Rhizosphere processes are quantitatively important components of terrestrial carbon and nutrient cycles

While there is an emerging view that roots and their associated microbes actively alter resource availability and soil organic matter (SOM) decomposition, the ecosystem consequences of such rhizosphere effects have rarely been quantified. Using a meta‐analysis, we show that multiple indices of microbially mediated C and nitrogen (N) cycling, including SOM decomposition, are significantly enhanced in the rhizospheres of diverse vegetation types. Then, using a numerical model that combines rhizosphere effect sizes with fine root morphology and depth distributions, we show that root‐accelerated mineralization and priming can account for up to one‐third of the total C and N mineralized in temperate forest soils. Finally, using a stoichiometrically constrained microbial decomposition model, we show that these effects can be induced by relatively modest fluxes of root‐derived C, on the order of 4% and 6% of gross and net primary production, respectively. Collectively, our results indicate that rhizosphere processes are a widespread, quantitatively important driver of SOM decomposition and nutrient release at the ecosystem scale, with potential consequences for global C stocks and vegetation feedbacks to climate.     

氮沉降对森林生态系统碳吸存的影响

工业化带来的大气氮沉降增加是影响森林生态系统碳吸存的重要因素。将森林碳库分为地上和地下两部分,从3个方面综述了国内外氮沉降对森林生态系统碳吸存影响的研究现状。(1)地上部分:氮限制的温带森林,氮沉降增加了地上部分碳吸存。氮丰富的热带森林,氮沉降对地上部分碳吸存没有影响。过量的氮输入会造成森林死亡率的上升,从而降低地上部分碳吸存。(2)地下部分:相比地上部分研究得少,表现为增加、降低和没有影响3种效果。(3)目前的结论趋向于认为氮沉降促进森林生态系统碳吸存,然而氮沉降所带来的森林生态系统碳吸存能力到底有多大依然无法确定,这也将成为未来氮碳循环研究的重点问题。分析了氮沉降影响森林生态系统碳吸存的机理,介绍了氮沉降对森林生态系统碳吸存影响的4种研究方法。探讨了该领域研究的不足及未来的研究方向。     

氮输入对植物光合固碳的影响研究进展

植物光合固碳(C)是生物固C的重要途径和生态系统C循环中的重要环节。在全球环境变化背景下,研究氮(N)输入对植物光合固C的影响,对于更好的认识C、N循环过程及生态系统对全球变化的响应过程等具有重要意义。N输入是否能够增加植物固C取决于生态系统类型以及生态系统的N饱和度;草原和湿地生态系统N输入的临界负荷值较高,干旱、半干旱荒漠地区较低;N输入可能改变植物光合固C在各器官的分配,主要由植物生理、自身生长节律和环境养分等决定。由于物种和生态系统类型的差异,N输入对植物固C的影响仍具有很大的不确定性,目前缺乏准确、定量表达N输入对生态系统光合和C同化物分配影响的数学表达方法和过程算法。未来应着重加强N输入下C同化物分配的生物地球化学模型和N、P富集下植物光合固C耦合模型研究,并应用同位素标记和分子生物学技术,从生态系统角度综合探讨N输入下植物光合固C的分配和转化特征。     

陆地生物圈模型的发展与应用

陆地生物圈与大气圈和水圈之间能量、水和碳氮等元素的交换和循环对整个地球系统产生了深刻的影响。陆地生物圈模型(TBM)是研究陆地生态系统如何响应和反馈全球变化的重要方法和工具。通过对从生态系统到区域和全球陆地生物圈不同空间尺度的植被动态、生物地球物理和生物地球化学循环过程、水循环和水文过程、自然干扰和人类活动等过程时间动态的模拟, 陆地生物圈模型被广泛地应用于评估和归因过去陆地生物圈的时空变化和预测陆地生物圈对未来全球变化的响应和反馈。该文简要回顾了陆地生物圈模型的发展, 总结了模型对陆地生态系统主要过程的刻画和模型在生态系统生态学的应用, 并对未来陆地生物圈模型的发展和应用进行了展望。     

Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils

It is proposed that carbon (C) sequestration in response to reactive nitrogen (N_r) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO₂ emissions. While studies have helped clarify the magnitude by which N_r deposition enhances C sequestration by forest vegetation, there remains a paucity of long‐term experimental studies evaluating how soil C pools respond. We conducted a long‐term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha^?1 yr^?1) in the boreal zone of northern Sweden to understand how atmospheric N_r deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non‐significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg^?1 N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg^?1 N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.     

微生物介导的碳氮循环过程对全球气候变化的响应

土壤是地球表层最为重要的碳库也是温室气体的源或汇。自工业革命以来,对土壤温室气体的容量、收支平衡和通量等已有较多研究和估算,但对关键过程及其源/汇的研究却十分有限。微生物是土壤碳氮转化的主要驱动者, 在生态系统碳氮循环过程中扮演重要的角色,对全球气候变化有着响应的响应、适应及反馈,然而其个体数量,群落结构和多样性如何与气候扰动相互关联、进而怎样影响生态系统过程的问题仍有待进一步探索。从微生物介导的碳氮循环过程入手,重点讨论微生物对气候变化包括温室气体(CO₂,CH₄,N₂O)增加、全球变暖、大气氮沉降等的响应和反馈,并由此提出削减温室气体排放的可能途径和今后发展的方向。     

海岛陆地生态系统固碳估算方法

陆地生态系统在调节全球碳平衡和减缓全球气候变化中起着重要作用。海岛作为一种特殊的生态系统,生物群落和环境与大陆基本相似。虽然海岛生态结构相对简单,物种的丰富程度比大陆低,但对全球碳循环也有一定的影响。在海岛陆地生态系统中,森林和灌草的种属相对较少,且不同纬度的海岛森林植被种属差异明显,可采用典型样地清查和生物量模型估算相结合的方法估算乔木层和灌草层的碳储量。采用模型估算固碳潜力时,根据海岛生态环境的特殊性,综合考虑岛陆面积、季节、风向、坡度、坡向、海拔、平均温度、降雨量、土壤理化性质等参数对其碳储量估算的影响。海岛植被生物多样性影响其土壤碳储存的生态服务功能,利用多元统计分析方法,建立岛陆植物物种丰度与土壤碳储量的空间回归模型,明确植物多样性的改变对岛陆土壤固碳能力的影响。此外,从土壤固碳的角度而言,海岛土壤-植物-微生物间相互作用是其重要的研究方向。利用现代分子生物学技术,研究海岛陆地生态系统的土壤-植物-微生物相互作用关系,有利于海岛土壤固碳潜力估算精度的提高。     

The role of the terrestrial biosphere in Holocene carbon cycle dynamics

Measurements of atmospheric CO₂ concentration, and its stable carbon isotope composition, from gas samples trapped in ice at Taylor Dome, Antarctica, indicate that the global carbon cycle has not been in steady state during the Holocene epoch. Inverse carbon cycle modelling has led to the hypothesized cumulative release from the terrestrial biosphere of 195 Gt C between 7 and 1 kyr before present (bp ). Here, three independent lines of evidence testing this hypothesis are critically examined: global reconstructions of terrestrial carbon reservoirs, vegetation–climate modelling, and high latitude subfossil plant stable carbon isotope records. Despite inherent uncertainties associated with each approach, it emerges that none strongly upholds the suggestion that terrestrial ecosystems released large amounts of carbon between 7 and 1 kyr bp . Consequently, our understanding of the processes involved in the exchange of CO₂ between the atmosphere, oceans and land biota continues to remain incomplete and to require further investigation.     

Quantifying photosynthetic capacity and its relationship to leaf nitrogen content for global-scale terrestrial biosphere models

Photosynthetic capacity and its relationship to leaf nitrogen content are two of the most sensitive parameters of terrestrial biosphere models (TBM) whose representation in global‐scale simulations has been severely hampered by a lack of systematic analyses using a sufficiently broad database. Here, we use data of qualitative traits, climate and soil to subdivide the terrestrial vegetation into functional types (PFT), and then assimilate observations of carboxylation capacity, V_max (723 data points), and maximum photosynthesis rates, A_max (776 data points), into the C₃ photosynthesis model proposed by Farquhar et al. to constrain the relationship of (V_max normalised to 25 °C) to leaf nitrogen content per unit leaf area for each PFT. In a second step, the resulting functions are used to predict per PFT from easily measurable values of leaf nitrogen content in natural vegetation (1966 data points). Mean values of thus obtained are implemented into a TBM (BETHY within the coupled climate–vegetation model ECHAM5/JSBACH) and modelled gross primary production (GPP) is compared with independent observations on stand scale. Apart from providing parameter ranges per PFT constrained from much more comprehensive data, the results of this analysis enable several major improvements on previous parameterisations. (1) The range of mean between PFTs is dominated by differences of photosynthetic nitrogen use efficiency (NUE, defined as divided by leaf nitrogen content), while within each PFT, the scatter of values is dominated by the high variability of leaf nitrogen content. (2) We find a systematic depression of NUE on certain tropical soils that are known to be deficient in phosphorous. (3) of tropical trees derived by this study is substantially lower than earlier estimates currently used in TBMs, with an obvious effect on modelled GPP and surface temperature. (4) The root‐mean‐squared difference between modelled and observed GPP is substantially reduced.     

The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems

The response of plants to elevated CO₂ is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO₂ concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO₂ stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO₂ in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO₂ would be to increase N-import to the grassland ecosystem through symbiotic N₂ fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N₂ fixation in legumes will be enhanced under elevated CO₂, ii) this enhancement of N₂ fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO₂ under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO₂, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO₂. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N₂ fixation usually promotes shoot growth (above-ground C storage) in elevated CO₂. However, the consequences of this larger N input under elevated CO₂ on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO₂ on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon.     

Biological nitrogen fixation: rates,patterns and ecological controls in terrestrial ecosystems

New techniques have identified a wide range of organisms with the capacity to carry out biological nitrogen fixation (BNF)—greatly expanding our appreciation of the diversity and ubiquity of N fixers—but our understanding of the rates and controls of BNF at ecosystem and global scales has not advanced at the same pace. Nevertheless, determining rates and controls of BNF is crucial to placing anthropogenic changes to the N cycle in context, and to understanding, predicting and managing many aspects of global environmental change. Here, we estimate terrestrial BNF for a pre-industrial world by combining information on N fluxes with ¹⁵N relative abundance data for terrestrial ecosystems. Our estimate is that pre-industrial N fixation was 58 (range of 40–100) Tg N fixed yr⁻¹; adding conservative assumptions for geological N reduces our best estimate to 44 Tg N yr⁻¹. This approach yields substantially lower estimates than most recent calculations; it suggests that the magnitude of human alternation of the N cycle is substantially larger than has been assumed.     

森林生态系统碳氮循环功能耦合研究综述

在大气CO2浓度升高和氮沉降增加等全球变化背景下,森林生态系统减缓CO2浓度升高的作用及其对全球变化的响应和反馈存在诸多不确定性.森林生态系统碳氮循环相互作用及功能耦合规律的研究是揭示这些不确定性的基础,也是反映森林生态系统生物产量与养分之间作用规律,涉及林地持久生产力（sustainability of long-term site productivity）的生态学机理问题.森林生态系统碳氮循环的耦合作用表现在林冠层光合作用的碳固定过程,森林植物组织呼吸、土壤凋落物与土壤有机质分解、地下部分根系周转与呼吸等碳释放过程,这些过程存在反馈机理和非线性作用,最终决定森林生态系统的碳平衡.着重在生态系统尺度上,综述了碳氮循环耦合作用研究的一些进展与存在的问题,对今后研究方向进行了展望.     

Is carbon within the global terrestrial biosphere becoming more oxidized? Implications for trends in atmospheric O2

Measurements of atmospheric O₂ and CO₂ concentrations serve as a widely used means to partition global land and ocean carbon sinks. Interpretation of these measurements has assumed that the terrestrial biosphere contributes to changing O₂ levels by either expanding or contracting in size, and thus serving as either a carbon sink or source (and conversely as either an oxygen source or sink). Here, we show how changes in atmospheric O₂ can also occur if carbon within the terrestrial biosphere becomes more reduced or more oxidized, even with a constant carbon pool. At a global scale, we hypothesize that increasing levels of disturbance within many biomes has favored plant functional types with lower oxidative ratios and that this has caused carbon within the terrestrial biosphere to become increasingly more oxidized over a period of decades. Accounting for this mechanism in the global atmospheric O₂ budget may require a small increase in the size of the land carbon sink. In a scenario based on the Carnegie–Ames–Stanford Approach model, a cumulative decrease in the oxidative ratio of net primary production (NPP) (moles of O₂ produced per mole of CO₂ fixed in NPP) by 0.01 over a period of 100 years would create an O₂ disequilibrium of 0.0017 and require an increased land carbon sink of 0.1 Pg C yr⁻¹ to balance global atmospheric O₂ and CO₂ budgets. At present, however, it is challenging to directly measure the oxidative ratio of terrestrial ecosystem exchange and even more difficult to detect a disequilibrium caused by a changing oxidative ratio of NPP. Information on plant and soil chemical composition complement gas exchange approaches for measuring the oxidative ratio, particularly for understanding how this quantity may respond to various global change processes over annual to decadal timescales.     

Nitrogen fixation and the mass balances of carbon and nitrogen in ecosystems

Ecosystems with high rates of nitrogen fixation often have high loss rates through leaching or possibly denitrification. However, there is no formal theoretical context to examine why this should be the case nor of how nitrogen accumulates in such open systems. Here, we propose a simple model coupling nitrogen inputs and losses to carbon inputs and losses. The nitrogen balance of this model system depends on plant (nitrogen fixer) growth rate, its carrying capacity, N fixed/C fixed, residence time of nitrogen and carbon in biomass, litter decay rate, litter N/C, and fractional loss rate of mineralized nitrogen. The model predicts the requirements for equilibrium in a nitrogen-fixing system, and the conditions on nitrogen fixation and losses in order for the system to accumulate nitrogen and carbon. In particular, the accumulation of nitrogen and carbon in a nitrogen-fixing system depend on an interaction between residence time in vegetation and litter decay rate in soil. To reflect a possible increased uptake of soil nitrogen and decreased respiratory cost of symbiotic nitrogen fixers, the model was then modified so that fixation rate decreased and growth rate increased as nitrogen capital accumulated. These modifications had only small effects on carbon and nitrogen accumulation. This suggests that switching from uptake of atmospheric nitrogen to mineral soil nitrogen as nitrogen capital accumulates simply results in a trade-off between energetic limitations and soil nitrogen limitations to carbon and nitrogen accumulation. Experimental tests of the model are suggested.     

植被恢复对土壤碳氮循环的影响研究进展

植被恢复重建是治理水土流失的主要手段之一,能够有效地促进侵蚀土壤发育、提高土壤肥力、增强土壤微生物活性,进一步影响土壤碳氮循环.因此,植被的恢复重建过程对土壤有机碳库、氮库累积以及温室气体的排放具有一定作用.本文综述了植被恢复对土壤碳、氮循环过程的影响以及土壤质量与植被修复之间的协同效应,并提出了今后进一步研究的方向.对评价植被恢复在应对全球气候变化中所起的作用具有借鉴与参考价值,对促进土壤肥力改善和退化生态系统的恢复及可持续发展也有重要的现实意义.     

Belowground carbon flux links biogeochemical cycles and resource‐use efficiency at the global scale

Nutrient limitation is pervasive in the terrestrial biosphere, although the relationship between global carbon (C) nitrogen (N) and phosphorus (P) cycles remains uncertain. Using meta‐analysis we show that gross primary production (GPP) partitioning belowground is inversely related to soil‐available N : P, increasing with latitude from tropical to boreal forests. N‐use efficiency is highest in boreal forests, and P‐use efficiency in tropical forests. High C partitioning belowground in boreal forests reflects a 13‐fold greater C cost of N acquisition compared to the tropics. By contrast, the C cost of P acquisition varies only 2‐fold among biomes. This analysis suggests a new hypothesis that the primary limitation on productivity in forested ecosystems transitions from belowground resources at high latitudes to aboveground resources at low latitudes as C‐intensive root‐ and mycorrhizal‐mediated nutrient capture is progressively replaced by rapidly cycling, enzyme‐derived nutrient fluxes when temperatures approach the thermal optimum for biogeochemical transformations.     

陆地和淡水生态系统新型微生物氮循环研究进展

氮生物地球化学循环是地球物质循环的重要枢纽,是决定陆地生态系统生产力水平、水资源安全、温室气体生成排放的关键过程。氮循环是由微生物介导的一系列复杂过程,不同形态、价态氮化合物的转化分别由相应的功能微生物驱动完成。随着厌氧氨氧化、完全氨氧化等新型氮转化过程的相继报道和发现更新了人们对氮循环的认识。本文综述了陆地和淡水生态系统中厌氧氨氧化(anammox)、硝酸盐异化还原为铵(DNRA)、完全氨氧化(comammox)等新型氮循环过程的发生机制、热区分布及环境效应,并总结了这三种氮循环的相互关系。     

Carbon accumulation and sequestration of lakes in China during the Holocene

Understanding the responses of lake systems to past climate change and human activity is critical for assessing and predicting the fate of lake carbon (C) in the future. In this study, we synthesized records of the sediment accumulation from 82 lakes and of C sequestration from 58 lakes with direct organic C measurements throughout China. We also identified the controlling factors of the long‐term sediment and C accumulation dynamics in these lakes during the past 12 ka (1 ka = 1000 cal yr BP). Our results indicated an overall increasing trend of sediment and C accumulation since 12 ka, with an accumulation peak in the last couple of millennia for lakes in China, corresponding to terrestrial organic matter input due to land‐use change. The Holocene lake sediment accumulation rate (SAR) and C accumulation rate (CAR) averaged (mean ± SE) 0.47 ± 0.05 mm yr^?1 and 7.7 ± 1.4 g C m^?2 yr^?1 in China, respectively, comparable to the previous estimates for boreal and temperate regions. The SAR for lakes in the East Plain of subtropical China (1.05 ± 0.28 mm yr^?1) was higher than those in other regions (P < 0.05). However, CAR did not vary significantly among regions. Overall, the variability and history of climate and anthropogenic interference regulated the temporal and spatial dynamics of sediment and C sequestration for lakes in China. We estimated the total amount of C burial in lakes of China as 8.0 ± 1.0 Pg C. This first estimation of total C storage and dynamics in lakes of China confirms the importance of lakes in land C budget in monsoon‐influenced regions.     

农林复合系统固碳潜力研究进展

农林复合系统是解决当前资源枯竭、农林用地紧张和实现环境保护的一种可持续土壤管理模式。自《京都议定书》签订以来, 农林复合系统因其较高的固碳潜力引起了科学家的广泛关注。深入理解农林复合系统的固碳过程及其对气候变化、环境条件的改变和管理措施的响应, 是准确地预测农林复合系统在全球变化情景下固碳潜力的关键。该文综述了农林复合系统的概念和分类, 探讨了农林复合系统相比单一系统的固碳潜力及固碳机理, 分析了农林复合系统固碳潜力的测定方法和当前面临的挑战, 综述了气候因子、环境条件和人为管理措施对农林复合系统固碳潜力的影响。我国农林复合系统的固碳潜力相比全球其他区域还处于较低水平, 为提高我国农林复合系统的固碳潜力, 未来需要加强以下四个方面的工作: 扩大农林复合系统的分布面积、加强农林复合系统的合理配置和管理、选择适宜的物种组合和优化系统的群体结构。     

施氮和降水格局改变对土壤CH4和CO2通量的影响

氮沉降增加和降水格局改变是全球变化的两项重要内容,但是同时考虑上述两因素对温室气体CH4和CO2通量影响的原位双因子模拟研究还相当有限.本研究以长白山温带阔叶红松林土壤为研究对象,采用静态箱法研究了外施氮源(50 kg N·hm-2·a-1)和增减30％降水对土壤CH4和CO2通量的影响.结果表明:施氮能抑制土壤CH4吸收,有时甚至能将土壤对CH4的吸收转为释放,但这种抑制效应只能维持5d左右,且能在一定程度上改变CH4通量和环境因子(温度、土壤pH、粘粒含量)的相关关系.降水改变未能显著影响土壤CH4通量.对CO2通量而言,施氮能降低土壤CO2排放,长白山阔叶红松林连续施氮第4年的平均抑制效应为27.4％.长期连续施氮的平均抑制效应随施氮时间延长而逐渐增大,一定年限后达到最大值.单次施氮的抑制效应随时间延长逐渐减弱,并在1个月的施氮周期末期基本消失.施氮的抑制效应和土壤充水孔隙度(WFPS)呈显著负相关关系,且升温能增强施氮对CO2释放的抑制效应并延长抑制时间.施氮、降水有可能改变土壤呼吸的温度敏感性.本研究表明,长白山森林土壤氮素尚未达到一定阈值,未来氮沉降增加将抑制CO2的释放和CH4的吸收,因此总体来看施氮抑制土壤碳排放.