黄土高原退耕地土壤团聚体有机碳来源和稳定性研究

The origin and stability of organic carbon in total soils and aggregates, as well as their response to environmental changes and human disturbances, are the urgent scientific questions in studying the C sequestration in revegetated farmland, and also important scientific basis for understanding the feedbacks between soil organic carbon cycling and climate system. In this project, by comparing the revegetated farmland and adjacent farmland, the original organic carbon and new input carbon in total soils and aggregates after land-use change will be separated through measuring natural abundance of 13C in soil organic matter. By conducting field and laboratory cultivation experiments, the mineralization of original and new input organic carbon in total soils and aggregates will be determined. The objectives of this project is to understand the origin and the redistribution of soil organic carbon in soils and aggregates and their dynamics during the grain for green project, to study how new input carbon affect the mineralization of original carbon in total soils and aggregates, to clarify the stability of soil organic carbon in both soils and aggregates derived from cropland and revegetated land and their response to environmental changes and anthropogenic disturbance. Based on these studies, we try to understand the capacity of carbon sequestration in revegetated farmland and their sensitivity to environmental factors and human disturbances, which would provide scientific evidences for assessing soil responses and precise predicting carbon sequestration in soils after the grain for green project in the Loess Plateau.

农地退耕后土壤和团聚体不同来源有机碳的稳定性及其对环境变化和人为扰动响应是退耕后土壤碳汇功能研究中需要迫切解决的科学问题，也是认识土壤有机碳循环与气候系统反馈作用的重要基础。本项目选取黄土高原退耕农地和未退耕农地为研究对象，利用土壤有机质13C自然丰度的差异辨识退耕地土壤和团聚体原有有机碳和新输入有机碳的分布和矿化，通过野外定位试验和室内模拟实验，研究不同水热变化和人为扰动下土壤和团聚体不同来源有机碳的矿化过程，以明确退耕过程中土壤和团聚体中新输入有机碳和原有有机碳的再分配特征，揭示新输入有机碳对原有有机碳矿化的影响及其制约因素，阐明退耕后土壤和团聚体原有有机碳和新输入有机碳的稳定性及其对环境因子变化和人为扰动的响应特征和适应机理，在此基础上揭示退耕后土壤碳汇功能及其对环境因子的敏感性，为退耕后土壤效应评价和土壤碳汇变化的准确预测提供科学依据。

项目围绕黄土高原退耕后土壤和团聚体碳氮的来源和稳定性，在黄土高原从南到北选取不同退耕年限和植被的样地，采集0-10和10-20 cm土层土壤样品，通过测定土壤和团聚体有机碳含量和储量、δ13C和δ15N丰度、实验室控制水热条件矿化过程监测，重点研究了农地退耕中土壤和团聚体新输入和原有碳氮的再分布、退耕地土壤和团聚体碳氮的矿化特征、不同退耕模式的土壤碳汇强度和稳定性，得到如下主要结论：（1）退耕刺槐林地中，δ13C和δ15N丰度以大团聚体最低，粉粘粒最高。新输入的有机碳和全氮主要集中在大团聚体和微团聚体中，但不同地点刺槐林地大团聚体和微团聚体的贡献不同。（2）农地退耕显著降低了土壤和团聚体的δ13C和δ15N丰度，增加了来源于林地的碳氮含量，这种影响随退耕年限的延长而增大。不同地点δ13C和δ15N丰度差异显著，而且0-10 cm土层显著低于10-20 cm土层。随粘粒含量降低，土壤、大团聚体和微团聚体δ13C和δ15N丰度逐渐增加，来源于林地的碳氮含量则逐渐降低，而且不同团聚体中来源于林地和农地碳氮的差异逐渐减小。0-10 cm土层土壤和团聚体来源于林地的碳氮的增幅大于10-20 cm土层，表明刺槐林地团聚体碳氮固存能力在0-10 cm土层更明显。因此，黄土高原退耕刺槐林地土壤碳氮的积累存在较大的时空变异性。（3）农地退耕显著增加了土壤碳氮矿化量，并受退耕年限和地点的影响。刺槐林地0-10 cm土层碳氮总矿化量是10-20 cm土层的1.43倍。土壤碳氮累积矿化量随着新输入碳氮的增加而增加，表明退耕刺槐林地新输入有机碳促进了碳氮的矿化，降低了其稳定性。项目研究结果阐明了黄土高原退耕地土壤结构特征、土壤和团聚体碳氮分布特征，辨识了退耕地土壤和团聚体碳氮来源特征，揭示了退耕地土壤碳氮矿化特征及影响因素，为深入认识退耕条件下土壤碳氮固存机制和稳定性提供了科学依据。