The Responses Of The Ecosystem Gas Exchange And Soil Respiration To Water And Nitrogen In Stipa Breviflora Steppe

As the main Chinese grassland region and part of the world’s largest contiguous arid and semi-arid steppe ecosystem, the desert steppe of Inner Mongolian plays an increasingly important role in environmental conservation and global climate change. However, this ecosystem has been severely degraded in recent decades due to poor management and increasing human pressures. As a typical desert steppe. Stipa breviflora steppe is mainly located in the drought area in Inner Mongolia. Since nitrogen is an important limiting factor in desert steppe of Inner Mongolian, the application of nitrogen may be a useful approach to restore degraded grasslands and increase carbon sequestration. However, the uptake of applied nitrogen mainly depends on water availability. In arid and semiarid ecosystems, rainfall is often the first limiting factor for plant growth and productivity, in which case nitrogen fertilization may only be effective at increasing rangeland production in wet years. Soil respiration is a very important indicator in evaluating soil surface carbon dioxide flux and carbon cycle of terrestrial ecosystems. However, limited studies can be found to address how the water and nitrogen affect the soil respiration and the ecosystem gas exchange in desert steppe. In the present study, the main objectives were to investigate the response of soil respiration rate and the ecosystem gas exchange to soil water content and nitrogen fertilizer application and to study the interactive effects of water and nitrogen on soil respiration rate and the ecosystem gas exchange in the Stipa breviflora steppe. A comparative study of different water and nitrogen treatments was conducted in the Desert Steppe of Siziwang County in Inner Mongolia. P. R. China. The randomized complete block design was used with three replications and two nitrogen levels and three water treatments. The nitrogen rates were0and100kg N ha-1. The three water treatments consisted of control (local annual average rainfall of58mm),70%of control and130%of control.Under the nature condition, the soil respiration rate was measured by using LI-8100(Li-8100, Li-COR, Lincoln, NE, USA) in the Stipa breviflora steppe in2011. The relationship between soil water content, applied nitrogen rate and soil respiraiion rate was established by the regression analysis. The results showed that:1) water addition significant increased the soil respiration rate (P<0.05), and the maximum value of soil respiration was observed during the period of the highest soil water content (in the early of August) in the whole growing season:2) in the whole growing season, supplementary nitrogen showed a negative effect on soil respiration rate in the same precipitation, whereas nitrogen addition slightly promoted soil respiration rate in the season of relatively low precipitation (from end of May to middle of June and October);3) Compared with linearly and exponentially model, a better significant quadratic function model was observed for the relationship between soil water content and soil respiration rate. The actual change of soil respiration rate was better explained by a quadratic model.Under the nature condition, the ecosystem gas exchange was measured by using LI-6400(Li-6400, Li-COR, Lincoln, NE, USA) in the Stipa breviflora steppe in2012. Experimental factors are water supplied and N addition under natural conditions. The results were showed that the whole growing season net ecosystem CO2exchange (NEE), the total ecosystem productivity (GEP), the value of ecosystem respiration (ER) were tested increased at first and then decreased, and in the growing season (August) reached at maximum. Under N. W treatments NEE has been increased, but the other treatments NEE has been reduced. Under N, WN treatments ER has been increased, but the other treatments ER has been reduced. Under N, W, WN treatments GEP has been increased, but the other treatments GEP has been reduced. NEE, ER, GEP reached at maximum at the N treatment. The changing global climate and the predicted increasing frequency of extreme weather in Inner Mongolia in the coming years may result in changes in resource availabilities. Therefore, our research results have important implications for better managing grassland in Inner Mongolia.