牦牛主要放牧行为对青藏高原东缘高寒草甸土壤N2O排放的影响机制

Yak is one of the dominant livestock species on the Qinghai-Tibetan Plateau (QTP), and soil nitrogen transformation in alpine grassland ecosystems is greatly influenced by yak grazing, which is mainly represented in three different forms: defoliation, trampling and excreta (urine and dung) deposition. However, grazing has been predominantly studied in experimental trials with the combined effect of all forms, the effect of individual form on soil nitrogen transformation, nitrous oxide (N2O) emission and plant growth are poorly studied. The proposed research is to be conducted on a typical alpine meadow in eastern QTP, including a control without defoliation (CD), selective defoliation (SD), non-selective defoliation (ND), and a control without trampling (CT), low-intensity trampling (LT), high-intensity trampling (HT), as well as a control without excreta deposition (CE), yak urine deposition (UE), yak dung deposition (DE), and yak urine plus dung deposition (UDE) treatments. There are four main themes within the proposed research. The first is to evaluate nitrogen dynamics under different grazing effects; the second is to investigate the plant growth and its nitrogen storage; the third is to monitor in situ N2O emission using the static chamber technique; and the fourth is to explore the impact of different grazing effects on the main nitrogen transformation processes and the related microorganisms using 15N tracing technique and modern molecular techniques. To know the key factors affecting N2O emission and plant growth in different treatments; to understand the possible relationships between microorganisms and nitrogen transformation processes and N2O emissions; and to propose a possible measure of yak grazing that can both favor plant growth and decrease N2O emission. The results of the research will be explored to provide insights into the mechanisms affecting nitrogen transformation and to further understand the specificity of nitrogen dynamics in alpine grassland ecosystems. The proposed research will benefit the mitigation of greenhouse gas emission and the sustainable development of grazed grassland ecosystems in the QTP.

牦牛放牧是影响青藏高原高寒草地生态系统氮循环的主要因素之一。放牧牦牛通过采食、践踏和排泄等主要行为来影响土壤氮素动态和植物生长，但以往大多数研究反映的都是多种行为的综合作用，而有关三种主要放牧行为对土壤氮转化、N2O排放和植物生长影响程度和作用机制差异的认识尚不清楚。本项目拟以青藏高原东缘典型高寒草甸生态系统为研究对象，研究不同放牧行为影响下土壤氮素动态变化、植物生长及其氮储量和N2O排放；同步运用15N成对标记法和分子生物学技术，分别测定土壤氮初级转化速率和氮转化相关微生物的特性。揭示不同放牧行为影响土壤N2O排放的关键控制因素，阐明微生物在驱动土壤氮转化和N2O产生过程中的作用机理；提出在促进植物生长的前提下减少高寒草甸土壤N2O排放的建议性放牧管理措施，为完善放牧活动影响下高寒草地土壤氮循环研究、减少温室气体排放和维持草地生态系统的可持续发展提供科学依据。

放牧牦牛通过采食、践踏和排泄等主要行为来影响土壤氮循环过程，但以往大多数研究反映的都是多种行为的综合作用，而有关单独放牧行为对土壤氮循环关键过程影响机制的认知还尚不清楚。鉴于此，本项目以青藏高原东緣典型高寒草甸生态系统为研究对象，研究不同采食强度、不同践踏强度和不同排泄行为对土壤N2O排放、土壤氮转化过程、土壤氮转化相关微生物的影响。原位观测发现，重度采食降低了N2O排放，而轻度采食无显著影响；践踏增加了N2O排放，且重度践踏的促进效果更大；排泄处理均增加了N2O排放，但尿液和尿液+粪便排泄处理之间无显著差异。在模拟不同放牧行为处理两年后，采食降低了土壤净矿化速率，但对硝化酶活性无显著影响，重度采食降低了净硝化速率和反硝化酶活性；践踏对净矿化速率和硝化酶活性无显著影响，但显著降低了净硝化速率，重度践踏降低而轻度践踏增加了反硝化酶活性；粪便排泄增加了净矿化和净硝化速率，降低了反硝化酶活性，但对硝化酶活性无影响；尿液排泄对净矿化和净硝化速率无影响，增加了硝化酶活性，但对反硝化酶活性无影响；尿液+粪便排泄对净矿化和净硝化速率及硝化酶活性均无影响，但显著降低了反硝化酶活性。不同放牧行为对AOA-amoA、AOB-amoA、nirS、nirK基因丰度有不同程度的影响，但对nosZ基因丰度均无影响。同时，践踏显著增加了表层0-10 cm土壤碳储量，采食并不影响0-30 cm土壤碳储量，相较于践踏和采食，排泄显著降低了10-30 cm土壤碳储量。此外还发现，不论是地上植被还是草层被破坏的情况下，翻耕均增加土壤N2O排放，但植物残体覆盖的影响结果却不尽一致，相较于翻耕，翻耕耦合植物残体覆盖会降低N2O排放；再者，翻耕会降低土壤碳储量，单一植物残体覆盖对土壤碳固存的影响较弱，翻耕后覆盖植物残体则更有利于土壤碳的封存。上述研究结果为完善牲畜放牧影响下草地土壤氮循环研究提供了理论依据。再者，这些结果还可为减少温室气体排放、提高草地生态系统固碳能力和维持草地生态系统的可持续发展提供科学依据。