高寒草地土壤温室气体排放对放牧的响应研究进展

高寒草地生态系统具有独特的地理环境和气候特征,对放牧干扰十分敏感,在全球温室气体通量中贡献突出,研究高寒草地放牧对土壤温室气体排放的影响机制具有重要意义。本文总结高寒草地温室气体源/汇特征、不同放牧方式对土壤微环境和微生物群落结构的影响,发现高寒草地主要是CO₂源、CH₄汇、N₂O源。放牧通过家畜选择性采食、践踏和排泄物返还等多重机制作用于地上植物、土壤结构、温度、湿度和养分,进而影响地下微生物及温室气体通量。本文旨在为高寒草地生态系统健康发展和管理及缓解全球气候变化提供科学依据,并对未来研究方向进行展望。     

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。     

全球变化野外控制试验及其在土壤微生物生态学研究中的应用

全球变化是近几十年世界广泛关注的热点之一.土地利用变化和化石能源消耗已引起如温室气体增多、气温升高、降水格局改变等多种形式的变化.这些变化对整个生态系统过程,特别是陆地生态系统碳氮循环过程有着深远影响.自20世纪70年代以来,世界各地已开展大量野外控制试验用以模拟单因子和多因子气候变化的影响,这些研究对解释生态系统响应和适应全球变化的内在机制提供了重要的基础.本文梳理了全球变化控制试验的发展历程,介绍了不同因子模拟控制试验的研究概况及不足之处,重点阐述CO₂倍增、增温、降水和模拟氮沉降等全球变化控制试验在土壤微生物生态学研究中的应用,探析土壤微生物及其介导的生态学过程对全球变化的响应和反馈,并对未来野外控制试验需关注的问题和研究方向进行了展望,为认识气候变化对地下生态系统的影响提供参考.     

氮沉降对热带亚热带森林土壤氮循环微生物过程的影响研究进展

近年来,高速的城市化和工业化建设导致全球大气氮沉降量逐年递增,其中热带亚热带地区氮沉降量显著高于全球平均水平,而大部分热带亚热带森林土壤趋近氮饱和状态,氮沉降增加将持续向土壤输入外源活性氮,极易导致土壤氮过剩,进而破环整个森林生态系统氮循环的平衡。我国热带亚热带地区经济发展快速,氮沉降增加导致的土壤养分失衡和林地退化等生态问题日益凸显,森林土壤氮循环对大气氮沉降的响应及适应机制已引起了学术界的广泛关注。研究表明氮循环各环节均由特定的功能微生物驱动完成,明确氮沉降增加对热带亚热带森林土壤氮循环功能微生物及其介导的关键过程的影响,对评价未来氮沉降增加背景下全球森林土壤氮循环的响应及驱动机制有重要作用,可为促进我国热带亚热带地区森林修复、生态环境的改善与提升提供科学支撑。鉴于此,本文综述了热带亚热带森林土壤氮循环主要过程（如固氮、硝化、反硝化、厌氧氨氧化等）及其功能微生物群落丰度、活性、组成等对氮沉降增加的响应,同时分析了这些功能微生物的群落特征与主要环境因子（如NH₄⁺、NO₃^-、有机碳、pH、含水量等）的关联性。在此基础上探讨了氮沉降增加下功能微生物对热带亚热带森林土壤氮循环的调控作用,重点探讨了功能微生物如何通过改变丰度与群落组成而影响氮循环过程,并对目前研究中存在的主要问题与未来研究重点进行了简要剖析。     

马尾松幼苗根系和光合碳供应对土壤性质变化和温室气体排放的驱动作用

森林植被受全球气候变化、森林经营活动及病虫害等多种干扰,导致林地光合碳供应水平及根系输入量发生变化。在此背景下,土壤性质及土壤温室气体排放的响应及其机理是预测森林碳汇功能变化及森林可持续经营的重要依据。以2年生马尾松盆栽苗为对象,通过单株/盆和3株/盆栽植密度控制根系输入量、通过环割和截干控制光合碳向地下的供应能力,模拟森林植被干扰导致的根系输入量及光合碳供应变化对土壤理化性质、微生物群落结构及温室气体排放的影响。结果表明,苗木根系非结构性碳水化合物(TNC)含量和氮含量比单株/盆低;3株/盆的土壤速效氮含量比单株/盆低,土壤革兰氏阳性菌、厌氧菌、放线菌及丛枝菌根真菌丰富度均比单株/盆显著增加,3株/盆的土壤二氧化碳(CO₂)排放速率较高,但土壤氧化亚氮(N₂O)排放速率差异不显著。无论是单株/盆还是3株/盆,环割和截干处理后,根系生物量、根系长度及表面积均比对照显著下降;根系TNC含量显著下降。土壤和根系氮含量都有增加趋势;土壤微生物生物量碳(SMBC)含量降低,而土壤微生物生物量氮(SMBN)则提高。环割和截干后,土壤中各种微生物组成丰富度均有下降趋势,土壤CO₂排放速率显著下降,土壤N₂O排放速率则显著提高。根系输入量及光合碳供应对土壤细菌和真菌含量均有显著影响,土壤细菌含量与根系生物量、SMBC和SMBN显著正相关;土壤真菌含量与土壤温度显著负相关,与根系生物量、SMBC和SMBN显著正相关。相关分析表明,土壤CO₂排放通量与土壤温度、土壤湿度及根系生物量显著正相关,与土壤硝态氮显著负相关;土壤N₂O排放通量与土壤温度和土壤湿度显著正相关。以上研究表明,根系输入量与地上光合碳供应共同作用,改变土壤理化性质及微生物环境,进而影响土壤温室气体排放。     

森林土壤微生物对干旱和氮沉降的响应

干旱和氮沉降深刻影响着人类世森林生态系统的生命活动与物质循环,进而影响全球碳平衡、并反馈作用于气候变化。土壤微生物驱动元素的生物地球化学循环和关键土壤生态过程,在气候变化生物学研究方面具有核心地位和全球重要性。本文综述了干旱和氮沉降对森林土壤细菌和菌根真菌的影响。提出未来应加强全球变化多因子交互作用对土壤微生物多样性、活性与生态功能的研究;建立野外长期定位站,强化亚热带森林生态系统与全球变化研究;注重土壤生物之间互作及网络研究;利用微生物大数据建立相关的机理模型等。从认识微生物多样性和群落组成对全球变化的响应与适应,逐步发展为调控利用微生物群落服务于森林的优化管理、生态资源的合理保护与可持续利用,为充分发挥微生物减缓全球气候变化的作用提供理论基础。     

高寒生态系统微生物群落研究进展

高寒生态系统分布在高纬度或高海拔、气候寒冷的地区,包括北极苔原、高山苔原、青藏高原等.高寒生态系统对气候变化非常敏感,其土壤中储存大量的有机碳,对全球的碳平衡起关键作用.微生物是生物地球化学循环的主要驱动者,微生物群落对气候变化的响应和反馈影响生态系统的功能与稳定性.本文回顾了高寒生态系统微生物群落组成、多样性与空间分布,以及微生物群落对气候变化(增温、氮沉降、火干扰)的响应,为拓展我国高寒生态系统微生物研究提供基础.     

氮添加对巴音布鲁克高寒湿地土壤微生物量和酶活性的影响

随着全球氮沉降速率的快速增加,已对陆地生态系统微生物群落活性和代谢产生了深刻的影响。因此迫切需要了解全球气候变化敏感区土壤中微生物量和酶活性对氮添加的响应。为此,以中亚干旱区巴音布鲁克高寒湿地为研究对象,在保护良好的高寒湿地选择沼泽(S)、沼泽草甸(SM)和草甸(M)3种湿地类型布设野外原位氮添加试验(施氮浓度分别为0、8、16 kg N hm^-2 a^-1),探究短期氮添加对土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳/氮(MBC/MBN)、微生物商(QMB)、土壤蛋白酶、脲酶、碱性磷酸酶、H₂O₂酶和蔗糖酶活性的影响。结果表明:(1)高寒湿地不同湿地类型土壤微生物量和酶活性存在显著差异,其中SM土壤MBC、MBN、MBC\\N、QMB较S和M区高,对酶活性而言,SM和M区土壤蛋白酶和碱性磷酸酶活性较高,M区H₂O₂酶和脲酶活性较高。(2)氮添加显著增加了3种湿地类型中土壤MBC和MBN,其中MBC增加了7.00%-119.00%,MBN增加了8.03%-38.26%。氮添加仅显著增加了S和SM区土壤MBC/N和QMB (增加了24.68%-113.10%),但抑制了M区土壤MBC/N和QMB (抑制了8.93%-10.36%)。(3)氮添加显著增加了3种湿地类型土壤中脲酶、蛋白酶和H₂O₂酶活性,分别增加了7.25%-59.63%、4.71%-58.55%和34.70%-157.27%。但是氮添加对土壤碱性磷酸酶活性无显著影响。对蔗糖酶而言,N1处理增加了S区土壤蔗糖酶活性(增加了58.58%),而N2处理显著降低了22.72%。氮添加对SM和M区蔗糖酶活性无显著影响。(4)结构方程模型的结果显示,氮添加直接增加了土壤微生物量和酶活性。而随着湿地类型的变化(S-SM-M)直接和间接(通过pH)增加了酶活性;湿地类型的变化还通过影响pH、有机碳和有效养分间接增加了土壤微生物量。总之,氮添加和湿地类型可直接或间接的影响着土壤微生物量和酶活性。其中,土壤pH和有机碳是微生物量和酶活性变化的主要影响因素。本研究可为中亚干旱区高寒湿地应对未来气候变化的措施的制定提供技术参考。     

陆地生态系统土壤CO2排放对模拟增温的响应特征及影响因素

全球变暖已经成为不争的事实,陆地生态系统碳循环的研究受到了各界广泛关注,是当前全球变化研究中的重点。土壤CO₂排放是陆地生态系统与大气间二氧化碳交换的最大通量之一,当前陆地生态系统中土壤CO₂排放如何响应全球气候变暖及其影响因素仍不清楚,限制了对土壤碳循环过程及影响机制的深入认识。旨在明确全球变暖背景下陆地生态系统中土壤CO₂排放格局及影响因素。基于Web of Science、PubMed和中国知网等中英文期刊数据库,充分收集全球范围内的相关野外试验文献81篇,提取出65个研究位置和213组相关研究数据,采用Meta分析方法探讨陆地生态系统土壤CO₂排放对增温的响应特征,分析其与海拔、气候、土壤含水量、容重(BD)、pH、全氮(TN)和土壤有机碳(SOC)的相关关系。结果表明:陆地生态系统中土壤CO₂排放对增温整体有显著的正向响应,在农、林、草生态系统中,增温使土壤CO₂排放分别显著增加13.1%、18.0%、5.9% (P<0.05),森林生态系统对增温响应的正效应最强烈;增温能在短时期内促进土壤呼吸,但随着增温持续时间增加,土壤呼吸对温度的敏感性会降低,对温度变化产生适应性,从而使其对增温的响应能力减弱;响应特征受到环境因子、土壤特性以及其他试验条件等的影响,绝大多数条件下对增温表现出显著的正响应特征,不同影响因子之间共同作用、相互影响。增温通常能够改变植物生物量、土壤养分含量及微生物数量和活性,从而影响到植被根际呼吸和土壤呼吸速率。相关分析表明,海拔对土壤CO₂排放有显著负向影响,而年均气温、年均降水量、土壤含水量和仪器嵌入土壤深度则对土壤CO₂排放产生显著正向影响。这些结果对于理解全球土壤CO₂排放的时空变化格局有重要意义,也为准确评价全球变暖背景下土壤碳汇功能及其持续性提供理论依据。     

土壤微生物群落对全球气候变化响应的研究进展

全球气候变化对陆地生态系统过程和功能产生重要影响,土壤微生物群落在陆地生态系统几乎所有的生物地球化学循环过程起到关键作用。本文针对气候变化对土壤微生物的影响研究结果,主要从土壤微生物活性(土壤呼吸与酶活性)和微生物群落结构对大气CO₂升高、增温、降水变化、氮沉降等全球变化单因子和多因子的直接或间接响应进行综述,并进一步阐述参与土壤碳氮循环过程的功能微生物对气候变化的响应机制与适应规律。全球变化因子改变了土壤微生物的群落组成,呈现降低、增加和无影响3种效应,且不同功能微生物也呈现不同的敏感性。多个全球变化因子对土壤微生物群落结构的交互效应可能存在加性、协同、拮抗作用,产生加和的、相互促进或抵消的整体效果。然而,目前对多种全球变化因子如三因子或四因子的组合作用,以及多因子的高阶交互作用研究较少;已有的研究地理分布不均匀,且时间和空间大尺度的研究不足;缺乏综合生态系统模型对全球变化的影响进行模拟和预测。最后指出今后的研究发展方向：进行多种全球变化因子、长时间、多生态系统点位、大空间尺度的土壤微生物群落动态研究;探究多种全球变化因子的高阶交互作用;建立综合响应的生态系统模...     

Microbial mediation of biogeochemical cycles revealed by simulation of global changes with soil transplant and cropping

Despite microbes'' key roles in driving biogeochemical cycles, the mechanism of microbe-mediated feedbacks to global changes remains elusive. Recently, soil transplant has been successfully established as a proxy to simulate climate changes, as the current trend of global warming coherently causes range shifts toward higher latitudes. Four years after southward soil transplant over large transects in China, we found that microbial functional diversity was increased, in addition to concurrent changes in microbial biomass, soil nutrient content and functional processes involved in the nitrogen cycle. However, soil transplant effects could be overridden by maize cropping, which was attributed to a negative interaction. Strikingly, abundances of nitrogen and carbon cycle genes were increased by these field experiments simulating global change, coinciding with higher soil nitrification potential and carbon dioxide (CO₂) efflux. Further investigation revealed strong correlations between carbon cycle genes and CO₂ efflux in bare soil but not cropped soil, and between nitrogen cycle genes and nitrification. These findings suggest that changes of soil carbon and nitrogen cycles by soil transplant and cropping were predictable by measuring microbial functional potentials, contributing to a better mechanistic understanding of these soil functional processes and suggesting a potential to incorporate microbial communities in greenhouse gas emission modeling.     

Global change could amplify fire effects on soil greenhouse gas emissions

Background

Little is known about the combined impacts of global environmental changes and ecological disturbances on ecosystem functioning, even though such combined impacts might play critical roles in shaping ecosystem processes that can in turn feed back to climate change, such as soil emissions of greenhouse gases.

Methodology/Principal Findings

We took advantage of an accidental, low-severity wildfire that burned part of a long-term global change experiment to investigate the interactive effects of a fire disturbance and increases in CO₂ concentration, precipitation and nitrogen supply on soil nitrous oxide (N₂O) emissions in a grassland ecosystem. We examined the responses of soil N₂O emissions, as well as the responses of the two main microbial processes contributing to soil N₂O production – nitrification and denitrification – and of their main drivers. We show that the fire disturbance greatly increased soil N₂O emissions over a three-year period, and that elevated CO₂ and enhanced nitrogen supply amplified fire effects on soil N₂O emissions: emissions increased by a factor of two with fire alone and by a factor of six under the combined influence of fire, elevated CO₂ and nitrogen. We also provide evidence that this response was caused by increased microbial denitrification, resulting from increased soil moisture and soil carbon and nitrogen availability in the burned and fertilized plots.

Conclusions/Significance

Our results indicate that the combined effects of fire and global environmental changes can exceed their effects in isolation, thereby creating unexpected feedbacks to soil greenhouse gas emissions. These findings highlight the need to further explore the impacts of ecological disturbances on ecosystem functioning in the context of global change if we wish to be able to model future soil greenhouse gas emissions with greater confidence.     

模拟氮沉降对马尾松土壤微生物群落结构及温室气体释放的影响

以2年生马尾松(Pinus massoniana)盆栽苗土壤为对象,通过施氮肥模拟氮沉降对土壤理化性质、微生物群落结构及温室气体释放的影响,探明氮沉降对森林土壤温室气体释放的驱动机制。结果表明,模拟氮沉降处理显著提高了土壤速效氮含量和苗木根系氮含量;土壤微生物碳(SMBC)含量比对照显著下降78%,而土壤微生物氮(SMBN)则提高2.6倍。模拟氮沉降处理显著降低土壤中微生物群落总含量。施氮肥对马尾松土壤N_2O和CO_2的释放速率均有显著影响,增施氮肥不仅显著提高了土壤N_2O的释放速率,而且CO_2释放速率短期内也显著提高,但伴随微生物群落的下降,施肥后期CO_2释放速率表现下降趋势。相关分析表明,土壤CO_2和N_2O释放与土壤pH值、土壤温度、土壤湿度、土壤速效氮含量及SMBC、SMBN相关;逐步回归分析表明,土壤硝态氮含量的变化是驱动土壤温室气体释放的主导因子。3株种植单位土壤体积内根系生物量较高,增加了土壤水分的消耗速率和氮的吸收固定,因而减少N_2O的释放速率。以上研究阐明了氮沉降或过量施肥对土壤氮含量、土壤pH值、根系生物量及氮含量、土壤微生物群落结构等因子的影响,这些因子直接或间接影响土壤温室气体释放速率。氮沉降及施用氮肥是加快土壤温室气体(CO_2和N_2O)排放进程的重要因素。     

Moderate precipitation reduction enhances nitrogen cycling and soil nitrous oxide emissions in a semi-arid grassland

The ongoing climate change is predicted to induce more weather extremes such as frequent drought and high-intensity precipitation events, causing more severe drying-rewetting cycles in soil. However, it remains largely unknown how these changes will affect soil nitrogen (N)-cycling microbes and the emissions of potent greenhouse gas nitrous oxide (N₂O). Utilizing a field precipitation manipulation in a semi-arid grassland on the Loess Plateau, we examined how precipitation reduction (ca. −30%) influenced soil N₂O and carbon dioxide (CO₂) emissions in field, and in a complementary lab-incubation with simulated drying-rewetting cycles. Results obtained showed that precipitation reduction stimulated plant root turnover and N-cycling processes, enhancing soil N₂O and CO₂ emissions in field, particularly after each rainfall event. Also, high-resolution isotopic analyses revealed that field soil N₂O emissions primarily originated from nitrification process. The incubation experiment further showed that in field soils under precipitation reduction, drying-rewetting stimulated N mineralization and ammonia-oxidizing bacteria in favor of genera Nitrosospira and Nitrosovibrio, increasing nitrification and N₂O emissions. These findings suggest that moderate precipitation reduction, accompanied with changes in drying-rewetting cycles under future precipitation scenarios, may enhance N cycling processes and soil N₂O emissions in semi-arid ecosystems, feeding positively back to the ongoing climate change.     

Microbes drive global soil nitrogen mineralization and availability

Soil net nitrogen mineralization rate (N_min), which is critical for soil nitrogen availability and plant growth, is thought to be primarily controlled by climate and soil physical and/or chemical properties. However, the role of microbes on regulating soil N_min has not been evaluated on the global scale. By compiling 1565 observational data points of potential net N_min from 198 published studies across terrestrial ecosystems, we found that N_min significantly increased with soil microbial biomass, total nitrogen, and mean annual precipitation, but decreased with soil pH. The variation of N_min was ascribed predominantly to soil microbial biomass on global and biome scales. Mean annual precipitation, soil pH, and total soil nitrogen significantly influenced N_min through soil microbes. The structural equation models (SEM) showed that soil substrates were the main factors controlling N_min when microbial biomass was excluded. Microbe became the primary driver when it was included in SEM analysis. SEM with soil microbial biomass improved the N_min prediction by 19% in comparison with that devoid of soil microbial biomass. The changes in N_min contributed the most to global soil NH₄⁺‐N variations in contrast to climate and soil properties. This study reveals the complex interactions of climate, soil properties, and microbes on N_min and highlights the importance of soil microbial biomass in determining N_min and nitrogen availability across the globe. The findings necessitate accurate representation of microbes in Earth system models to better predict nitrogen cycle under global change.     

Winter climate change effects on soil C and N cycles in urban grasslands

Despite growing recognition of the role that cities have in global biogeochemical cycles, urban systems are among the least understood of all ecosystems. Urban grasslands are expanding rapidly along with urbanization, which is expected to increase at unprecedented rates in upcoming decades. The large and increasing area of urban grasslands and their impact on water and air quality justify the need for a better understanding of their biogeochemical cycles. There is also great uncertainty about the effect that climate change, especially changes in winter snow cover, will have on nutrient cycles in urban grasslands. We aimed to evaluate how reduced snow accumulation directly affects winter soil frost dynamics, and indirectly greenhouse gas fluxes and the processing of carbon (C) and nitrogen (N) during the subsequent growing season in northern urban grasslands. Both artificial and natural snow reduction increased winter soil frost, affecting winter microbial C and N processing, accelerating C and N cycles and increasing soil : atmosphere greenhouse gas exchange during the subsequent growing season. With lower snow accumulations that are predicted with climate change, we found decreases in N retention in these ecosystems, and increases in N₂O and CO₂ flux to the atmosphere, significantly increasing the global warming potential of urban grasslands. Our results suggest that the environmental impacts of these rapidly expanding ecosystems are likely to increase as climate change brings milder winters and more extensive soil frost.     

Effects of multiple global change treatments on soil N2O fluxes

Global environmental changes are expected to alter ecosystem carbon and nitrogen cycling, but the interactive effects of multiple simultaneous environmental changes are poorly understood. Effects of these changes on the production of nitrous oxide (N₂O), an important greenhouse gas, could accelerate climate change. We assessed the responses of soil N₂O fluxes to elevated CO₂, heat, altered precipitation, and enhanced nitrogen deposition, as well as their interactions, in an annual grassland at the Jasper Ridge Global Change Experiment (CA, USA). Measurements were conducted after 6, 7 and 8?years of treatments. Elevated precipitation increased N₂O efflux, especially in combination with added nitrogen and heat. Path analysis supported the idea that increased denitrification due to increased soil water content and higher labile carbon availability best explained increased N₂O efflux, with a smaller, indirect contribution from nitrification. In our data and across the literature, single-factor responses tended to overestimate interactive responses, except when global change was combined with disturbance by fire, in which case interactive effects were large. Thus, for chronic global environmental changes, higher order interactions dampened responses of N₂O efflux to multiple global environmental changes, but interactions were strongly positive when global change was combined with disturbance. Testing whether these responses are general should be a high priority for future research.     

A new estimation of global soil greenhouse gas fluxes using a simple data-oriented model

Soil greenhouse gas fluxes (particularly CO₂, CH₄, and N₂O) play important roles in climate change. However, despite the importance of these soil greenhouse gases, the number of reports on global soil greenhouse gas fluxes is limited. Here, new estimates are presented for global soil CO₂ emission (total soil respiration), CH₄ uptake, and N₂O emission fluxes, using a simple data-oriented model. The estimated global fluxes for CO₂ emission, CH₄ uptake, and N₂O emission were 78 Pg C yr⁻¹ (Monte Carlo 95% confidence interval, 64–95 Pg C yr⁻¹), 18 Tg C yr⁻¹ (11–23 Tg C yr⁻¹), and 4.4 Tg N yr⁻¹ (1.4–11.1 Tg N yr⁻¹), respectively. Tropical regions were the largest contributor of all of the gases, particularly the CO₂ and N₂O fluxes. The soil CO₂ and N₂O fluxes had more pronounced seasonal patterns than the soil CH₄ flux. The collected estimates, including both the previous and the present estimates, demonstrate that the means of the best estimates from each study were 79 Pg C yr⁻¹ (291 Pg CO₂ yr⁻¹; coefficient of variation, CV = 13%, N = 6) for CO₂, 21 Tg C yr⁻¹ (29 Tg CH₄ yr⁻¹; CV = 24%, N = 24) for CH₄, and 7.8 Tg N yr⁻¹ (12.2 Tg N₂O yr⁻¹; CV = 38%, N = 11) for N₂O. For N₂O, the mean of the estimates that was calculated by excluding the earliest two estimates was 6.6 Tg N yr⁻¹ (10.4 Tg N₂O yr⁻¹; CV = 22%, N = 9). The reported estimates vary and have large degrees of uncertainty but their overall magnitudes are in general agreement. To further minimize the uncertainty of soil greenhouse gas flux estimates, it is necessary to build global databases and identify key processes in describing global soil greenhouse gas fluxes.     

Combined global change effects on ecosystem processes in nine U.S. topographically complex areas

Concurrent changes in climate, atmospheric nitrogen (N) deposition, and increasing levels of atmospheric carbon dioxide (CO₂) affect ecosystems in complex ways. The DayCent-Chem model was used to investigate the combined effects of these human-caused drivers of change over the period 1980–2075 at seven forested montane and two alpine watersheds in the United States. Net ecosystem production (NEP) increased linearly with increasing N deposition for six out of seven forested watersheds; warming directly increased NEP at only two of these sites. Warming reduced soil organic carbon storage at all sites by increasing heterotrophic respiration. At most sites, warming together with high N deposition increased nitrous oxide (N₂O) emissions enough to negate the greenhouse benefit of soil carbon sequestration alone, though there was a net greenhouse gas sink across nearly all sites mainly due to the effect of CO₂ fertilization and associated sequestration by plants. Over the simulation period, an increase in atmospheric CO₂ from 350 to 600 ppm was the main driver of change in net ecosystem greenhouse gas sequestration at all forested sites and one of two alpine sites, but an additional increase in CO₂ from 600 to 760 ppm produced smaller effects. Warming either increased or decreased net greenhouse gas sequestration, depending on the site. The N contribution to net ecosystem greenhouse gas sequestration averaged across forest sites was only 5–7 % and was negligible for the alpine. Stream nitrate (NO₃ ^?) fluxes increased sharply with N-loading, primarily at three watersheds where initial N deposition values were high relative to terrestrial N uptake capacity. The simulated results displayed fewer synergistic responses to warming, N-loading, and CO₂ fertilization than expected. Overall, simulations with DayCent-Chem suggest individual site characteristics and historical patterns of N deposition are important determinants of forest or alpine ecosystem responses to global change.