Isotopic evidence for increased carbon and nitrogen exchanges between peatland plants and their symbiotic microbes with rising atmospheric CO2 concentrations since 15,000 cal. year BP

Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO₂ by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer-term (decadal to century) feedbacks between climate, CO₂ and plant N uptake could emerge to reduce ecosystem-level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N₂ fixation—the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ¹⁵N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem-level N cycling responses to rising atmospheric CO₂ concentrations. We find that pre-industrial increases in global atmospheric CO₂ concentrations corresponded with a decrease in the δ¹⁵N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ¹⁵N of Sphagnum decreases with increasing N₂-fixation rates. These findings suggest that plant-microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO₂ concentrations, highlighting an ecosystem-level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome.     

Soil respiration and the global carbon cycle

Soil respiration is the primary path by which CO₂fixed by land plants returns to the atmosphere. Estimated at approximately 75 × 10¹⁵gC/yr, this large natural flux is likely to increase due changes in the Earth's condition. The objective of this paper is to provide a brief scientific review for policymakers who are concerned that changes in soil respiration may contribute to the rise in CO₂in Earth's atmosphere. Rising concentrations of CO₂in the atmosphere will increase the flux of CO₂from soils, while simultaneously leaving a greater store of carbon in the soil. Traditional tillage cultivation and rising temperature increase the flux of CO₂from soils without increasing the stock of soil organic matter. Increasing deposition of nitrogen from the atmosphere may lead to the sequestration of carbon in vegetation and soils. The response of the land biosphere to simultaneous changes in all of these factors is unknown, but a large increase in the soil carbon pool seems unlikely to moderate the rise in atmospheric CO₂during the next century.     

氮添加对森林土壤有机碳库固存及CO2排放的影响研究进展

氮添加会引起土壤理化性质和养分有效性的改变。受此影响,森林植物的地上碳同化能力和地下碳分配格局也会相应地发生变化,总体表现为促进植物生长固碳,增加凋落物和植物根系沉积碳输入土壤,并改变上述植物源有机质的数量和化学成分。与此同时,土壤微生物的群落结构和生态功能也会受到氮添加的影响,由于土壤中的有机碳分解、转化和稳定等过程均受到微生物的驱动,因此,氮添加所引起的底物供应差异和微生物响应会影响森林土壤有机碳的矿化,并最终影响森林土壤有机碳库固存、稳定和CO₂排放。但目前关于氮添加对森林土壤有机碳库固存能力和CO₂排放特征的影响机制仍不清楚,为此,以森林土壤的碳循环过程为线索,综述了氮添加对底物供应、土壤有机碳激发效应、微生物碳代谢等过程的影响,并尝试梳理在氮添加影响下森林土壤有机碳分解、转化和稳定的微生物驱动机制。这有助于预测氮添加对森林土壤"氮促碳汇"的实际效果,以便研究人员在未来氮沉降日益严重背景下更好地预测森林土壤的碳循环特征,寻找提高森林土壤有机碳库固存能力和降低CO₂排放相关途径提供参考。同时,还分析了目前相关研究中存在的问题,并对该领域未来的研究热点进行了展望。     

Faster turnover of new soil carbon inputs under increased atmospheric CO2

Rising levels of atmospheric CO₂ frequently stimulate plant inputs to soil, but the consequences of these changes for soil carbon (C) dynamics are poorly understood. Plant‐derived inputs can accumulate in the soil and become part of the soil C pool (“new soil C”), or accelerate losses of pre‐existing (“old”) soil C. The dynamics of the new and old pools will likely differ and alter the long‐term fate of soil C, but these separate pools, which can be distinguished through isotopic labeling, have not been considered in past syntheses. Using meta‐analysis, we found that while elevated CO₂ (ranging from 550 to 800 parts per million by volume) stimulates the accumulation of new soil C in the short term (<1 year), these effects do not persist in the longer term (1–4 years). Elevated CO₂ does not affect the decomposition or the size of the old soil C pool over either temporal scale. Our results are inconsistent with predictions of conventional soil C models and suggest that elevated CO₂ might increase turnover rates of new soil C. Because increased turnover rates of new soil C limit the potential for additional soil C sequestration, the capacity of land ecosystems to slow the rise in atmospheric CO₂ concentrations may be smaller than previously assumed.     

Biodiversity,Nitrogen Deposition,and CO<Subscript>2</Subscript> Affect Grassland Soil Carbon Cycling but not Storage

Grasslands are globally widespread and capable of storing large amounts of carbon (C) in soils, and are generally experiencing increasing atmospheric CO₂, nitrogen (N) deposition, and biodiversity losses. To better understand whether grasslands will act as C sources or sinks in the future we measured microbial respiration in long-term laboratory incubations of soils collected from a grassland field experiment after 9 years of factorial treatment of atmospheric CO₂, N deposition, and plant species richness on a deep and uniformly sandy soil. We fit microbial soil respiration rates to three-pool models of soil C cycling to separate treatment effects on decomposition and pool sizes of fast, slow, and resistant C pools. Elevated CO₂ decreased the mean residence time (MRT) of slow C pools without affecting their pool size. Decreasing diversity reduced the size and MRT of fast C pools (comparing monocultures to plots planted with 16 species), but increased the slow pool MRT. N additions increased the size of the resistant pool. These effects of CO₂, N, and species-richness treatments were largely due to plant biomass differences between the treatments. We found no significant interactions among treatments. These results suggest that C sequestration in sandy grassland soils may not be strongly influenced by elevated CO₂ or species losses. However, high N deposition may increase the amount of resistant C in these grasslands, which could contribute to increased C sequestration.     

Nitrogen deposition and carbon sequestration in alpine meadows

Nitrogen deposition experiments were carried out in alpine meadow ecosystems in Qinghai-Xizang Plateau in China, in order to explore the contribution of nitrogen deposition to carbon sequestration in alpine meadows. Two methods were used in this respect. First, we used the allocation of ¹⁵N tracer to soil and plant pools. Second, we used increased root biomass observed in the nitrogen-amended plots. Calculating enhanced carbon storage, we considered the net soil CO₂ emissions exposed to nitrogen deposition in alpine meadows. Our results show that nitrogen deposition can enhance the net soil CO₂ emissions, and thus offset part of carbon uptake by vegetation and soils. It means that we have to be cautious to draw a conclusion when we estimate the contribution of nitrogen deposition to carbon sequestration based on the partitioning of ¹⁵N tracer in terrestrial ecosystems, in particular in N-limited ecosystems. Even if we assess the contribution of nitrogen deposition to carbon sequestration based on increased biomass exposed to nitrogen deposition in terrestrial ecosystems, likewise, we have to consider the effects of nitrogen deposition on the soil CO₂ emissions.     

Nonlinear root-derived carbon sequestration across a gradient of nitrogen and phosphorous deposition in experimental mesocosms

Enhanced sequestration of plant‐carbon (C) inputs to soil may mitigate rising atmospheric carbon dioxide (CO₂) concentrations and related climate change but how this sequestration will respond to anthropogenic nitrogen (N) and phosphorous (P) deposition is uncertain. We couple isotope, soil C fractionation and mesocosm techniques to assess the sequestration of plant‐C inputs, and their partitioning into C pools with different sink potentials, under an experimental gradient of N and P deposition (0, 10, 30, 60 and 100 kg N ha^?1 yr^?1; and 0, 2, 6, 12 and 20 kg P ha^?1 yr^?1). We hypothesized that N deposition would increase sequestration, with the majority of the C being sequestered in faster cycling soil pools because N deposition has been shown to accelerate the turnover of these pools while decelerating the turnover of slower cycling pools. In contrast to this hypothesis, sequestration into all soil C pools peaked at intermediate levels of N deposition. Given that P amendment has been shown to cause a net loss of soil C, we postulated that P deposition would decrease sequestration. This expectation was not supported by our data, with sequestration generally being greater under P deposition. When soils were amended simultaneously with N and P, neither the shape of the sequestration relationship across the deposition gradient, nor the observed sequestration at the majority of the deposition rates, was statistically predictable from the effects of N and P in isolation. The profound nonlinearities we observed, both for total sequestration responses and the partitioning of C into soil pools with different sink potentials, suggests that the rates of N and P deposition to ecosystems will be the critical determinant of whether they enhance or decrease the long‐term sequestration of fresh plant‐C inputs to soils.     

Terrestrial ecosystems and the carbon cycle

The terrestrial biosphere plays an important role in the global carbon cycle. In the 1994 Intergovernmental Panel Assessment on Climate Change (IPCC), an effort was made to improve the quantification of terrestrial exchanges and potential feedbacks from climate, changing CO₂, and other factors; this paper presents the key results from that assessment, together with expanded discussion. The carbon cycle is the fluxes of carbon among four main reservoirs: fossil carbon, the atmosphere, the oceans, and the terrestrial biosphere. Emissions of fossil carbon during the 1980s averaged 5.5 Gt y^?1. During the same period, the atmosphere gained 3.2 Gt C y^?1 and the oceans are believed to have absorbed 2.0 Gt C y^?1. The regrowing forests of the Northern Hemisphere may have absorbed 0.5 Gt C y^?1 during this period. Meanwhile, tropical deforestation is thought to have released an average 1.6 Gt C y^?1 over the 1980s. While the fluxes among the four pools should balance, the average 198Ds values lead to a ‘missing sink’ of 1.4 Gt C y^?1 Several processes, including forest regrowth, CO₂ fertilization of plant growth (c. 1.0 Gt C y^?1), N deposition (c. 0.6 Gt C y^?1), and their interactions, may account for the budget imbalance. However, it remains difficult to quantify the influences of these separate but interactive processes. Uncertainties in the individual numbers are large, and are themselves poorly quantified. This paper presents detail beyond the IPCC assessment on procedures used to approximate the flux uncertainties. Lack of knowledge about positive and negative feedbacks from the biosphere is a major limiting factor to credible simulations of future atmospheric CO₂ concentrations. Analyses of the atmospheric gradients of CO₂ and ¹³ CO₂ concentrations provide increasingly strong evidence for terrestrial sinks, potentially distributed between Northern Hemisphere and tropical regions, but conclusive detection in direct biomass and soil measurements remains elusive. Current regional-to-global terrestrial ecosystem models with coupled carbon and nitrogen cycles represent the effects of CO₂ fertilization differently, but all suggest longterm responses to CO₂ that are substantially smaller than potential leaf- or laboratory whole plant-level responses. Analyses of emissions and biogeochemical fluxes consistent with eventual stabilization of atmospheric CO₂ concentrations are sensitive to the way in which biospheric feedbacks are modeled by c. 15%. Decisions about land use can have effects of 100s of Gt C over the next few centuries, with similarly significant effects on the atmosphere. Critical areas for future research are continued measurements and analyses of atmospheric data (CO₂ and ¹³CO₂) to serve as large-scale constraints, process studies of the scaling from the photosynthetic response to CO₂ to whole-ecosystem carbon storage, and rigorous quantification of the effects of changing land use on carbon storage.     

Soil carbon sequestrations by nitrogen fertilizer application, straw return and no-tillage in China's cropland

Soil as the largest global carbon pool has played a great role in sequestering the atmospheric carbon dioxide (CO₂). Although global carbon sequestration potentials have been assessed since the 1980s, few investigations have been made on soil carbon sequestration (SCS) in China's cropland. China is a developing country and has a long history of agricultural activities. Estimation of SCS potentials in China's cropland is very important for assessing the potential measures to prevent the atmospheric carbon rise and predicting the atmospheric CO₂ concentration in future. After review of the available results of the field experiments in China, relationships between SCS and nitrogen fertilizer application, straw return and no‐tillage (NT) practices were established for each of the four agricultural regions. According to the current agricultural practices and their future development, estimations were made on SCS by nitrogen fertilizer application, straw return and NT in China's cropland. In the current situation, nitrogen fertilizer application, straw return and zero tillage can sequester 5.96, 9.76 and 0.800 Tg C each year. Carbon sequestration potential will increase to 12.1 Tg C yr⁻¹ if nitrogen is fertilized on experts' recommendations. The carbon sequestration potentials of straw return and NT can reach 34.4 and 4.60 Tg C yr⁻¹ when these two techniques are further popularized. In these measures, straw return is the most promising one. Full popularization of straw return can reduce 5.3% of the CO₂ emission from fossil fuel combustion in China in 1990, which meets the global mean CO₂ reduction requested by the Kyoto Protocol (5.2%). In general, if more incentive policies can be elaborated and implemented, the SCS in China's cropland will be increased by about two times. So, popularization of the above‐mentioned agricultural measures for carbon sequestration can be considered as an effective tool to prevent the rapid rise of the atmospheric CO₂ in China.     

大气CO2浓度升高对土壤碳库稳定性的影响

工业革命以来,大气CO₂浓度持续上升,升高的CO₂浓度会改变植物光合产物积累、土壤碳库的碳输入和碳输出过程,进而通过影响有机碳组成和周转特征来调控土壤碳库动态变化。土壤碳库是陆地生态系统碳库的重要组成部分,其碳储量的微小变化都会对大气CO₂浓度和气候变化产生巨大影响。但目前关于CO₂浓度升高对土壤碳库动态和稳定性的影响还不清楚,很大程度上限制了预测陆地生态系统碳循环对气候变化的反馈。系统综述国内外大气CO₂浓度升高对植被生产力、植被碳输入和土壤碳库影响的研究进展,旨在揭示土壤碳库物理、化学组成以及周转特征对CO₂浓度升高的响应过程和机理,探讨CO₂升高情境下土壤微生物特征对土壤碳库稳定性的影响和驱动机制,为深入理解全球变化下的土壤碳循环特征提供理论支撑。     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

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.     

Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment

Soil fungi couple plant and ecosystem resource demands to pools of soil resources. Research on these organisms is needed to predict how rising atmospheric CO₂ will influence forest ecosystem processes and soil carbon (C) sequestration potential. We examined the influence of free‐air‐CO₂‐enrichment (FACE) on mycorrhizal and extraradical rhizomorph dynamics over a 5‐year period in a loblolly pine forest using minirhizotrons. Standing crop of mycorrhizal root tips varied greatly spatially and through time. Summed across all years, CO₂ enrichment increased mycorrhizal root tip production by 194% in deep soil (15–30 cm) but did not influence mycorrhizal production in shallow soil (0–15 cm). Production and mortality of soil rhizomorph length was 27% and 25% greater in CO₂‐enriched plots compared with controls over a 5‐year period beginning in January of 2000 and running through autumn 2004. Effects of atmospheric CO₂ enrichment on longevity of mycorrhizal root tips and rhizomorphs varied with soil depth (mycorrhizae and rhizomorphs) and with diameter (rhizomorphs). For instance, survival of mycorrhizal tips was reduced in CO₂‐enriched plots in deep soil (15–30 cm depth) but was increased in shallower soil (0–15 cm). Rhizomorph turnover was accelerated in shallow soil but effects on survivorship in deep soil varied according to diameter. A drought in 2002 coupled with loss of leaf area to an ice storm late in 2002 were followed by reductions in rhizomorph and mycorrhizal production, increases in mortality, and decreases in standing crop during 2003 and 2004. These effects tended to be more severe in CO₂‐enriched plots. Positive effects of atmospheric CO₂ enrichment on mycorrhizal fungi, primarily observed in deeper soil, are probably contributing to the prolonged stimulation of NPP by CO₂ enrichment at the Duke FACE study site.     

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 Management Affects Carbon Sequestration in North American Cropland Soils

Agricultural soils in North America can be a sink for rising atmospheric CO₂ concentrations through the formation of soil organic matter (SOM) or humus. Humification is limited by the availability of nutrients such as nitrogen (N). Recommended management practices (RMPs) that optimize N availability foster humus formation. This review examines the management practices that contribute to maximizing N availability for optimizing sequestration of atmospheric CO₂ into soil humus. Farming practices that enhance nutrient use, reduce or eliminate tillage, and increase crop intensity, together, affect N availability and, therefore, C sequestration. N additions, from especially, livestock manure and leguminous cover crops are necessary for increasing grain and biomass yields and returning crop residues to the soil thereby increasing soil organic carbon (SOC) concentration. Conservation tillage practices enhance also the availability of N and increase SOC concentration. Increase in cropping intensity and/or crop rotations produce higher quantity and quality of residues, increase availability of N, and therefore foster increase in C sequestration. The benefit of C sequestration from N additions may be negated by CO₂ and N₂O emissions associated with production and application of N fertilizers. More studies need to be conducted to ascertain the benefits of adding N via manuring versus N fertilizer additions. Furthermore, site specific adaptive research is needed to identify RMPs that optimize soil N use efficiency while improving crop yield and C sequestration thereby curbing greenhouse gas (GHG) emissions. Due to the wide range of climate in North America, there is a large range of C sequestration potential in agricultural soils through N management. Humid croplands may have the potential to sequester 8–298 Tg C yr^?1 while dry croplands may sequester 1–35 Tg C yr^?1. These estimates, however, are highly uncertain and wide-ranging. Clearly, more research is needed to quantify, more precisely, the C sequestration potential across different N management scenarios especially in Mexico and Canada.     

Nitrogen deposition and plant species interact to influence soil carbon stabilization

Anthropogenic nitrogen (N) deposition effects on soil organic carbon (C) decomposition remain controversial, while the role of plant species composition in mediating effects of N deposition on soil organic C decomposition and long‐term soil C sequestration is virtually unknown. Here we provide evidence from a 5‐year grassland field experiment in Minnesota that under elevated atmospheric CO₂ concentration (560 ppm), plant species determine whether N deposition inhibits the decomposition of soil organic matter via inter‐specific variation in root lignin concentration. Plant species producing lignin‐rich litter increased stabilization of soil C older than 5 years, but only in combination with elevated N inputs (4 g m^?2 year^?1). Our results suggest that N deposition will increase soil C sequestration in those ecosystems where vegetation composition and/or elevated atmospheric CO₂ cause high litter lignin inputs to soils.     

氮输入对滨海盐沼湿地碳循环关键过程的影响及机制

滨海盐沼湿地是缓解全球变暖的有效蓝色碳汇, 但是近岸海域富营养化导致的大量氮输入对盐沼湿地稳定性和碳汇功能构成严重威胁。潮汐作用下大量氮输入对盐沼湿地植物光合碳输入、植物-土壤碳分配和土壤碳输出等碳循环关键过程产生深刻影响, 进而影响盐沼湿地碳汇功能评估的准确性。该文从植物光合固碳、植物-土壤系统碳分配、土壤有机碳分解、土壤可溶性有机碳释放、盐沼湿地土壤碳库5个方面综述了氮输入对盐沼湿地碳循环关键过程的影响。在此基础上, 针对当前研究的不足, 提出今后的研究中, 需要进一步探究氮输入对盐沼湿地植物光合固碳及碳分配过程的影响、盐沼湿地土壤有机碳分解的微生物机制、盐沼湿地土壤可溶性有机碳产生和横向流动的影响、以及氮类型对盐沼湿地土壤碳库的影响。以期为揭示氮输入对盐沼湿地碳汇形成过程与机制提供基础资料和理论依据, 为评估未来近岸海域水体富营养化影响下滨海盐沼湿地碳库的潜在变化提供新思路。     

Plant diversity positively affects short‐term soil carbon storage in experimental grasslands

Increasing atmospheric CO₂ concentration and related climate change have stimulated much interest in the potential of soils to sequester carbon. In ‘The Jena Experiment’, a managed grassland experiment on a former agricultural field, we investigated the link between plant diversity and soil carbon storage. The biodiversity gradient ranged from one to 60 species belonging to four functional groups. Stratified soil samples were taken to 30 cm depth from 86 plots in 2002, 2004 and 2006, and organic carbon contents were determined. Soil organic carbon stocks in 0–30 cm decreased from 7.3 kg C m^?2 in 2002 to 6.9 kg C m^?2 in 2004, but had recovered to 7.8 kg C m^?2 by 2006. During the first 2 years, carbon storage was limited to the top 5 cm of soil while below 10 cm depth, carbon was lost probably as short‐term effect of the land use change. After 4 years, carbon stocks significantly increased within the top 20 cm. More importantly, carbon storage significantly increased with sown species richness (log‐transformed) in all depth segments and even carbon losses were significantly smaller with higher species richness. Although increasing species diversity increased root biomass production, statistical analyses revealed that species diversity per se was more important than biomass production for changes in soil carbon. Below 20 cm depth, the presence of one functional group, tall herbs, significantly reduced carbon losses in the beginning of the experiment. Our analysis indicates that plant species richness and certain plant functional traits accelerate the build‐up of new carbon pools within 4 years. Additionally, higher plant diversity mitigated soil carbon losses in deeper horizons. This suggests that higher biodiversity might lead to higher soil carbon sequestration in the long‐term and therefore the conservation of biodiversity might play a role in greenhouse gas mitigation.     

Global atmospheric change effects on terrestrial carbon sequestration: Exploration with a global C- and N-cycle model (CQUESTN)

A model of the interacting global carbon and nitrogen cycles (CQUESTN) is developed to explore the possible history of C-sequestration into the terrestrial biosphere in response to the global increases (past and possible future) in atmospheric CO₂ concentration, temperature and N-deposition. The model is based on published estimates of pre-industrial C and N pools and fluxes into vegetation, litter and soil compartments. It was found necessary to assign low estimates of N pools and fluxes to be compatible with the more firmly established C-cycle data. Net primary production was made responsive to phytomass N level, and to CO₂ and temperature deviation from preindustrial values with sensitivities covering the ranges in the literature. Biological N-fixation could be made either unresponsive to soil C:N ratio, or could act to tend to restore the preindustrial C:N of humus with different N-fixation intensities. As for all such simulation models, uncertainties in both data and functional relationships render it more useful for qualitative evaluation than for quantitative prediction.With the N-fixation response turned off, the historic CO₂ increase led to standard-model sequestration into terrestrial ecosystems in 1995AD of 1.8 Gt C yr^–1. With N-fixation restoring humus C:N strongly, C sequestration was 3 Gt yr^–1 in 1995. In both cases C:N of phytomass and litter increased with time and these increases were plausible when compared with experimental data on CO₂ effects. The temperature increase also caused net C sequestration in the model biosphere because decrease in soil organic matter was more than offset by the increase in phytomass deriving from the extra N mineralised. For temperature increase to reduce system C pool size, the biosphere leakiness to N would have to increase substantially with temperature. Assuming a constant N-loss coefficient, the historic temperature increase alone caused standard-model net C sequestration to be about 0.6 Gt C in 1995. Given the disparity of plant and microbial C:N, the modelled impact of anthropogenic N-deposition on C-sequestration depends substantially on whether the deposited N is initially taken up by plants or by soil microorganisms. Assuming the latter, standard-model net sequestration in 1995 was 0.2 Gt C in 1995 from the N-deposition effect alone. Combining the effects of the historic courses of CO₂, temperature and N-deposition, the standard-model gave C-sequestration of 3.5 Gt in 1995. This involved an assumed weak response of biological N-fixation to the increased carbon status of the ecosystem. For N-fixation to track ecosystem C-fixation in the long term however, more phosphorus must enter the biological cycle. New experimental evidence shows that plants in elevated CO₂ have the capacity to mobilize more phosphorus from so-called unavailable sources using mechanisms involving exudation of organic acids and phosphatases.     

Modelling carbon balances of coastal arctic tundra under changing climate

Rising air temperatures are believed to be hastening heterotrophic respiration (R_h) in arctic tundra ecosystems, which could lead to substantial losses of soil carbon (C). In order to improve confidence in predicting the likelihood of such loss, the comprehensive ecosystem model ecosys was first tested with carbon dioxide (CO₂) fluxes measured over a tundra soil in a growth chamber under various temperatures and soil‐water contents (θ). The model was then tested with CO₂ and energy fluxes measured over a coastal arctic tundra near Barrow, Alaska, under a range of weather conditions during 1998–1999. A rise in growth chamber temperature from 7 to 15 °C caused large, but commensurate, rises in respiration and CO₂ fixation, and so no significant effect on net CO₂ exchange was modelled or measured. An increase in growth chamber θ from field capacity to saturation caused substantial reductions in respiration but not in CO₂ fixation, and so an increase in net CO₂ exchange was modelled and measured. Long daylengths over the coastal tundra at Barrow caused an almost continuous C sink to be modelled and measured during most of July (2–4 g C m^?2 d^?1), but shortening daylengths and declining air temperatures caused a C source to be modelled and measured by early September (～1 g C m^?2 d^?1). At an annual time scale, the coastal tundra was modelled to be a small C sink (4 g C m^?2 y^?1) during 1998 when average air temperatures were 4 °C above normal, and a larger C sink (16 g C m^?2 y^?1) during 1999 when air temperatures were close to long‐term normals. During 100 years under rising atmospheric CO₂ concentration (C_a), air temperature and precipitation driven by the IS92a emissions scenario, modelled R_h rose commensurately with net primary productivity (NPP) under both current and elevated rates of atmospheric nitrogen (N) deposition, so that changes in soil C remained small. However, methane (CH₄) emissions were predicted to rise substantially in coastal tundra with IS92a‐driven climate change (from ～20 to ～40 g C m^?2 y^?1), causing a substantial increase in the emission of CO₂ equivalents. If the rate of temperature increase hypothesized in the IS92a emissions scenario had been raised by 50%, substantial losses of soil C (～1 kg C m^?2) would have been modelled after 100 years, including additional emissions of CH₄.