氮沉降对气候过渡带森林土壤有机碳稳定性的影响

In recent decades, atmospheric nitrogen (N) deposition has been increasing dramatically. N deposition and the changes of plants and soil microbes induced by it could profoundly impact the stability of forest soil organic carbon, and then affect the soil carbon cycling in forest ecosystems. In the studies of N deposition, mimicking N deposition by adding N directly onto the understory plants and/or forest floors, which neglects the processes of the N retention and N transformation occurring in canopy that are critical for forest ecosystems, could not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Therefore, in this study, an innovative method that simulates N deposition by adding N directly onto canopy will be employed to examine the effects of atmospheric N deposition on the stability of soil organic carbon in the deciduous broad-leaved mixed forest in the transitional zone from a northern subtropical to warm temperate climate. Sequential fumigation-incubation procedures, density fractionation and chemical fractionation methods based on soil aggregate fractionation will be employed to analyze the stability of soil organic carbon. Meanwhile, the chemical properties of leaf litter and fine root, soil microbial community structure, enzymes activities, and soil physiochemical properties will be analyzed. After that, the effects of N deposition on the stability of soil organic carbon will be revealed from the aspects of soil microbial utilization, physical protection, and chemical oxidability in the deciduous broad-leaved mixed forest in the transitional zone from a northern subtropical to warm temperate climate. Ultimately, this study will provide a scientific basis for accurately predicting how the soil carbon cycling in forest ecosystems responds to the intensifying atmospheric N deposition in the future.

近年来大气氮沉降不断加剧，氮沉降及其引发的植物和土壤微生物的变化极有可能引起森林土壤有机碳稳定性的变化，从而影响森林土壤碳循环。在氮沉降对森林生态系统影响的研究中，以往的林下氮添加方式忽略了林冠氮截留和氮转化等重要过程，难以真实反映氮沉降对森林生态系统的影响。据此，本项目拟借助国内首个“林冠模拟氮沉降”野外控制实验平台，以北亚热带-暖温带过渡带的落叶阔叶混交林为研究对象，采用连续熏蒸培养法和基于团聚体分级的密度分组法和化学分组法分析土壤有机碳的稳定性，监测凋落物及细根化学性质、土壤微生物群落结构、酶活性和土壤理化性质的变化，从微生物利用性、物理保护性和化学氧化性多角度揭示氮沉降对气候过渡带森林土壤有机碳稳定性的影响，为准确预测未来森林土壤碳循环对大气氮沉降增加如何响应提供科学依据。

大气氮沉降增加是全球变化的重要特征之一，它可通过改变植物养分吸收和土壤养分有效性等过程引发土壤有机碳（SOC）稳定性的变化，从而影响土壤碳循环。在森林生态系统中以往的模拟氮沉降研究大部分都是基于林下氮添加，该方式忽略了林冠氮截留与转化等重要过程，无法真实反映氮沉降对森林生态系统结构和功能的影响。本项目借助国内首个“林冠模拟氮沉降”野外控制实验平台，开展了林冠和林下模拟大气氮沉降对北亚热带-暖温带过渡带的落叶阔叶混交林土壤有机碳稳定性的影响研究。结果发现，林冠和林下氮添加对0-20cm土层土壤微生物生物量及群落结构无显著影响，且没有显著影响土壤易分解有机碳（LOC）浓度，但降低了10-20cm土层土壤LOC对SOC的贡献率；9年的林冠和林下氮添加没有改变土壤团聚体粒径分布比例，林冠高氮添加可以促进有机碳在团聚体各粒径中的固存，林冠低氮添加和林下氮添加处理对土壤团聚体各粒径有机碳含量无显著影响；林冠和林下氮添加对土壤总的易氧化有机碳（ROC）含量及ROC对SOC的贡献率均无显著影响，林冠高氮添加则有利于土壤ROC在团聚体各粒径中的积累；氮添加能够提高土壤氨基糖的累积，林冠高氮添加显著增加了0-10cm和10-20cm土层土壤中胞壁酸、氨基葡萄糖和总氨基糖的浓度，林下高氮添加增加了0-10cm土层氨基半乳糖、胞壁酸、氨基葡萄糖和总氨基糖的浓度，10-20cm的氨基糖则无显著变化；林冠低氮添加增加了0-10cm土层中的胞壁酸浓度，林下低氮添加对0-10cm和10-20cm土层中的氨基糖均无显著影响。上述结果表明，不同SOC组分对氮沉降响应有差异，微生物残留物C更为敏感，林冠氮添加对土壤碳稳定性的影响大于林下氮添加，且高氮添加效应高于低氮添加。因此我们提出在研究氮沉降对森林土壤有机碳循环的影响时需重视林冠氮截留过程的影响。该研究从微生物利用性、物理保护性和化学氧化性等多角度揭示氮沉降对气候过渡带森林土壤有机碳稳定性的影响，为准确预测未来森林土壤有机碳循环对大气氮沉降增加的响应提供科学依据。