Implications of increased carbon dioxide levels for carbon input and turnover in soils

The complexity of the plant-soil system in its interaction with the changing climate is discussed. It is shown that processes at the level of organic matter inputs into the soil and the fluxes and pools involved in the global cycle are not known in sufficient detail to allow an estimation of the future quantitative shifts. Even the direction in which the level of stored carbon in the soil organic matter pool will develop is not clear. The importance of the nitrogen cycle, which is intimately coupled to the carbon cycle through the turnover of soil organic matter is underlined. In its turn, the mineralisation of soil organic matter takes place at a rate which is highly dependent on the nature of inputs and the availability of mineral nutrients.Aspects of shifts in temperature, changes in cultivation practices (reduced tillage) and unintended spreading of inputs in chemical N-fertilizers are of great importance at a regional and global scale.The complexity of the interactions in the process of mineralisation do require further studies to clarify the point whether a substantial and durable additional storage of carbon in soil organic matter is likely, or that shifts in temperature will cause an overriding acceleration of the mineralisation, and trigger a corresponding net release of carbon.     

Depth-dependent effects of ericoid mycorrhizal shrubs on soil carbon and nitrogen pools are accentuated under arbuscular mycorrhizal trees

Plant mycorrhizal associations influence the accumulation and persistence of soil organic matter and could therefore shape ecosystem biogeochemical responses to global changes that are altering forest composition. For instance, arbuscular mycorrhizal (AM) tree dominance is increasing in temperate forests, and ericoid mycorrhizal (ErM) shrubs can respond positively to canopy disturbances. Yet how shifts in the co-occurrence of trees and shrubs with different mycorrhizal associations will affect soil organic matter pools remains largely unknown. We examine the effects of ErM shrubs on soil carbon and nitrogen stocks and indicators of microbial activity at different depths across gradients of AM versus ectomycorrhizal (EcM) tree dominance in three temperate forest sites. We find that ErM shrubs strongly modulate tree mycorrhizal dominance effects. In surface soils, ErM shrubs increase particulate organic matter accumulation and weaken the positive relationship between soil organic matter stocks and indicators of microbial activity. These effects are strongest under AM trees that lack fungal symbionts that can degrade organic matter. In subsurface soil organic matter pools, by contrast, tree mycorrhizal dominance effects are stronger than those of ErM shrubs. Ectomycorrhizal tree dominance has a negative influence on particulate and mineral-associated soil organic matter pools, and these effects are stronger for nitrogen than for carbon stocks. Our findings suggest that increasing co-occurrence of ErM shrubs and AM trees will enhance particulate organic matter accumulation in surface soils by suppressing microbial activity while having little influence on mineral-associated organic matter in subsurface soils. Our study highlights the importance of considering interactions between co-occurring plant mycorrhizal types, as well as their depth-dependent effects, for projecting changes in soil carbon and nitrogen stocks in response to compositional shifts in temperate forests driven by disturbances and global change.     

Impact of priming on global soil carbon stocks

Fresh carbon input (above and belowground) contributes to soil carbon sequestration, but also accelerates decomposition of soil organic matter through biological priming mechanisms. Currently, poor understanding precludes the incorporation of these priming mechanisms into the global carbon models used for future projections. Here, we show that priming can be incorporated based on a simple equation calibrated from incubation and verified against independent litter manipulation experiments in the global land surface model, ORCHIDEE. When incorporated into ORCHIDEE, priming improved the model's representation of global soil carbon stocks and decreased soil carbon sequestration by 51% (12 ± 3 Pg C) during the period 1901–2010. Future projections with the same model across the range of CO₂ and climate changes defined by the IPCC‐RCP scenarios reveal that priming buffers the projected changes in soil carbon stocks — both the increases due to enhanced productivity and new input to the soil, and the decreases due to warming‐induced accelerated decomposition. Including priming in Earth system models leads to different projections of soil carbon changes, which are challenging to verify at large spatial scales.     

氮沉降对森林土壤有机质和凋落物分解的影响及其微生物学机制

氮沉降持续增加背景下土壤C∶N∶P化学计量比和pH环境等的改变及其可能的土壤微生物学机制已经成为陆地生态系统与全球变化研究的新生长点和科学研究前沿.以生态化学计量学和土壤微生物生态学为理论基础,综述了氮沉降对森林土壤有机质和凋落物分解的影响及其微生物学机制的基本理论、最新进展、研究热点与难点,旨在促进全球变化背景下陆地生态系统地下生态学的研究.氮沉降持续增加会导致森林生态系统磷循环加速,导致磷限制.氮沉降不但改变森林土壤有机质和凋落物的C∶N∶P化学计量比和降低土壤pH值,而且改变土壤微生物生物量碳氮磷、细菌、真菌和放线菌的组成以及影响碳氮磷分解的关键酶活性.氮沉降对森林土壤有机质和凋落物分解的影响表现为促进、抑制和无影响,其影响的差异可能来源于微生物效应的不同.叶片在凋落前有显著的氮磷养分回收,但是根无明显的养分回收,造成土壤有机质和凋落物的C∶N∶P化学计量比存在明显差异.基于DNA/RNA等分子生物学方法为土壤微生物生态学研究提供了强有力的手段,将促进氮沉降对森林土壤有机质和凋落物化学计量比改变的微生物学机制研究.     

土壤有机质对农田管理措施的动态响应

土壤有机质在农田肥力、环境保护、农业可持续发展等方面具有重要意义。它不仅决定农作物产量,而且在全球碳素循环中起着重要作用。由于大气CO2浓度升高与全球气候变暖等一系列环境问题的加剧,全球碳素循环受到越来越多的关注。农田具有大气CO2源和库的双重潜力。历史上由于人类对农田的过度开垦和耕种,造成土壤有机质含量大幅度下降,降低了农田的作物产量潜力;同时导致大量的碳以CO2形式由陆地生态系统排放到大气圈,加剧了全球温室效应。大量研究结果表明,诸如耕作、种植制度、施肥等农田管理措施能够显著地影响土壤有机质动态,而免耕、提高复种指数、合理的轮作换茬、有机肥料和化肥的施用以及弃耕农田还林还草等保护性管理措施则能够提高农田土壤有机质含量,使农田起到大气CO2汇的作用。综述了近年来农田管理对土壤有机质动态影响研究方面的进展。     

Increased litterfall in tropical forests boosts the transfer of soil CO2 to the atmosphere

Aboveground litter production in forests is likely to increase as a consequence of elevated atmospheric carbon dioxide (CO(2)) concentrations, rising temperatures, and shifting rainfall patterns. As litterfall represents a major flux of carbon from vegetation to soil, changes in litter inputs are likely to have wide-reaching consequences for soil carbon dynamics. Such disturbances to the carbon balance may be particularly important in the tropics because tropical forests store almost 30% of the global soil carbon, making them a critical component of the global carbon cycle; nevertheless, the effects of increasing aboveground litter production on belowground carbon dynamics are poorly understood. We used long-term, large-scale monthly litter removal and addition treatments in a lowland tropical forest to assess the consequences of increased litterfall on belowground CO(2) production. Over the second to the fifth year of treatments, litter addition increased soil respiration more than litter removal decreased it; soil respiration was on average 20% lower in the litter removal and 43% higher in the litter addition treatment compared to the controls but litter addition did not change microbial biomass. We predicted a 9% increase in soil respiration in the litter addition plots, based on the 20% decrease in the litter removal plots and an 11% reduction due to lower fine root biomass in the litter addition plots. The 43% measured increase in soil respiration was therefore 34% higher than predicted and it is possible that this 'extra' CO(2) was a result of priming effects, i.e. stimulation of the decomposition of older soil organic matter by the addition of fresh organic matter. Our results show that increases in aboveground litter production as a result of global change have the potential to cause considerable losses of soil carbon to the atmosphere in tropical forests.     

Effects of substrate availability on the temperature sensitivity of soil organic matter decomposition

Soil carbon is a major component in the global carbon cycle. Understanding the relationship between environmental changes and rates of soil respiration is critical for projecting changes in soil carbon fluxes in a changing climate. Although significant attention has been focused on the temperature sensitivity of soil organic matter decomposition, the factors that affect this temperature sensitivity are still debated. In this study, we examined the effects of substrate availability on the temperature sensitivity of soil respiration in several different kinds of soils. We found that increased substrate availability had a significant positive effect on temperature sensitivity, as measured by soil Q ₁₀ values, and that this effect was inversely proportional to original substrate availability. This observation can be explained if decomposition follows Michaelis–Menten kinetics. The simple Q ₁₀ model was most appropriate in soils with high substrate availability.     

Retention of soil organic matter by occlusion within soil minerals

Reviews in Environmental Science and Bio/Technology - The stabilization of soil organic matter is crucial for global carbon cycling processes as soil stores large amounts of organic carbon. The...     

Soil carbon sequestration and land‐use change: processes and potential

When agricultural land is no longer used for cultivation and allowed to revert to natural vegetation or replanted to perennial vegetation, soil organic carbon can accumulate. This accumulation process essentially reverses some of the effects responsible for soil organic carbon losses from when the land was converted from perennial vegetation. We discuss the essential elements of what is known about soil organic matter dynamics that may result in enhanced soil carbon sequestration with changes in land‐use and soil management. We review literature that reports changes in soil organic carbon after changes in land‐use that favour carbon accumulation. This data summary provides a guide to approximate rates of SOC sequestration that are possible with management, and indicates the relative importance of some factors that influence the rates of organic carbon sequestration in soil. There is a large variation in the length of time for and the rate at which carbon may accumulate in soil, related to the productivity of the recovering vegetation, physical and biological conditions in the soil, and the past history of soil organic carbon inputs and physical disturbance. Maximum rates of C accumulation during the early aggrading stage of perennial vegetation growth, while substantial, are usually much less than 100 g C m^?2 y^?1. Average rates of accumulation are similar for forest or grassland establishment: 33.8 g C m^?2 y^?1 and 33.2 g C m^?2 y^?1, respectively. These observed rates of soil organic C accumulation, when combined with the small amount of land area involved, are insufficient to account for a significant fraction of the missing C in the global carbon cycle as accumulating in the soils of formerly agricultural land.     

土壤有机质和外源有机物对甲烷产生的影响

对土壤有机质含量及组分、外源有机物和根系分泌对甲烷产生的影响作了综述。土壤产甲烷量和甲烷排放量随有机质含量增加而提高,与土壤中易矿化有机碳或沸水浸提有机碳含量呈显著相关。外源有机碳加入促进了土壤排放甲,刺激效果与外源有机碳的用量和组成有关。还原力强的有机物如纤维素和半纤维素较还原力弱的有机物如类脂和多糖能够产生更多的甲烷。甲醇、甲基化氨基酸等无其它微生物竞争利用的有机物能被产甲烷菌更多地转化成甲烷。植物根系分泌物也促进甲烷的产生,促进作用大小与植物种类及分泌物的数量和质量有关。外源有机物通过3种方式促进土壤甲烷产生;提高土壤的甲烷底物供应量,降低土壤氧化还原电位,刺激土壤原有有机碳的转化。     

草原土壤有机碳含量的控制因素

基于374个高寒草原和温带草原土壤样品的测试结果,运用多元逐步回归分析模型定量评估了土壤环境因子对土壤有机碳(SOC)含量的影响.结果表明:高寒草原土壤有机碳含量(20.18 kg C/m2)高于温带草原(9.23 kg C/m2).土壤理化生物学因子对高寒草原和温带草原SOC含量(10 cm)变化的贡献分别是87.84％和75.00％.其中,土壤总氮含量和根系对高寒草原SOC含量变化的贡献均大于对温带草原SOC含量变化的相应贡献.土壤水分是温带草原SOC含量变化的主要限制性因素,其对SOC含量变化的贡献达33.27％.高寒草原土壤C/N比显著高于温带草原土壤的相应值,揭示了青藏高原高寒草原较高的SOC含量是由于较低的土壤微生物活性所导致.     

生物标志物及其在生态系统研究中的应用

生物标志物是环境和地质体中记载着原始生物母质分子结构信息的有机化合物, 其含量可以指征特定生物来源对天然有机质的相对贡献, 其组成和同位素信息还可以记录有机质的转化及环境信息。与传统元素及组分分析相比, 生物标志物为研究天然有机质的来源、动态变化和转化特征提供了具有高度专一性和灵敏度的工具, 因此, 近年来被广泛地应用于生态学和生物地球化学研究中。特别是, 与生态系统观测以及控制实验相结合, 生物标志物在揭示微生物的活性与碳源变化、土壤有机碳的稳定机制及其对全球变化的响应等方面显示了广阔的应用前景。近些年开发的生物标志物单体同位素分析也在生态系统碳氮周转与食物网研究等方面显示了巨大的研究潜力。基于此, 该文综述了生态系统研究中常用的生物标志物的种类、分析方法和应用方向, 总结了生物标志物研究目前存在的问题, 并对未来的研究方向进行了展望, 旨在为使用生物标志物的生态学和环境科学研究者提供参考。     

氮肥用量和秸秆根茬碳投入对黄淮海平原典型农田土壤有机质积累的影响

土壤有机质与粮食安全和气候变化等重大问题密切相关,也是目前国内外研究热点。通过分析黄淮海平原10个小麦玉米长期定位试验资料,在不同施氮量下建立土壤有机质积累与碳投入的数学模型,探讨了土壤有机质积累与碳氮投入的关系。结果表明:土壤有机质积累与碳投入的关系受氮肥用量的影响显著,无论低量施氮(长期定位试验中的常规施肥量,化肥氮量≤330kg/(hm.2a))还是高量施氮(长期定位试验中的高量施肥量,化肥氮量330kg/(hm.2a)),秸秆根茬碳还田量与土壤有机质变化量之间均呈极显著正相关关系(P0.01);利用土壤有机质积累与碳投入的数学模型计算的维持土壤有机质平衡的碳投入量(MSC),高量施氮时为7254kg/(hm.2a),低量施氮为1297kg/(hm.2a),前者是后者的5.6倍。因此,大量施用氮肥降低了投入碳的积累,不利于土壤有机质的提升。     

人工油松林恢复过程中土壤理化性质及有机碳含量的变化特征

采用时空替代法,选取岷江上游大沟流域内不同恢复时期(12、18、25、35a)的人工油松林为研究对象,研究了植被恢复过程中土壤理化性质及有机碳含量的变化特征,同时探讨了它们之间的相互关系.研究结果表明沿恢复梯度,土壤质量得到了改善,主要表现为土壤粘粒含量、比表面积、有机质含量显著增加,土壤粉粒含量和pH值则显著下降.土壤有机质与土壤粘粒和比表面积呈显著正相关,与土壤容重呈显著负相关.此外,土壤有机碳含量沿恢复梯度显著增加,0-50 cm内土壤有机碳含量从5.59 kg/m2增加到12.64 kg/m2,土壤年平均固碳速率为0.31 kg/m2.     

Substrate and environmental controls on microbial assimilation of soil organic carbon: a framework for Earth system models

A mechanistic understanding of microbial assimilation of soil organic carbon is important to improve Earth system models’ ability to simulate carbon‐climate feedbacks. A simple modelling framework was developed to investigate how substrate quality and environmental controls over microbial activity regulate microbial assimilation of soil organic carbon and on the size of the microbial biomass. Substrate quality has a positive effect on microbial assimilation of soil organic carbon: higher substrate quality leads to higher ratio of microbial carbon to soil organic carbon. Microbial biomass carbon peaks and then declines as cumulative activity increases. The simulated ratios of soil microbial biomass to soil organic carbon are reasonably consistent with a recently compiled global data set at the biome level. The modelling framework developed in this study offers a simple approach to incorporate microbial contributions to the carbon cycling into Earth system models to simulate carbon‐climate feedbacks and explain global patterns of microbial biomass.     

植物凋落物影响土壤有机质分解的研究进展

植物凋落物是土壤动物和土壤微生物的主要生命物质和能量来源,其类型、组成以及物理化学等性质直接决定了土壤有机质的品质。对植物凋落物的类型、品质、物理性质、层效应和激发效应以及根际碳淀积与土壤有机质分解的关系进行了总结,可为研究植物凋落物对土壤有机质的影响提供理论参考,指出要在全球变暖背景下进一步加强凋落物分解过程中土壤微生物和酶活性变化的研究。     

Transplantation of organic surface horizons of boreal soils into warmer regions alters microbiology but not the temperature sensitivity of decomposition

Changes in soil carbon, the largest terrestrial carbon pool, are critical for the global carbon cycle, atmospheric CO₂ levels and climate. Climate warming is predicted to be most pronounced in the northern regions and therefore the large soil carbon pool residing in boreal forests will be subject to larger global warming impact than soil carbon pools in the temperate or the tropical forest. A major uncertainty in current estimates of the terrestrial carbon balance is related to decomposition of soil organic matter (SOM). We hypothesized that when soils are exposed to warmer climate the structure of the ground vegetation will change much more rapidly than the dominant tree species. This change will alter the quality and amount of litter input to the soil and induce changes in microbial communities, thus possibly altering the temperature sensitivity of SOM decomposition. We transferred organic surface soil sections from the northern borders of the boreal forest zone to corresponding forest sites in the southern borders of the boreal forest zone and studied the effects of warmer climate after an adaptation period of 2 years. The results showed that initially ground vegetation and soil microbial community structure and community functions were different in northern and southern forest sites and that 2 years of exposure to warmer climate was long enough to cause changes in these ecological indicators. The rate of SOM decomposition was approximately equally sensitive to temperature irrespective of changes in vegetation or microbial communities in the studied forest sites. However, as temperature sensitivity of the decomposition increases with decreasing temperature regime, the proportional increase in the decomposition rate in northern latitudes could lead to significant carbon losses from the soils.     

The temperature dependence of organic matter decomposition: seasonal temperature variations turn a sharp short‐term temperature response into a more moderate annually averaged response

The temperature dependence of organic matter decomposition is a critically important determinant of any long‐term changes of soil‐carbon stocks in response to global warming. Because of practical experimental constraints, most knowledge of this temperature dependence is based on short‐term studies. These studies generally show a strong temperature dependence of organic matter decomposition. At the same time, many modelling studies, especially global studies, or studies that investigate the effects of climate change, use longer time steps, such as annual. It is investigated here to what extent the use of short‐term temperature dependencies are appropriate, or how they may need to be modified, for application over longer time steps. The work indicated that for global applications, it is critically important to explicitly consider seasonal temperature variations. Across the globe, observed annual mean temperature and the annual temperature range are negatively correlated. Inclusion of this correlation means that the strong short‐term temperature dependence becomes much weaker when data are expressed as annual averages for the temperatures experienced across the globe. For short‐term responses, the temperature dependence of organic matter decomposition is greater at low than high temperature and deviates strongly from an assumption of a constant Q₁₀ across temperature. For annually averaged values, this pattern also weakens, and temperature dependencies change only slightly with temperature. Using short time steps for simulations leads to the expectation of more positive changes (sequestration) in soil carbon especially for cold regions of the globe than would be predicted for simulations at annual time steps without explicit consideration of seasonal temperature variations. These considerations help to reconcile some of the apparent differences in temperature dependencies obtained by different workers using different approaches.     

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.     

Carbon cycle confidence and uncertainty: Exploring variation among soil biogeochemical models

Emerging insights into factors responsible for soil organic matter stabilization and decomposition are being applied in a variety of contexts, but new tools are needed to facilitate the understanding, evaluation, and improvement of soil biogeochemical theory and models at regional to global scales. To isolate the effects of model structural uncertainty on the global distribution of soil carbon stocks and turnover times we developed a soil biogeochemical testbed that forces three different soil models with consistent climate and plant productivity inputs. The models tested here include a first‐order, microbial implicit approach (CASA‐CNP), and two recently developed microbially explicit models that can be run at global scales (MIMICS and CORPSE). When forced with common environmental drivers, the soil models generated similar estimates of initial soil carbon stocks (roughly 1,400 Pg C globally, 0–100 cm), but each model shows a different functional relationship between mean annual temperature and inferred turnover times. Subsequently, the models made divergent projections about the fate of these soil carbon stocks over the 20^th century, with models either gaining or losing over 20 Pg C globally between 1901 and 2010. Single‐forcing experiments with changed inputs, temperature, and moisture suggest that uncertainty associated with freeze‐thaw processes as well as soil textural effects on soil carbon stabilization were larger than direct temperature uncertainties among models. Finally, the models generated distinct projections about the timing and magnitude of seasonal heterotrophic respiration rates, again reflecting structural uncertainties that were related to environmental sensitivities and assumptions about physicochemical stabilization of soil organic matter. By providing a computationally tractable and numerically consistent framework to evaluate models we aim to better understand uncertainties among models and generate insights about factors regulating the turnover of soil organic matter.