logic of carbon (C) and nitrogen (N) interactions in ecosystems predicts further increases or decreases in productivity (positive feedback) in response to high and low fertility in the soil, respectively; but the potential for development of feedback in ecosystems recovering from disturbance is less well understood. Furthermore, this logic has been challenged in grassland ecosystems where frequent fires or grazing may reduce the contribution of aboveground litter inputs to soil organic matter pools and nutrient supply for plant growth, relative to forest ecosystems. Further, if increases in plant productivity increase soil C content more than soil N content, negative feedback may result from increased microbial demand for N making less available for plant growth. We used a field experiment to test for feedback in an establishing grassland by comparing aboveground net primary productivity (ANPP) and belowground pools and fluxes of C and N in soil with enriched, ambient, and reduced N availability. For eight years annual N enrichment increased ANPP, root N, and root tissue quality, but root C:N ratios remained well above the threshold for net mineralization of N. There was no evidence that N enrichment increased root biomass, soil C or N accrual rates, or storage of C in total, microbial, or mineralizable pools within this time frame. However, the net nitrogen mineralization potential (NMP) rate was greater following eight years of N enrichment, and we attributed this to N saturation of the microbial biomass. Grassland developing under experimentally imposed N limitation through C addition to the soil exhibited ANPP, root biomass and quality, and net NMP rate similar to the ambient soil. Similarity in productivity and roots in the reduced and ambient N treatments was attributed to the potentially high nitrogen-use efficiency (NUE) of the dominant C4 grasses, and increasing cover of legumes over time in the C-amended soil. Thus, in a developing ecosystem, positive feedback between soil N supply and plant productivity may promote enhanced long-term N availability and override progressive N limitation as C accrues in plant and soil pools. However, experimentally imposed reduction in N availability did not feed back to reduce ANPP, possibly due to shifts in NUE and functional group composition.     

Influences of a calcium gradient on soil inorganic nitrogen in the Adirondack Mountains, New York

Studies of the long-term impacts of acidic deposition in Europe and North America have prompted growing interest in understanding the dynamics linking the nitrogen (N) and calcium (Ca) cycles in forested watersheds. While it has been shown that increasing concentrations of nitrate (NO3-) through atmospheric deposition or through nitrification can increase Ca loss, the reciprocal effects of Ca on N transformation processes have received less attention. We studied the influence of soil Ca availability on extractable inorganic N (NO3- + NH4+) across a Ca gradient in the Adirondack Mountains, New York, USA. Our results did not show the direct Ca-N interaction that we had expected, but instead showed that exchangeable Ca coupled with soil moisture, soil organic matter, and ambient temperature accounted for 61% of the variability in extractable inorganic N across 11 sites over two growing seasons. Soil Ca concentrations were, however, positively related to sugar maple (Acer saccharum) and American basswood (Tilia americana) basal areas and negatively related to American beech (Fagus grandifolia) basal area. Based on litter chemistry differences among these tree species and reported potential N mineralization values, we suggest that the influence of Ca on soil inorganic N is through a multistep pathway: reciprocal interactions between soil Ca concentrations and species composition, which in turn affect the quality of litter available for N mineralization. If chronic soil Ca depletion continues, as reported in some forested ecosystems, potential shifts in biotic communities could result in considerable alterations of N cycling processes.     

Integrating soil carbon cycling with that of nitrogen and phosphorus in the watershed model SWAT: Theory and model testing

In this paper we describe and test a sub-model that integrates the cycling of carbon (C), nitrogen (N) and phosphorus (P) in the Soil Water Assessment Tool (SWAT) watershed model. The core of the sub-model is a multi-layer, one-pool soil organic carbon (S_C) algorithm, in which the decomposition rate of S_C and input rate to S_C (through decomposition and humification of residues) depend on the current size of S_C. The organic N and P fluxes are coupled to that of C and depend on the available mineral N and P, and the C:N and N:P ratios of the decomposing pools. Tillage explicitly affects the soil organic matter turnover rate through tool-specific coefficients. Unlike most models, the turnover of soil organic matter does not follow first order kinetics. Each soil layer has a specific maximum capacity to accumulate C or C saturation (S_x) that depends on texture and controls the turnover rate. It is shown in an analytical solution that S_x is a parameter with major influence in the model C dynamics. Testing with a 65-yr data set from the dryland wheat growing region in Oregon shows that the model adequately simulates the S_C dynamics in the topsoil (top 0.3 m) for three different treatments. Three key model parameters, the optimal decomposition and humification rates and a factor controlling the effect of soil moisture and temperature on the decomposition rate, showed low uncertainty as determined by generalized likelihood uncertainty estimation. Nonetheless, the parameter set that provided accurate simulations in the topsoil tended to overestimate S_C in the subsoil, suggesting that a mechanism that expresses at depth might not be represented in the current sub-model structure. The explicit integration of C, N, and P fluxes allows for a more cohesive simulation of nutrient cycling in the SWAT model. The sub-model has to be tested in forestland and rangeland in addition to agricultural land, and in diverse soils with extreme properties such high or low pH, an organic horizon, or volcanic soils.     

Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.     

Stoichiometric controls of nitrogen and phosphorus cycling in decomposing beech leaf litter

Resource stoichiometry (C:N:P) is an important determinant of litter decomposition. However, the effect of elemental stoichiometry on the gross rates of microbial N and P cycling processes during litter decomposition is unknown. In a mesocosm experiment, beech (Fagus sylvatica L.) litter with natural differences in elemental stoichiometry (C:N:P) was incubated under constant environmental conditions. After three and six months, we measured various aspects of nitrogen and phosphorus cycling. We found that gross protein depolymerization, N mineralization (ammonification), and nitrification rates were negatively related to litter C:N. Rates of P mineralization were negatively correlated with litter C:P. The negative correlations with litter C:N were stronger for inorganic N cycling processes than for gross protein depolymerization, indicating that the effect of resource stoichiometry on intracellular processes was stronger than on processes catalyzed by extracellular enzymes. Consistent with this, extracellular protein depolymerization was mainly limited by substrate availability and less so by the amount of protease. Strong positive correlations between the interconnected N and P pools and the respective production and consumption processes pointed to feed-forward control of microbial litter N and P cycling. A negative relationship between litter C:N and phosphatase activity (and between litter C:P and protease activity) demonstrated that microbes tended to allocate carbon and nutrients in ample supply into the production of extracellular enzymes to mine for the nutrient that is more limiting. Overall, the study demonstrated a strong effect of litter stoichiometry (C:N:P) on gross processes of microbial N and P cycling in decomposing litter; mineralization of N and P were tightly coupled to assist in maintaining cellular homeostasis of litter microbial communities.     

大气CO2升高对土壤碳循环影响的研究进展

土壤有机碳是陆地碳库的重要组成部分,其积累和分解的变化直接影响全球的碳平衡。据估计,全球土壤（表层1m）有机碳积累总量相当于大气中碳总量的2～3倍。土壤是温室气体的源或汇,土壤碳库的变化将影响大气C02的浓度,因此,土壤碳库对人类活动的响应也是全球碳循环和全球变化研究的热点。在全球变化的大背景下,大气CO2升高导致植被生态系统碳平衡的改变进而对土壤碳循环产生影响。总结了陆地生态系统碳循环对大气C02浓度升高响应的主要生物学机制及过程,简述了大气C02浓度升高对影响土壤碳输入和输出的各因素的研究进展,并指出未来研究的主要方向。在大气C02浓度升高条件下,陆地生态系统碳循环的变化主要反映在以下几个方面：1）不同类型植物群落的净初级生产力（NPP）显著增加,但湿地植物的净初级生产力也有可能降低;2）光合产物向根系分配的数量增加,地上／地下生物量降低,根系形态发生变化,根系周转速率和根系分泌等过程的碳流量提高;3）植物含氮量降低,C／N提高,次生代谢产物增加,微生物生长受到抑制,植物残体分解速率降低;4）土壤呼吸速率显著增加,提高幅度受植物类型与土壤状况的影响;5）进入土壤的植物残体及分泌物的数量和性质影响土壤酶的活性,脱氢酶和转化酶活性增加,酚氧化酶和纤维素酶受植物类型与环境条件的影响;6）土壤中真菌的数量的增加幅度要高于细菌;7）CH4释放量增加,在植物的生长期表现更为明显。由于陆地生态系统碳循环的复杂性,研究结果仍有很大的不确定性。大气C02浓度升高与全球变化的其它表现间的交互作用将是今后研究的重点,同时由于土壤碳循环是一个由微生物介导的生物地球化学循环过程,因此,加强陆地生态系统碳循环的微生物机制研究也将为全面理解碳循环的过程提供更加准确的研究理论基础。     

冻融作用下寒温带针叶林土壤碳氮矿化过程研究

以大兴安岭落叶松林土壤为研究对象,设置8℃恒温和-5-8℃冻融循环（1个冻融循环为在-5℃培养24 h,后在8℃培养24 h）2个处理,进行30 d的室内培养实验,探讨了寒温带针叶林土壤在冻融交替时期的碳氮矿化过程及其相互关系。结果表明,培养温度和培养时间对土壤碳矿化速率和碳矿化累积量均有显著影响。第1次和第5次冻融循环后,冻融处理土壤的碳矿化速率显著高于恒温培养下土壤的碳矿化速率;第7次和第15次冻融循环后,冻融土壤碳矿化累积量显著低于恒温土壤的碳矿化累积量。土壤氮矿化速率没有受到培养温度、培养时间以及二者交互作用的影响,但培养时间和培养温度对土壤净氮矿化累积量有显著的影响。第5、7、15次冻融循环后,冻融处理的土壤无机氮净矿化累积量低于恒温培养的土壤无机氮净矿化累积量。经过30 d的培养,恒温处理下的土壤碳、氮矿化累积量（碳累积量：92.82μg·g-1,氮累积量73.76 mg·kg-1）是冻融处理下（碳累积量：65.51μg·g-1,氮累积量33.45 mg·kg-1）的1.42倍和2.21倍。土壤碳矿化累积量与土壤净氮矿化累积量均为正相关关系,但在相同的碳释放量下冻融循环处理土壤累积的无机氮较少。以上结果表明,冻融循环减少了大兴安岭寒温带落叶松林土壤碳排放和无机氮的累积,有利于土壤碳的固持和减少养分的流失。     

Elevated CO2 stimulates net accumulations of carbon and nitrogen in land ecosystems: a meta-analysis

The capability of terrestrial ecosystems to sequester carbon (C) plays a critical role in regulating future climatic change yet depends on nitrogen (N) availability. To predict long-term ecosystem C storage, it is essential to examine whether soil N becomes progressively limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. A critical parameter to indicate the long-term progressive N limitation (PNL) is net change in ecosystem N content in association with C accumulation in plant and soil pools under elevated CO2. We compiled data from 104 published papers that study C and N dynamics at ambient and elevated CO2. The compiled database contains C contents, N contents, and C:N ratio in various plant and soil pools, and root:shoot ratio. Averaged C and N pool sizes in plant and soil all significantly increase at elevated CO2 in comparison to those at ambient CO2, ranging from a 5% increase in shoot N content to a 32% increase in root C content. The C and N contents in litter pools are consistently higher in elevated than ambient CO2 among all the surveyed studies whereas C and N contents in the other pools increase in some studies and decrease in other studies. The high variability in CO2-induced changes in C and N pool sizes results from diverse responses of various C and N processes to elevated CO2. Averaged C:N ratios are higher by 3% in litter and soil pools and 11% in root and shoot pools at elevated relative to ambient CO2. Elevated CO2 slightly increases root:shoot ratio. The net N accumulation in plant and soil pools at least helps prevent complete down-regulation of, and likely supports, long-term CO2 stimulation of C sequestration. The concomitant C and N accumulations in response to rising atmospheric CO2 may reflect intrinsic nature of ecosystem development as revealed before by studies of succession over hundreds to millions of years.     

Incorporation of plant carbon into the soil animal food web of an arable system

We used stable isotopes to examine the incorporation of plant carbon into the belowground food web of an agricultural system. Plots were established and planted with maize (Zea mays) in a rye field (Secale cereale) near G?ttingen (northern Germany) in May 1999. In October 1999, April 2000, and October 2000, meso- and macrofauna and maize and rye litter were collected in each plot and analyzed for 13C and 15N content. 15N signatures suggested that the soil animal species analyzed span three trophic levels with the trophic position of species varying little in time. The species investigated formed a continuum from primary to secondary decomposers to predators. On average, predator species differed from primary and secondary decomposers by 3.9 sigma15N suggesting that they fed on a mixed diet of both decomposer groups. The combined analysis of 13C and 15N signatures allowed us to identify links between prey and consumer species. In October 1999, shortly after maize residues had been incorporated into the plots, maize-born carbon was present in each of the animal species investigated, including top predators. The incorporation of maize carbon into the belowground food web increased during the following 12 months but the concentration of maize-born carbon never exceeded 50% in any of the species. Furthermore, the ranks of the incorporation of maize-born carbon of the species changed little. The results suggest that the belowground food web relies heavily on carbon originating from plant residues from before the recent two growing seasons. In most species the amount of maize-born carbon increased continuously; however, in some species it decreased during winter, suggesting that these species switched to a diet based more on C3 plants during winter, or predominantly metabolized carbon incorporated during the last growing season. The study documents that the combined analysis of 13C and 15N signatures in soil invertebrate species, after replacement of C3 by C4 plants, is a powerful tool to better understand the structure of the belowground food web and the flux of carbon through it.     

Tree species and mycorrhizal associations influence the magnitude of rhizosphere effects

Previous research on the effects of tree species on soil processes has focused primarily on the role of leaf litter inputs. We quantified the extent to which arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree species influence soil microbial activity and nutrient availability through rhizosphere effects. Rhizosphere soil, bulk soil, and fine roots were collected from 12 monospecifc plots (six AM and six ECM tree species) planted on a common soil at the Turkey Hill Plantations in Dryden, New York. Rhizosphere effects were estimated by the percentage difference between rhizosphere and bulk soil samples for several assays. Rhizosphere effects on soil microbes and their activities were significant for ECM species but in only a few cases for AM species. In AM tree species, microbial biomass, net N mineralization, and phosphatase enzyme activity in the rhizosphere were 10-12% greater than in bulk soil. In ECM tree species, rhizosphere effects for microbial biomass, C mineralization rates, net N mineralization, and phosphatase activity were 25-30% greater than bulk soil, and significantly greater than AM rhizosphere effects. The magnitude of rhizosphere effects was negatively correlated with the degree of mycorrhizal colonization in AM tree species (r = -0.83) and with fine root biomass (r = -0.88) in ECM tree species, suggesting that different factors influence rhizosphere effects in tree species forming different mycorrhizal associations. Rhizosphere effects on net N mineralization and phosphatase activity were also much greater in soils with pH < 4.3 for both AM and ECM tree species, suggesting that soil pH and its relation to nutrient availability may also influence the magnitude of rhizosphere effects. Our results support the idea that tree roots stimulate nutrient availability in the rhizosphere, and that systematic differences between AM and ECM may result in distinctive rhizosphere effects for C, N, and P cycling between AM and ECM tree species.     

Long-term aboveground and belowground consequences of red wood ant exclusion in boreal forest

Despite their ubiquity, the role of ants in driving ecosystem processes both aboveground and belowground has been seldom explored, except within the nest. During 1995 we established 16 ant exclusion plots of approximately 1.1 x 1.1 m, together with paired control plots, in the understory layer of a boreal forest ecosystem in northern Sweden that supports high densities of the mound-forming ant Formica aquilonia, a red wood ant species of the Formica rufa group. Aboveground and belowground measurements were then made on destructively sampled subplots in 2001 and 2008, i.e., 6 and 13 years after set-up. While ant exclusion had no effect on total understory plant biomass, it did greatly increase the relative contribution of herbaceous species, most likely through preventing ants from removing their seeds. This in turn led to higher quality resources entering the belowground subsystem, which in turn stimulated soil microbial biomass and activity and the rates of loss of mass and carbon (C) and nitrogen (N) from litter in litterbags placed in the plots. This was accompanied by losses of approximately 15% of N and C stored in the humus on a per area basis. Ant exclusion also had some effects on foliar stable isotope ratios for both C and N, most probably as a consequence of greater soil fertility. Further, exclusion of ants had multitrophic effects on a microbe-nematode soil food web with three consumer trophic levels and after six years promoted the bacterial-based relative to the fungal-based energy channel in this food web. Our results point to a major role of red wood ants in determining forest floor vegetation and thereby exerting wide-ranging effects on belowground properties and processes. Given that the boreal forest occupies 11% of the Earth's terrestrial surface and stores more C than any other forest biome, our results suggest that this role of ants could potentially be of widespread significance for biogeochemical nutrient cycling, soil nutrient capital, and sequestration of belowground carbon.