不同碎化程度牦牛粪便斑块对藏北高寒草原土壤氮转化和N2O排放的影响机制

Yak dung deposition is an important pathway of nutrient return and redistribution on alpine grasslands in the Qinghai-Tibetan Plateau. Soil nitrogen transformations in alpine grassland ecosystems is supposed to be greatly influenced by yak dung return, although fragmentizing yak dung into small pats may be more beneficial to plant growth compared with large pat, the effects of fragmented yak dung pats on soil nitrogen transformations and nitrous oxide emissions are still unknown. The proposed research is to be conducted on a typical semiarid alpine steppe in northern Tibet, China, placing original yak dung pat (ODP), four equal parts of original dung pat (FDP), nine equal parts of original dung pat (NDP), sixteen equal parts of original dung pat (SDP) and a control (CK) on different underlying surfaces, including PVC plate, soil and soil-plant system. There are four main themes within the proposed research. The first is to evaluate nitrogen dynamics in the dung and soil during yak dung decomposition; the second is to investigate the plant growth and its nitrogen storage; the third is to monitor in situ N2O emissions by using static chamber technique and the fourth is to explore the influences of different fragmented yak dung pats on the main nitrogen transformation processes and the related microorganisms by using 15N tracing technique and modern molecular biotechnology, and then to understand the possible relationships between these microorganisms and nitrogen transformation processes; to know the key affecting factors of different treatments on N2O emissions and plant growth, so as to propose a measure of fragmentizing yak dung pat can both induce plant growth and decrease N2O emission. The fruits of the research will be exposed to provide deep insights into the nitrogen transformation mechanisms and to further understand the specificity of nitrogen dynamics in alpine grassland ecosystems. The sustainable development of grazed grassland ecosystems in the Qinghai-Tibet Plateau is also be expected to benefit from the proposed research.

牦牛粪便排泄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，碎化牦牛粪便斑块可能会改善土壤氮素供应和环境条件，继而可能比原状斑块更利于植物生长，但其对土壤氮转化和N2O排放的影响机制尚不清楚。本项目拟以藏北高原高寒草原为研究对象，研究不同碎化程度牦牛粪便斑块位于不同介质（PVC板框、土壤和土壤—植物体系）表面时粪便降解过程中粪便和土壤氮素动态变化、斑块区域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。阐明微生物在驱动土壤氮转化过程中的作用机理；揭示不同碎化程度粪便斑块对斑块区域N2O排放影响的关键控制因素；提出在促进植物生长的前提下减少高寒草原土壤N2O排放的碎化牦牛粪便施用措施，为合理施用牦牛粪便及完善放牧牲畜排泄物影响下高寒草原土壤氮循环研究、减少温室气体排放和维持草原生态系统的可持续发展提供科学依据。

牦牛粪便排泄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，碎化粪便斑块可能会改善土壤氮素供应和环境条件，继而可能比原状斑块更利于植物生长，但其对氮素迁移转化和N2O排放的影响机制尚不清楚。在这一背景下，本项目以藏北高原高寒草原为研究对象，将不同碎化程度牦牛粪便斑块位于砾石表面和土壤—植物系统，研究了粪便降解过程中粪便和土壤氮素动态变化、斑块区域植物生长及其氮储量和N2O排放规律。研究发现，1）斑块碎化增加了可溶性有机碳（DOC）和有机氮（DON）的淋溶，但是却降低了NO3−-N的淋溶，碎化程度最大的处理（1/16碎化斑块）也可降低NH4+-N的淋溶；2）斑块碎化促进了粪便自身N2O排放，不过仅原状斑块和1/4碎化斑块显著增加了土壤N2O排放，1/8和1/16碎化斑块则未影响土壤N2O排放；3）不同碎化程度牦牛粪便斑块对粪便自身和土壤的硝化、反硝化功能基因也有显著影响，这一影响与粪便施加后的降解时间和下垫面类型密切相关；4）牛粪斑块碎化有利于提升粪便施加后的环境友好性，碎化程度最大的1/16碎化斑块处理是个值得推荐的草地放牧牦牛粪便管理措施，其基于氮吸收量的N2O累积排放量最低；5）牛粪和牛尿均可显著增加土壤N2O排放，硝化抑制剂以及其与脲酶抑制剂的结合施用可以降低这一排放，牛羊排泄物的N2O排放系数分别仅为IPCC（政府间气候变化专门委员会）2006年所公布缺省值的30%和26%。以上研究结果为完善放牧牲畜排泄物影响下高寒草原土壤氮循环研究和维持草原生态系统的可持续发展提供科学依据，此外，这些结果还将为指导高寒草地施肥、提高草地生态系统生产力以及维护生态安全屏障功能提供基础资料和技术支持。