Abstract:

The carbon cycle of restoring steppe in degraded land is one of the important parts of the global terrestrial carbon cycle. The study of soil respiration in restoring steppe plays a very important role in accurately evaluating the carbon budget of restoring steppe and its function in the global carbon budget. As we know, temperature and soil water content are two important environmental factors in understanding the soil respiration process in terrestrial ecosystems. To quantify their allocation and the relationship between soil respiration and temperature and soil moisture, we conducted field experiments to investigate soil respiration from June to September 2001. The effects of heat and water factors on soil respiration of restoring Leymus chinensis steppe in degraded land was studied at Baiyinxile pasture (43°55′N, 116°31′E, with an altitude of about 1 200 m) in Inner Mongolia. This region, un-grazed in June, has a typical temperate and semi-arid climate and the community structure of the vegetation consisted mainly of two species, Leymus chinensis and Stipa grandis; the soil type was classified as chestnut. Daily variation was investigated by taking measurements once an hour from 6∶00 to 19∶00 every ten days during the whole growing season using an enclosed chamber method. The results indicated that environmental factors affecting soil respiration ranged as soil water content > temperature, and the soil water content in the upper 10 cm of the soil was of real significance in all of the environmental factors. We analyzed the relationship between daily average soil respiration rate (6∶00-19∶00) and daily average soil water content (%) using the partial correlation analysis of the SPSS and the coefficients showed that soil water content of 7.5% and 18.4% were two thresholds affecting the soil respiration rate. Soil water content could be divided into three levels: <7.5%, 7.5%-18.4% and >18.4% based on their effects on soil respiration. When soil water content was less than 7.5%, soil temperature was the dominant factor of soil respiration. Soil respiration (Rs) could be expressed as an exponential relationship with soil temperature (t) at the depth of 5 cm and their relationship was Rs=4.07t1.098 (w<7.5%, R2=0.49, n=80, p<[KG-*2]<0.01). When soil water content was more than 7.5%, soil respiration was affected by soil temperature (t), soil moisture (w)and the interaction of the two, and their relationship was Rs=-147.693+5.11w+6.012t+1.214wt (w>7.5%, R2=0.81, n=69,<[KG-*2]<0.01). When soil water content was close to the water wilting coefficient, about 6.0%, plant root respiration had few contributions to soil respiration. When soil water content was more than 7.5%, soil respiration mainly comes from the metabolism and other living activities of the plants and microbes. Thus, the contributions of soil and plants respectively to soil respiration might be explained from environmental factors. This research will help to develop soil respiration models and to understand accurately the carbon budget of terrestrial ecosystems. Our research implied that drought in the growing season in the year of 2001 tended to reduce the soil respiration rate but temperature did not. There were only three times when the soil water content was more than 18.4%, and so, because of the limitation of the data, the threshold of soil water content of 7.5% must be verified in future of experiments.