Mechanisms of Carbon Sequestration in Soil Aggregates

Soil and crop management practices have a profound impact on carbon (C) sequestration, but the mechanisms of interaction between soil structure and soil organic C (SOC) dynamics are not well understood. Understanding how an aggregate stores and protects SOC is essential to developing proper management practices to enhance SOC sequestration. The objectives of this article are to: (1) describe the importance of plants and soil functions on SOC sequestration, (2) review the mechanisms of SOC sequestration within aggregates under different vegetation and soil management practices, (3) explain methods of assessing distribution of SOC within aggregates, and (4) identify knowledge gaps with regards to SOC and soil structural dynamics. The quality and quantity of plant residues define the amount of organic matter and thus the SOC pool in aggregates. The nature of plant debris (C:N ratio, lignin content, and phenolic compound content) affects the rate of SOC sequestration. Mechanisms of interaction of aggregate dynamics with SOC are complex and embrace a range of spatial and temporal processes within macro- ( > 250 μ m e.c.d.) and microaggregates ( < 250 μ m e.c.d.). A relevant mechanism for SOC sequestration within aggregates is the confinement of plant debris in the core of the microaggregates. The C-rich young plant residues form and stabilize macroaggregates, whereas the old organic C is occluded in the microaggregates. Interactions of clay minerals with C rich humic compounds in correlation with clay mineralogy determine the protection and storage of SOC. Principal techniques used to assess the C distribution in aggregates include the determination of total organic C in different aggregate size fractions, isotopic methods to assess the turnover and storage of organic C in aggregates, and computed tomography and X-ray scattering to determine the internal porosity and inter-aggregate attributes. The literature is replete with studies on soil and crop management influences on total organic C and soil aggregation. However, research reports on the interactions of SOC within aggregates for C sequestration are scanty. Questions still remain on how SOC interacts physically and chemically with aggregates, and research is needed to understand the mechanisms responsible for the dynamics of aggregate formation and stability in relation to C sequestration.     

Linking soil biodiversity and vegetation: implications for a changing planet

Soil biota are intimately tied to plant communities through herbivory and symbiosis and indirectly by the decomposition of dead organic plant material. Through both roots and aboveground organic material (e.g., leaves and wood), plants provide substantial inputs of organic matter to soil systems. Plants are the basis for most biotic soil food webs that comprise an enormous diversity of species whose multiple interactions function to help regulate nutrient cycling, which in turn influences plant growth. Many factors govern the biogeography of soil biota, including the physical and chemical properties of soil, climate, the composition and type of vegetation, and interactions with other soil biota. Despite awareness of factors influencing soil communities, no single factor allows predictions of soil animal diversity or distribution. However, research is showing that plants can have unique soil biotic communities. Degradation of soil, which removes predators and biotic regulation that occurs in less managed ecosystems, can result in increased pathogens and pests that affect humans, other animals and plants. Global changes such as land use, desertification, and soil pollution all have been shown to alter soil animal diversity and abundance. Because of our dependence on soils and plant production, studies linking soil biotic communities to primary productivity are needed to assure long-term soil sustainability.     

Grass invasion causes rapid increases in ecosystem carbon and nitrogen storage in a semiarid shrubland

Accurately predicting terrestrial carbon (C) and nitrogen (N) storage requires understanding how plant invasions alter cycling and storage. A common, highly successful type of plant invasion occurs when the invasive species is of a distinctly different functional type than the native dominant plant, such as shrub encroachment throughout the western United States and annual grass invasions in Mediterranean shrublands, as studied here. Such invasions can dramatically transform landscapes and have large potential to alter C and N cycling by influencing storage in multiple pools. We used a manipulation of non‐native annual grass litter within a shrub‐dominated habitat in southern California (coastal sage scrub, CSS) to study how grass invasion alters ecosystem C and N storage. We added, removed, or left unchanged grass litter in areas of high and low invasion, then followed soil and vegetation changes. Grass litter greatly increased C and N storage in soil, aboveground native and non‐native biomass. Aboveground litter storage increased due to the greater inputs and slower decomposition of grass litter relative to shrub litter; shading by grass litter further reduced decomposition of both non‐native and native litter, which may be due to reduced photodegradation. Soil C and N pools in areas of high litter increased ～20% relative to low litter areas in the two years following manipulation and were generally sinks for C and N, while areas with low litter were sources. We synthesize our results into a C cycle of invaded and uninvaded areas of CSS and link changes in storage to increases in the soil fungi : bacteria ratio, increased plant inputs, and decreased litter loss. Overall, we show that grasses, especially through their litter, control important abiotic and biotic mechanisms governing C and N storage, with widespread implications for C sequestration and N storage in semiarid systems undergoing grass or shrub invasions.     

Multi‐nutrient vs. nitrogen‐only effects on carbon sequestration in grassland soils

Human activities have greatly increased the availability of biologically active forms of nutrients [e.g., nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg)] in many soil ecosystems worldwide. Multi‐nutrient fertilization strongly increases plant productivity but may also alter the storage of carbon (C) in soil, which represents the largest terrestrial pool of organic C. Despite this issue is important from a global change perspective, key questions remain on how the single addition of N or the combination of N with other nutrients might affect C sequestration in human‐managed soils. Here, we use a 19‐year old nutrient addition experiment on a permanent grassland to test for nutrient‐induced effects on soil C sequestration. We show that combined NPKMg additions to permanent grassland have ‘constrained’ soil C sequestration to levels similar to unfertilized plots whereas the single addition of N significantly enhanced soil C stocks (N‐only fertilized soils store, on average, 11 t C ha^?1 more than unfertilized soils). These results were consistent across grazing and liming treatments suggesting that whilst multi‐nutrient additions increase plant productivity, soil C sequestration is increased by N‐only additions. The positive N‐only effect on soil C content was not related to changes in plant species diversity or to the functional composition of the plant community. N‐only fertilized grasslands show, however, increases in total root mass and the accumulation of organic matter detritus in topsoils. Finally, soils receiving any N addition (N only or N in combination with other nutrients) were associated with high N losses. Overall, our results demonstrate that nutrient fertilization remains an important global change driver of ecosystem functioning, which can strongly affect the long‐term sustainability of grassland soil ecosystems (e.g., soils ability to deliver multiple ecosystem services).     

Soil physical properties associated with the invasive spotted knapweed and native grasses are similar

Centaurea maculosa Lam. (spotted knapweed), a Eurasian perennial forb, has invaded disturbed and undisturbed semiarid grasslands in the western United States. In the past, success in controlling C. maculosa and restoring invaded areas has been limited. Most research has addressed chemical aspects of invasive species interactions with soils, while potential impacts of altered soil physical properties on C. maculosa's success has not been studied. We hypothesized that the persistence of C. maculosa in semiarid rangelands might reflect an ability to alter site conditions. The objective of this study was to compare selected soil physical properties under C. maculosa-dominated and native perennial grass-dominated areas on semiarid grassland. We used six field sites in western Montana containing adjacent plots dominated by C. maculosa and by native perennial grasses. Soil physical properties including particle size fractions, bulk density, and hydraulic and thermal properties, as well as total organic carbon content, of near-surface soils were measured for each vegetation type. Soil physical properties seldom differed between C. maculosa- and native grass-dominated areas. When soil physical properties differed, the differences were inconsistent within and among sites. Presence of C. maculosa did not alter surface soil characteristics at our six sites, thus its persistence on these semi-arid grasslands cannot be explained by an ability to alter near-surface soil characteristics.     

Plant litter: Its dynamics and effects on plant community structure

We discuss the dynamics of plant litter, the effects of litter on the chemical and physical environment, the direct and indirect effects of plant litter on plant populations and communities, and different adaptative traits that may be related to litter accumulation. The production of litter depends primarily on the site productivity, but other properties of the environment, as well as chance, may introduce important variation. The existence of time lags between the production of plant organs and their transformation into litter appears as a relevant character of litter dynamics seldom included in models. Herbivory, and other processes that destroy biomass or reduce productivity, may reduce the amount of litter produced. The destruction of litter encompasses a complex of interactions. The main processes, including physical and chemical degradation, consumption by invertebrates and decomposition, are differentially affected by the environment and by the physical and chemical characteristics of the litter itself. The relative importance of those processes varies among systems. Litter alters the physical and chemical environment directly and indirectly. The decomposition of litter may release both nutrients and phytotoxic substances into the soil. The physical changes produced by litter also alter the activity of decomposers, resulting in an indirect effect on the chemical environment. The accumulated litter intercepts light, shading seeds and seedlings, and reduces the thermal amplitude in the soil. By reducing maximum soil temperatures, and creating a barrier to water vapor diffusion, litter reduces evaporation from the soil. However, litter may also diminish water availability when it retains a large proportion of rainfall. Litter creates a physical barrier for seedling and sprout emergence and to seeds reaching the soil.     

Plant-soil Interactions in Temperate Grasslands

We present a conceptual model in which plant-soil interactions in grasslands are characterized by the extent to which water is limiting. Plant-soil interactions in dry grasslands, those dominated by water limitation (belowground-dominance), are fundamentally different from plant-soil interactions in subhumid grasslands, where resource limitations vary in time and space among water, nitrogen, and light (indeterminate dominance). In the belowground-dominance grasslands, the strong limitation of soil water leads to complete (though uneven) occupation of the soil by roots, but insufficient resources to support continuous aboveground plant cover. Discontinuous aboveground plant cover leads to strong biological and physical forces that result in the accumulation of soil materials beneath individual plants in resource islands. The degree of accumulation in these resource islands is strongly influenced by plant functional type (lifespan, growth form, root:shoot ratio, photosynthetic pathway), with the largest resource islands accumulating under perennial bunchgrasses. Resource islands develop over decadal time scales, but may be reduced to the level of bare ground following death of an individual plant in as little as 3 years. These resource islands may have a great deal of significance as an index of recovery from disturbance, an indicator of ecosystem stability or harbinger of desertification, or may be significant because of possible feedbacks to plant establishment. In the grasslands in which the dominant resource limiting plant community dynamics is indeterminate, plant cover is relatively continuous, and thus the major force in plant-soil interactions is related to the feedbacks among plant biomass production, litter quality and nutrient availability. With increasing precipitation, the over-riding importance of water as a limiting factor diminishes, and four other factors become important in determining plant community and ecosystem dynamics: soil nitrogen, herbivory, fire, and light. Thus, several different strategies for competing for resources are present in this portion of the gradient. These strategies are represented by different plant traits, for example root:shoot allocation, height and photosynthetic pathway type (C3 vs. C4) and nitrogen fixation, each of which has a different influence on litter quality and thus nutrient availability. Recent work has indicated that there are strong feedbacks between plant community structure, diversity, and soil attributes including nitrogen availability and carbon storage. Across both types of grasslands, there is strong evidence that human forces that alter plant community structure, such as invasions by nonnative annual plants or changes in grazing or fire regime, alters the pattern, quantity, and quality of soil organic matter in grassland ecosystems. The reverse influence of soils on plant communities is also strong; in turn, alterations of soil nutrient supply in grasslands can have major influences on plant species composition, plant diversity, and primary productivity.     

Dynamics of soil organic carbon storage following restoration of grassland on Yunwu Mountain

Grassland recovery and reconstruction are critical to ecological restoration in the Chinese Loess Plateau (CLP). Investigating changes in soil organic carbon density (SOCD), soil organic carbon (SOC) storage, and the rate of SOC sequestration is very important to assess the effect of ecological recovery and estimate the capacity of soil carbon sequestration. Here, we present the data of SOCD, SOC storage, and SOC sequestration rate from grasslands conversion from farmlands in the CLP. Our results indicate that: (1) The average SOCD (0–100 cm) in sites continued cultivation (CC), cultivation abandonment at 1999 (AC-99) and cultivation abandonment at 1989 (AC-89) is 6.00, 21.64 and 22.23 kg m^?2, respectively. SOCD in sites AC-99 and AC-89 is significantly higher than that in site CC and the average SOCD of China (10.53 kg m^?2), which indicates that vegetation restoration is benefit to increase soil carbon storage as well as preserve soil and water in this area. (2) The SOC storage (0–100 cm) in sites CC, AC-99 and AC-89 is 60.02, 216.35 and 222.32 kg m^?2, respectively. Results of ANOVA indicate that SOC storage of AC-99 is significantly higher than that of CC, while SOC storage of AC-89 is significantly higher than that of AC-99 at the depth of 0–50 cm (P < 0.001). It suggests that the capability of soil carbon sequestration increases after vegetation restoration, which is mainly due to the increase of plant roots. (3) The rate of SOC sequestration varies at different depths, which is high at the depth of 0–50 cm while low at the depth of 50–100 cm. This is probably due to the accumulation of plant root in the surface layer, which is the main controlling factor of SOC in this area. Our results indicate that the SOCD and SOC storage increase with vegetation restoration in our study site significantly.     

Soil carbon saturation: concept,evidence and evaluation

Current estimates of soil C storage potential are based on models or factors that assume linearity between C input levels and C stocks at steady-state, implying that SOC stocks could increase without limit as C input levels increase. However, some soils show little or no increase in steady-state SOC stock with increasing C input levels suggesting that SOC can become saturated with respect to C input. We used long-term field experiment data to assess alternative hypotheses of soil carbon storage by three simple models: a linear model (no saturation), a one-pool whole-soil C saturation model, and a two-pool mixed model with C saturation of a single C pool, but not the whole soil. The one-pool C saturation model best fit the combined data from 14 sites, four individual sites were best-fit with the linear model, and no sites were best fit by the mixed model. These results indicate that existing agricultural field experiments generally have too small a range in C input levels to show saturation behavior, and verify the accepted linear relationship between soil C and C input used to model SOM dynamics. However, all sites combined and the site with the widest range in C input levels were best fit with the C-saturation model. Nevertheless, the same site produced distinct effective stabilization capacity curves rather than an absolute C saturation level. We conclude that the saturation of soil C does occur and therefore the greatest efficiency in soil C sequestration will be in soils further from C saturation.

Effects of Exotic Plant Invasions on Soil Nutrient Cycling Processes

Although it is generally acknowledged that invasions by exotic plant species represent a major threat to biodiversity and ecosystem stability, little attention has been paid to the potential impacts of these invasions on nutrient cycling processes in the soil. The literature on plant–soil interactions strongly suggests that the introduction of a new plant species, such as an invasive exotic, has the potential to change many components of the carbon (C), nitrogen (N), water, and other cycles of an ecosystem. I have reviewed studies that compare pool sizes and flux rates of the major nutrient cycles in invaded and noninvaded systems for invasions of 56 species. The available data suggest that invasive plant species frequently increase biomass and net primary production, increase N availability, alter N fixation rates, and produce litter with higher decomposition rates than co-occurring natives. However, the opposite patterns also occur, and patterns of difference between exotics and native species show no trends in some other components of nutrient cycles (for example, the size of soil pools of C and N). In some cases, a given species has different effects at different sites, suggesting that the composition of the invaded community and/or environmental factors such as soil type may determine the direction and magnitude of ecosystem-level impacts. Exotic plants alter soil nutrient dynamics by differing from native species in biomass and productivity, tissue chemistry, plant morphology, and phenology. Future research is needed to (a) experimentally test the patterns suggested by this data set; (b) examine fluxes and pools for which few data are available, including whole-site budgets; and (c) determine the magnitude of the difference in plant characteristics and in plant dominance within a community that is needed to alter ecosystem processes. Such research should be an integral component of the evaluation of the impacts of invasive species.     

Evaluation of carbon accrual in afforested agricultural soils

Afforestation of agricultural lands can provide economically and environmentally realistic C storage to mitigate for elevated CO₂ until other actions such as reduced fossil fuel use can be taken. Soil carbon sequestration following afforestation of agricultural land ranges from losses to substantial annual gains. The present understanding of the controlling factors is inadequate for understanding ecosystem dynamics, modeling global change and for policy decision‐makers. Our study found that planting agricultural soils to deciduous forests resulted in ecosystem C accumulations of 2.4 Mg C ha⁻¹ yr⁻¹ and soil accumulations of 0.35 Mg C ha⁻¹ yr⁻¹. Planting to conifers showed an average ecosystem sequestration of 2.5 and 0.26 Mg C ha⁻¹ yr⁻¹ in the soils but showed greater field to field variability than when planted to deciduous forest. Path analysis showed that Ca was positively related to soil C accumulations for both conifers and deciduous afforested sites and played a significant role in soil C accumulations in these sites. Soil N increases were closely related to C accumulation and were two times greater than could be explained by system N inputs from atmospheric deposition and natural sources. Our results suggest that the addition of Ca to afforested sites, especially conifers, may be an economical means to enhance soil C sequestration even if it does not result in increasing C in aboveground pools. The mechanism of N accumulation in these aggrading stands needs further investigation.     

A synthesis of the effect of grazing exclusion on carbon dynamics in grasslands in China

Grazing exclusion (GE) is considered to be an effective approach to restore degraded grasslands and to improve their carbon (C) sequestration. However, the C dynamics and related controlling factors in grasslands with GE have not been well characterized. This synthesis examines the dynamics of soil C content and vegetation biomass with the recovery age through synthesizing results of 51 sites in grasslands in China. The results illustrate increases in soil C content and vegetation biomass with GE at most sites. Generally, both soil C content and vegetation biomass arrive at steady state after 15 years of GE. In comparison, the rates of increase in above‐ and belowground biomass declined exponentially with the age of GE, whereas soil C content declined in a milder (linear) way, implying a lagged response of soil C to the inputs from plant biomass. Mean annual precipitation (MAP) and the rate of soil nitrogen (N) change were the main factors affecting the rate of soil C content change. MAP played a major role at the early stage, whereas the rate of soil N change was the major contributor at the middle and late stages. Our results imply that the national grassland restoration projects in China may be more beneficial for C sequestration in humid regions with high MAP. In addition, increased soil N supply to grasslands with GE at the latter recovery stage may enhance ecosystem C sequestration capacity.     

Influence of nutrients,disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.     

土壤有机碳动态:风蚀效应

土壤风蚀是引起土壤退化最广泛的形式和原因之一。土壤风蚀对土壤碳动态的影响机制一方面是土壤风蚀引起土壤退化使土壤生产力下降,输入土壤的碳数量减少;另一方面是富含有机碳的细粒物质直接移出系统。风蚀土壤碳的去向包括:(1)就近沉积,(2)沉积于水渠和河流,输入水体;(3)以粉尘形式运移,在远离风蚀区的地域沉积;(4)氧化释放至大气。风蚀引起土壤碳的迁移和沉积不仅导致土壤有机碳在地域间的再分布,使土壤性状的空间异质性增加,也显著改变了土壤系统中碳矿化的生物学过程。土壤有机碳的保持可以促进团聚体的形成,使土壤物理稳定性增加,减缓风蚀。对易风蚀土地进行退耕还林还草、实行保护性耕作等措施可以有效增加土壤碳的固存。     

Application of a 15N tracer to simulate and track the fate of atmospherically deposited N in the coastal forests of the Waquoit Bay Watershed, Cape Cod, Massachusetts

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling. Received: 11 July 1996 / Accepted: 5 December 1996     

Biotic interactions with mycorrhizal systems as extended nutrient acquisition strategies shaping forest soil communities and functions

Plant nutrient acquisition strategies involving ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) associations, are key plant functional traits leading to distinct carbon (C) and nutrient dynamics in forests. Yet, little is known about how these strategies influence the structure and functioning of soil communities, and if such mycorrhizal effects may be more or less pronounced depending on the type of forest and various abiotic factors. Here we explore the potential interactions occurring between plant-EcM and plant-AM systems with the diverse soil organisms occurring in forest soils, and in the process draw attention to major issues that are worthy for future research directions. Based on these potential interactions, we suggest that EcM systems, especially those involving gymnosperms in colder climates, may select for a soil community with a narrow set of functions. These EcM systems may exhibit low functional redundancy, dominated by symbiotic interactions, where EcM fungi maintain low pH and high C/N conditions in order to tightly control nutrient cycling and maintain the dominance of EcM trees. By contrast, AM systems, particularly those involving deciduous angiosperm trees in mild and warmer climates, may facilitate a functionally more diverse and redundant soil community tending towards the dominance of competitive and antagonistic interactions, but also with a range of symbiotic interactions that together maintain diverse plant communities. We propose that the contrasting belowground interactions in AM and EcM systems act as extended nutrient acquisition traits that contribute greatly to the prevailing nutrient and C dynamics occurring in these systems.     

Spatial and temporal variation of soil moisture in dependence of multiple environmental parameters in semi-arid grasslands

Grazing of grasslands changes soil physical and chemical properties as well as vegetation characteristics, such as vegetation cover, species composition and biomass production. In consequence, nutrient allocation and water storage in the top soil are affected. Land use and management changes alter these processes. Knowledge on the impacts of grazing management on nutrient and water fluxes is necessary because of the global importance of grasslands for carbon sequestration. Soil water in semi-arid areas is a limiting factor for matter fluxes and the intrinsic interaction between soil, vegetation and atmosphere. It is therefore desirable to understand the effects of grazing management and stocking rate on the spatial and temporal distribution of soil moisture. In the present study, we address the question how spatio-temporal soil moisture distribution on grazed and ungrazed grassland sites is affected by soil and vegetation properties. The study took place in the Xilin river catchment in Inner Mongolia (PR China). It is a semi-arid steppe environment, which is characterized by still moderate grazing compared to other regions in central Inner Mongolia. However, stocking rates have locally increased and resulted in a degradation of soils and vegetation also in the upper Xilin River basin. We used a multivariate geostatistical approach to reveal spatial dependencies between soil moisture distribution and soil or vegetation parameters. Overall, 7 soil and vegetation parameters (bulk density, sand, silt and clay content, mean weight diameter, mean carbon content of the soil, vegetation cover) and 57 soil moisture data sets were recorded on 100 gridded points on four sites subject to different grazing intensities. Increasing stocking rates accelerated the influence of soil and vegetation parameters on soil moisture. However, the correlation was rather weak, except for a site with high stocking rate where higher correlations were found. Low nugget ratios indicate spatial dependency between soil or plant parameters and soil moisture on a long-term ungrazed site. However, the effect was not found for a second ungrazed site that had been excluded from grazing for a shorter period. Furthermore the most important soil and vegetation parameters for predicting soil moisture distribution varied between different grazing intensities. Therefore, predicting soil moisture by using secondary variables requires a careful selection of the soil or vegetation parameters.     

氮沉降增加情景下植物-土壤-微生物交互对自然生态系统土壤有机碳的调控研究进展

大气氮沉降增加倾向于促进受氮限制陆地生态系统地上生物量,但是对地下碳过程和土壤碳截存的影响结果迥异,导致陆地生态系统“氮促碳汇”的评估存在很大的不确定性。大气氮沉降输入直接影响微生物活性或间接影响底物质量,改变凋落物和土壤有机质(SOM)的分解速率和分解程度,进而影响土壤有机碳(SOC)的积累与损耗过程。过去相关研究主要集中在土壤碳转化过程和碳储量动态方面,缺乏植物-微生物-SOM交互作用的理解,对土壤碳截存调控的生物化学和微生物学机理尚不清楚。本文以地下碳循环过程为主线,分别综述了氮沉降增加对植物地下碳分配、SOC激发效应、微生物群落碳代谢过程的影响,深入分析SOM化学稳定性与微生物群落动态的关系。该领域研究的薄弱环节体现在:(1)增氮倾向于降低根系的生长和周转,对根际沉积碳分配(数量和格局)的影响及驱动因素不明确;(2)虽然认识到氮素有效性影响土壤激发效应的方向和强度,但是氧化态NO-3和还原态NH+4输入对有机质激发效应的差异性影响及潜在机理知之甚少;(3)微生物碳利用效率(CUE)是微生物群落碳代谢的关键表征,能够很好地解释土壤碳的积累与损耗过程;由于缺乏适宜的测定方法,难以准确量化土壤微生物的CUE及微生物生物量的周转时间;(4)增氮会抑制土壤真菌群落及其胞外酶活性,对细菌群落组成的影响尚未定论,有关SOM化学质量与土壤微生物群落活性、组成之间的耦合关系尚不清楚。未来研究应基于长期的氮添加控制实验平台,结合碳氧稳定性同位素示踪、有机质化学、分子生物学和宏基因组学等方法,深入分析植物同化碳的地下分配规律、微生物碳代谢和周转、有机质化学结构与功能微生物群落的耦合关系等关键环节。上述研究将有助于揭示植物-土壤-微生物交互作用对SOC动态的调控机制,完善陆地生态系统碳-氮耦合循环模型,有效降低区域陆地碳汇评估的不确定性,并可为陆地生态系统应对全球变化提供科学依据。     

Nitrogen limitation of decomposition and decay: How can it occur?

The availability of nitrogen (N) is a critical control on the cycling and storage of soil carbon (C). Yet, there are conflicting conceptual models to explain how N availability influences the decomposition of organic matter by soil microbial communities. Several lines of evidence suggest that N availability limits decomposition; the earliest stages of leaf litter decay are associated with a net import of N from the soil environment, and both observations and models show that high N organic matter decomposes more rapidly. In direct contrast to these findings, experimental additions of inorganic N to soils broadly show a suppression of microbial activity, which is inconsistent with N limitation of decomposition. Resolving this apparent contradiction is critical to representing nutrient dynamics in predictive ecosystem models under a multitude of global change factors that alter soil N availability. Here, we propose a new conceptual framework, the Carbon, Acidity, and Mineral Protection hypothesis, to understand the effects of N availability on soil C cycling and storage and explore the predictions of this framework with a mathematical model. Our model simulations demonstrate that N addition can have opposing effects on separate soil C pools (particulate and mineral‐protected carbon) because they are differentially affected by microbial biomass growth. Moreover, changes in N availability are frequently linked to shifts in soil pH or osmotic stress, which can independently affect microbial biomass dynamics and mask N stimulation of microbial activity. Thus, the net effect of N addition on soil C is dependent upon interactions among microbial physiology, soil mineralogy, and soil acidity. We believe that our synthesis provides a broadly applicable conceptual framework to understand and predict the effect of changes in soil N availability on ecosystem C cycling under global change.     

Comparative impacts of aboveground and belowground enemies on an invasive thistle

Most research examining how herbivores and pathogens affect performance of invasive plants focuses on aboveground interactions. Although important, the role of belowground communities remains poorly understood, and the relative impact of aboveground and belowground interactions is still debated. As well, most studies of belowground interactions have been carried out in controlled environments, so little is known about the role of these interactions under natural conditions or how these relationships may change across a plant's range. Using the invasive plant Cirsium arvense, we performed a reciprocal transplant experiment to test the relative impacts of above‐ and belowground interactions at three sites across a 509‐km latitudinal gradient in its invaded range in Ontario, Canada. At each site, C. arvense seedlings were protected with above‐ and/or belowground exclosures in a factorial design. Plant performance (biomass, height, stem thickness, number of leaves, length of longest leaf, maximum rhizome length) was greatest when both above‐ and belowground exclosures were applied and lowest when no exclosures were applied. When only one type of exclosure was applied, biomass generally improved more with belowground exclosures than with aboveground exclosures. Despite site‐to‐site differences in foliar damage, root damage, and mesofaunal populations, belowground interactions generally had a greater negative impact on performance than aboveground herbivory alone. These results stress the importance of including both aboveground enemy interactions and plant–soil interactions in studies of plant community dynamics and invader performance.