Responses Of Ecosystem CO₂ Exchange To Stocking Rate In Stipa Breviflora Desert Steppe

Grassland is one of the most important terrestrial ecosystems on the earth, which stores nearly a third of the organic carbon in the terrestrial ecosystem. Grazing as one of the most long used methods in grassland ecosystem, not only affects the structure and function of ecosystem, but has a far-reaching influence on the global carbon cycle.This study was based on Inner Mongolia desert steppe ten years of continuous grazing control experimental platform. Using a completely randomized experimental design, set up four different stocking rates, i.e. no grazing control (CK), light grazing (LG), moderate grazing (MG) and heavy grazing (HG). LI-6400 portable photosynthesis system and closed box method were used to observe the ecosystem CO2 exchange in the growing season (May to Oct.) and the non-growth season (Nov. to March) in 2014 to 2015. At the same time, we also measured the aboveground and belowground biomass, precipitation, soil temperature and moisture etc., aims to analyze the changes of aboveground biomass, underground biomass, soil temperature, soil moisture and carbon flux under four different stocking rates and the relationship between environmental factors and ecosystem CO2 exchange. The main results are as follows:1. The standing crop of desert steppe decreased with the increasing stocking rate, and CK plot was significantly higher than that of MG and HG plot (P< 0.05); Net aboveground biomass had the same trend. The 0-40 cm underground biomass in the vertical distribution performance a decrease with the increase of the depth, that is,0-10cm biomass was significantly higher than that of other layers. At the same time, with the increase of stocking rate, underground biomass gradually decreased, and CK plot was significantly higher than that of HG plot (P< 0.05). However, underground net primary productivity showed the trend of LG>CK>MG>HG.2. The stocking rate had no significant difference on the soil temperature, but had significant effect on soil moisture. In 2014, the soil moisture was LG>CK>MG>HG, and LG plot was significantly higher than MG and HG plot.3. With the increase of stocking rate, the gross ecosystem productivity (GEP) gradually decreased, and the ecosystem respiration (ER) showed a trend of CK>LG>HG>MG and CK plot was significantly higher than other plots and the absorption of net ecosystem exchange (NEE) was reduced, even in 2015 severe C release in heavy grazing, but in the whole growing season, NEE still was C absorption.4. Along with the increase of aboveground and underground standing crop, the absorption of GEP was increased and the release of ER was increased, while NEE changed from release to absorption. There was a linear relationship between ecosystem CO2 exchange and soil temperature, humidity, temperature and precipitation. With the increase of soil moisture and precipitation, NEE, ER and GEP all increased, and decreased with the increase of soil temperature and air temperature. The stepwise regression results showed that the temperature was the main factor affecting the NEE, ER and GEP.