放牧和开垦对典型草原土壤温室气体通量的影响研究

The rewetting of dry soils and the freeze-thaw cycles of the soils are short-term, transitional phenomena in terms of hydrology and thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem CO2 and CH4 flux is increasing recognized.The rewetting of dry soil in the growing season is the dorminant phenomenon of the temperate grassland in northern China and limit the mojorities of the ecosystem process, for exmaple biomass accumaltion and carbon cycle. Freeze-thaw cycle is another important natural phenomenon in early spring and late autumn-winter season of the temperate grassland which contribute great effects on the grassland soil CO2 and CH4 flux. However, besides this natural paremeters which can impact soil gas fluxes,the utilization ways and intensities such as grassland reclaimation, winter grazing and overgrazing etcs.also play important roles in soil gas fluxes. Our main objectives of this research were to :(1) how the rewetting of dry soils and thawing of freeze soils impact on soil CO2 and CH4 flux under different grassland management practices as grazing strategies and stocking rate, grassland convertion to cropland etc.(2)discuss the potential underlying mechanism and drivers of variation of soil CO2 and CH4 fluxes following rewetting and thawing;(3)identify the human disturbances of the grassland such as grazing strategies and grassland reclamation on the soil fluxes magnitude following rewetting and thawing;(4)to optimize the management of the land use/land cover in agro-pastoral transitional zone of north China to mitigate the soil green house gases.

通过非生长季节土壤冻融过程和生长季节土壤干湿交替过程两种自然现象对中国北方典型草原区不同类型生态系统土壤CH4和CO2通量的影响研究，了解放牧（放牧强度）和开垦等人类活动对生态系统土壤温室气体通量的影响效应及影响机制，确定半干旱草原区不同生态系统类型及利用强度下CO2和CH4通量的年度变化规律，以及与土壤冻融模式和生长季节降水模式的定量关系；探讨土壤养分含量及微生物区系等对土壤冻融循环次数、土壤干湿交替格局及强度的响应方式，阐明草地放牧强度和草地开垦等活动对土壤温室气体通量的影响效果，揭示北方典型草原区不同生态系统类型土壤温室气体通量变化的主要驱动因子及其对自然气候干扰的响应机制，为典型草原区生态系统的适应性管理提供理论依据。

陆地生态系统2/3以上的碳贮存在土壤中，土壤呼吸作用是陆地生态系统向大气输出碳的主要途径，是化石燃料排放量的10倍以上。全球草地面积占陆地面积的10%，草地碳储量变化对陆地生态系统的碳过程有重要影响。农业开垦、城市化、过度放牧和荒漠化等人类活动显著降低草地碳库功能及土壤吸收甲烷的能力。草地管理方式决定草地是温室气体的源或汇，能减缓草地温室气体排放。本项目通过放牧和开垦等人类活动对北方农牧交错区土壤CH4和CO2通量的影响，阐明各类型生态系统土壤呼吸的主要影响因子，确定CO2和CH4通量的季节和年际变化率；探讨土壤养分含量变化对草地利用的响应，分析土壤温室气体变化的主要驱动因子，为农牧交错区生态系统适应性管理提供依据。. 草地放牧生态系统CO2净交换（NEE）及其组分（GEP和Re）受气候变化和放牧率共同调控，放牧率通过改变地上生物量直接影响NEE大小，通过改变草地土壤温度、土壤含水量以及土壤养分状况等微环境间接影响NEE变化。草地中度放牧NEE生长季节累积量为1,005±45 g C m-2，比UG和HG高31%和98%。草地开垦成农田和多年生人工草地显著增加土壤呼吸，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。UG，MG，HG，CL和PP土地利用类型非生长季节土壤CO2排放分别为307，241，210，308 和374 mg C mg−2 。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。.北方农牧交错带放牧系统均是大气CH4净排放源，放牧强度调控CH4源的大小。各放牧强度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%。草地延迟放牧和中度放牧有利于土壤CH4的吸收。土壤CH4年吸收量可以分别抵消DG，MG和HG放牧率下绵羊、羊圈和粪尿等年CH4排放量的29.7 ± 5.6, 15.9 ± 4.3 和6.8 ± 1.0% 。不同放牧率下，绵羊日增重(g day–1)的CH4排放量分别是0.21 (DG), 0.32 (MG)和0.37 (HG) g CH4–C。延迟放牧（DG）能增加草地牧草供应量，提高家畜生产性能，