大气CO2升高对土壤碳循环影响的研究进展

土壤有机碳是陆地碳库的重要组成部分，其积累和分解的变化直接影响全球的碳平衡。据估计，全球土壤（表层1m）有机碳积累总量相当于大气中碳总量的2～3倍。土壤是温室气体的源或汇，土壤碳库的变化将影响大气C02的浓度，因此，土壤碳库对人类活动的响应也是全球碳循环和全球变化研究的热点。在全球变化的大背景下，大气CO2升高导致植被生态系统碳平衡的改变进而对土壤碳循环产生影响。总结了陆地生态系统碳循环对大气C02浓度升高响应的主要生物学机制及过程，简述了大气C02浓度升高对影响土壤碳输入和输出的各因素的研究进展，并指出未来研究的主要方向。在大气C02浓度升高条件下，陆地生态系统碳循环的变化主要反映在以下几个方面：1）不同类型植物群落的净初级生产力（NPP）显著增加，但湿地植物的净初级生产力也有可能降低；2）光合产物向根系分配的数量增加，地上／地下生物量降低，根系形态发生变化，根系周转速率和根系分泌等过程的碳流量提高；3）植物含氮量降低，C／N提高，次生代谢产物增加，微生物生长受到抑制，植物残体分解速率降低；4）土壤呼吸速率显著增加，提高幅度受植物类型与土壤状况的影响；5）进入土壤的植物残体及分泌物的数量和性质影响土壤酶的活性，脱氢酶和转化酶活性增加，酚氧化酶和纤维素酶受植物类型与环境条件的影响；6）土壤中真菌的数量的增加幅度要高于细菌；7）CH4释放量增加，在植物的生长期表现更为明显。由于陆地生态系统碳循环的复杂性，研究结果仍有很大的不确定性。大气C02浓度升高与全球变化的其它表现间的交互作用将是今后研究的重点，同时由于土壤碳循环是一个由微生物介导的生物地球化学循环过程，因此，加强陆地生态系统碳循环的微生物机制研究也将为全面理解碳循环的过程提供更加准确的研究理论基础。

Research progress on the effects of elevated C02 concentration on carbon cycling

Soil organic carbon is an important component of terrestrial carbon pools, and it has direct effects on the global carbon balance. It is estimated that the amount of organic carbon accumulated in the surface soil of 1 m is equivalent to 2 to 3 times of carbon in the atmosphere globally. Soils are acting as the source or net sink of greenhouse gases and the change of soil carbon stock will influence the concentration of atmospheric CO2. The response of soil carbon stock to human activities is also the hotspot of global carbon cycle and global change research. Under the background of global climate change, elevated CO2 will cause the change of vegetation carbon balance and affect the soil carbon cycle eventually. This paper summarized the biological mechanisms on the response of terrestrial carbon balance to elevated COz and recent advance in affecting soil carbon input and output factors. Elevated atmospheric CO2 concentration will influence terrestrial carbon cycling in several aspects： 1） Net primary productivity （NPP） of different plant communities will increase significantly, but the NPP of plant communities growing on wetland may decrease; 2） Photosynthetic products allocated to root will increase and thus aboveground/belowground biomass will decrease, root morphology changes and carbon flux induced by root turnover rate and root exudation will increase; 3） Plant N concentration will decrease and thus C/N ratio will increase, the increase of secondary metabolites will lead to the decomposition rate of plant residues reduced because of the microbial activity inhibited; 4） A significant increase in soil respiration rate will be influenced by plant types and soil environmental conditions; 5） The quantity and characteristics of plant residues and exudation into soil will affect soil enzyme activity, dehydrogenase and invertase activities will increase, meanwhile, phenol oxidase and cellulose activities will be influenced by plant types and environments; 6） The increase of soil fungi biomass will be bigger than the increase of bacteria biomass; 7） The amount of CH4 emissions will increase, especially at the plant growing season. Because of the complexity of terrestrial carbon cycling, the researches still remain a lot of uncertainties. In the future research, there is an urgent need to address the interaction among elevated atmospheric CO2 concentration and other global change factors including N deposition, climate warming, drought and so on. As soil carbon cycle is a microbe-mediated biogeochemistry process, therefore, it is indispensable to strengthen microbial mechanism study of terrestrial carbon cycling, which will also provide a comprehensive understanding of carbon cycle.