牲畜排泄物返还对藏北高寒草原土壤氨挥发和N2O排放的影响

Grazing livestock excreta is one of the most important nitrogen sources in natural grassland ecosystems. Nitrogen cycling in nitrogen-unsaturated grassland ecosystems, especially in alpine grassland ecosystems, is supposed to be greatly influenced by excretal returns. However, the impact of excretal returns on nitrogen transformation is still not clear. Previous studies have shown that the total amount of nitrogen excreted by the yak and Tibetan sheep in Qinghai-Tibet Plateau was not less than 370 million kilograms. The proposed research is to be conducted in a typical semiarid alpine steppe in Northern Tibet, China, which involves five treatments: yak dung (YD), yak urine (YU), Tibetan sheep dung (SD), Tibetan sheep urine (SU) and control (CK) where no excreta application. There are two main themes within the proposed research. The first is to evaluate seasonal variations of NH3 and N2O emissions and their key affecting factors from the five treatments through in situ monitoring ammonia volatilization, N2O fluxes and some environment variables. The second theme is to explore the influences of excretal returns on the main nitrogen transformation processes (nitrogen mineralization, nitrification and denitrification) and the related microorganisms and enzymatic activities by using 15N isotope dilution technique, acetylene inhibition technique and some biological methods, and then to understand the possible relationships between these influences and soil N2O emissions in different treatments. The fruits of the research will be exposed to provide deep insights into the nitrogen transformation mechanisms and to improve the accuracy of regional emission inventory for NH3 and N2O in alpine grassland ecosystems. The sustainable development of grazed grassland ecosystems in Qinghai-Tibet Plateau is also be expected to benefit from the proposed research.

牲畜排泄物是天然草地生态系统氮素输入的主要来源之一。作为氮非饱和的天然草地尤其是高海拔的高寒草地，牲畜排泄物返还势必会改变草地生态系统氮循环过程，但目前尚不清楚其作用机制。申请者前期研究发现青藏高原牦牛和藏绵羊排泄物氮量可超过37万吨/年。本项目拟以藏北高原高寒草原为研究对象，通过原位测定牦牛和藏绵羊排泄物返还条件下土壤NH3挥发和N2O排放通量，阐明牲畜排泄物返还条件下土壤NH3挥发和N2O排放的季节变化规律及其关键控制因素，明确牲畜排泄物对土壤氨挥发和N2O排放的影响程度；利用15N同位素稀释法、乙炔抑制技术和相关生物学手段，剖析牲畜排泄物返还对土壤氮矿化、硝化和反硝化速率的影响效应，建立土壤氮素转化过程及相关微生物和土壤酶活性与N2O排放之间的可能连接关系，研究结果将为深入了解高寒草原土壤氮素转化特征、提高NH3和N2O排放清单准确性以及维持生态系统可持续发展提供基础资料和理论依据。

牲畜排泄物是天然草地生态系统养分输入的主要来源之一，牲畜排泄物返还势必会改变草地生态系统碳氮循环过程和温室气体排放，在高海拔地区的天然高寒草地生态系统中这一现象可能会更加明显。在这样的背景下，本研究选择藏北申扎地区半干旱高寒草原为研究对象，通过野外控制试验和室内培养实验相结合的方法，探讨高寒草地土壤氮素转化过程和温室气体排放对牲畜排泄物施加的响应规律。野外原位观测发现，牦牛粪便处理N2O累积排放量显著高于藏绵羊粪便和对照处理，粪便施加后前期N2O排放量的不同导致了不同处理间N2O排放的差异，反硝化作用增强可能是牦牛粪便处理N2O排放量增加的主要原因，而藏绵羊粪便处理的N2O排放则主要与硝化作用有关。此外，牦牛粪便施加会显著降低植物的地上生物量，藏绵羊粪便添加则能略微提高植物生物量及植物体碳氮储量，不过并无显著效应。室内研究发现（1）牦牛和藏绵羊粪便在低土壤含水量时均能显著促进N2O排放，且前者的促进效果更大，但在高土壤含水量时，牦牛粪便则可大幅度促使深度反硝化的进行而使得其N2O排放量低于藏绵羊粪便处理；（2）牲畜粪便的添加能增加土壤的总N矿化速率和NH4+固持速率，但是牲畜粪便添加后土壤NO3-固持速率的增加和总硝化速率的减少会降低NO3-累积带来的淋溶风险；（3）牦牛粪便施加显著增加了CH4和CO2排放，而藏绵羊粪便施加则仅能略微降低CH4的氧化能力。牦牛尿液比等氮量的尿素更能促进土壤N2O和CO2排放，尿液和尿素添加均能显著降低CH4的氧化能力。研究揭示牲畜排泄物与高寒草地土壤养分供应和全球变化密切相关，研究结果为深入了解高寒草地土壤碳氮转化特征、提高高寒区域温室气体排放清单的准确性以及维持生态系统可持续发展提供科学依据。