放牧对草原土壤表层有机碳储量及稳定性的影响机制研究

Grassland soil stores 39% of soil organic carbon (SOC) of the land surface. Grasslands in China cover 40% of the land's surface area, and play a critical role in the global carbon budget. Composition and sources of SOC has high heterogeneity, the stability of the SOC fractions and variations of the SOC residence time are significantly different. Accurate evaluation of grassland ecosystem for SOC storage and carbon sequestration potential changes are benefit to understand the relationship between grassland carbon cycle and the intensity of grassland utilization, and global climate change. It is also significance to determine exact grassland management practice to enhance grassland carbon sequestration potentiality. Livestock grazing is the main biological factor to impact SOC storage. Grazing effects on grassland soil physical and chemical properties and biological chemistry process through herding behavior as defoliation, trampling and feces excretion, which regulates the grassland SOC sequestration potentiality, residence time and stability. The objectives of this study are: 1) quantify the relationship of SOC storage and grazing intensity; 2) determine the changes of soil particle size fraction of SOC, aggregate structure, biological activity pool (such as microbial biomass carbon and carbon mineralization potential) with the grazing intensity; 3) integrate vegetation production, plant litter and root biomass and the turnover rate, clarify the mechanism of grazing intensity effects on SOC storage, carbon fraction and turnover rate; 4) determine the response indicators and threshold values to predict SOC storage changes of the grassland affected by grazing intensity.

草地土壤有机碳储量约占陆地有机碳储量的39%，中国草地面积约占国土面积的40%，在全球碳平衡中具有重要作用。土壤有机碳来源和组成的异质性强，各组分的稳定性及周转滞留时间差异很大，准确评估草地土壤有机碳储量及稳定性对了解草地碳循环和草地利用强度、气候变化的关系，确定增强草地固碳潜力和碳储量的草地精准管理模式具有重要意义。放牧是影响草地土壤表层碳储量及稳定性的主要生物因子，采食、践踏和排泄粪尿等放牧行为通过影响草地土壤理化性状和生物化学过程，调控草地土壤固碳潜力、有机碳滞留时间及稳定性。草地土壤表层有机碳变化与放牧的定量研究，确定土壤有机质中粒径组分、团聚体结构、生物活性碳库（如微生物生物量碳和碳矿化潜力）随放牧强度变化的时空响应规律，结合植被生产力、凋落物和根量及周转速率，阐明放牧强度对草地土壤有机碳库组分、储量及周转速率的影响机制，确定草地土壤有机碳库变化对放牧响应的指示指标及阈值。

草地土壤有机碳储量约占陆地有机碳储量的39%，在全球碳平衡中具有重要作用。家畜采食、践踏和排泄粪尿等放牧行为通过影响草地土壤理化性状和生物化学过程，调控草地土壤固碳潜力和有机碳稳定性。草地土壤表层有机碳变化与放牧的定量研究，确定土壤有机质中粒径组分、团聚体结构、生物活性碳库（如微生物生物量碳和碳矿化潜力）随放牧强度变化的时空响应规律，结合植被生产力、凋落物和根量及周转速率，阐明放牧强度对土壤表层活性有机碳的影响机制，确定草地土壤有机碳库变化对放牧响应规律。.草地放牧显著降低土壤CO2和CH4通量的时空分布格局，生长季节降水量对非生长季节土壤CO2排放具有显著的滞后效应。气候的年际波动能减弱土地利用方式对土壤CO2排放的影响。放牧强度和草地植物种丰富度不改变土壤微生物群落结构。放牧通过家畜的选择性采食对土壤真菌群落的影响更大，其对土壤细菌和真菌群落的贡献率分别为25.42%和54.53%。土壤pH和有机碳对细菌群落结构的影响最大，土壤电导率和NO3—N对真菌群落结构的影响最大。在气候变化背景下，半干旱草地土壤微生物群落在放牧压力持续作用下，能够保持结构稳定。.北方典型草原放牧系统均是大气CH4净排放源，放牧强度调控CH4源的大小，不同放牧强度下，系统CH4收支分别为：–1.2 ± 0.1 （不放牧），5.7 ± 0.6 (延迟放牧), 12.2 ± 2.3(中牧), 16.5 ± 2.0 (重牧) kg CH4–C ha–1 year–1。非生长季节典型草原土壤的CH4吸收量占全年总土壤吸收量的37.7-50.0%，在草地CH4收支平衡估算中应该充分考虑其重要性。CH4年均吸收量的空间格局与年均土壤孔隙含水量呈正相关。土地利用变化通过降低土壤水分显著和增加沙粒含量促进了中国温带草原土壤CH4的年吸收。在氮限制的生态系统中，施用不超过100 kg N ha−1 yr−1的氮肥显著促进了土壤CH4的吸收。因此，草地利用变化和低施氮促进了中国温带草原土壤CH4的吸收。