| Understanding the seasonal dynamics of soil N pools, net N mineralization, and litter decomposition in grassland ecosystems, and their responses to environmental change and human disturbance, is very important to grasp the degradation mechanism of grassland, and could provide basic theory for grassland using in reason and ecosystem sustainable development. So based on the long-term grazing experiment set up by the Inner Mongolia Grassland Ecosystem Research Station, CAS, the seasonal responses of inorganic N dynamics, net N mineralization, and litter decomposition to grazing intensity and management regime were studied in a typical steppe in Inner Mongolia. The experiment site conducted in 2005, including 56 experiment plots. The experimental treatment was composed of two grazing systems (e.g., traditional grazing and haymaking shift yearly) and 7 levels of grazing intensities (i.e., 0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9.0sheep hm-2). All plots have been fenced against sheep migration between plots since the beginning of the experiment. The traditional system was managed with grazing and haymaking on two different areas. In the grazing area, sheep was allowed to graze continuously during the vegetation period from June to September. The mixed system was managed with a yearly shift between grazing and haymaking.(1) The seasonal patterns of soil inorganic N pools and net N mineralization and their linkages to ecosystem functioning from a grazing experiment were conducted in Inner Mongolia grassland from October in 2008 to September in 2009. Our findings demonstrated that, in the non-growing season and peak growing season, the net N mineralization rate was reduced in grazing plots, and the net N immobilization was observed in all grazed treatments. However, grazing increased the net N mineralization rate and cumulative N mineralization in the early growing season. The seasonally dependent effects of grazing on soil inorganic N pools and net N mineralization are mainly mediated by soil temperature and moisture, with soil moisture being predominant in the peak growing seasons. Grazing alterations of soil inorganic N and net N mineralization were closely linked to the changes in ANPP, biomass N contents and nitrogen use efficiency. The temporal scaling provides new insights into the interactions of grazing with soil moisture and temperature affecting the inorganic N pools and net N mineralization.The effects of urine and dung addition on soil inorganic N in different soil layers and plant functional traits were studied based on a control experiment in June and July, 2009. The results showed that soil inorganic N varied greatly with sampling time and soil layers. For the urine treatment, soil inorganic N increased sharply in a week, but varied little after 16 weeks. The urine addition had positive effects on plant aboveground biomass, height and total nitrogen content, while had negative effect on total carbon concentration. For dung treatment, however, the influence of dung addition on plant and soil was tardive. The effect on 0-5cm soil layer was more significant than that 5-10cm soil layer. The plant aboveground biomass and height increased with dung addition, and total nitrogen and carbon concentration were higher for the moderate dung treatment.(2) The seasonal dynamics of soil inorganic N content, soil temperature, soil moisture content, soil ammonium-N and nitrate-N,under the four grazing intensities (e.g., 0, 3.0, 6.0, and 9.0sheep hm-2) were measured from June to September in 2009. The results showed that the soil temperature increased first and then decreased, in contrast, soil moisture decreased first and then increased. The soil ammonium-N content was negatively correlated with soil moisture, but unrelated to soil temperature. The soil nitrate-N content was negatively correlated with soil temperature, but positively correlated with soil moisture. Under moderate grazing intensity, the total inorganic N content was negatively correlated with soil temperature, but positively correlated with soil moisture. In conclusion, the mixed management regime, e.g., alternation of moderate-grazing intensity and haymaking is more favorable for soil N turnover and inorganic N increase. The decomposition rate, weight loss rate, relative carbon and nitrogen loss rates of litter were determined under different management regimes and grazing intensities. The results showed that grazing could accelerate the litter decomposition and weight loss in both traditional and mixed grazing systems during a growing season. In the traditional grazing system (flat block) and mixed grazing system (slope block), the litter decomposition and weight loss rates at high grazing intensity (7.5sheep·hm-2) were increased by about 45% and 40%, respectively, compared to control. And the litter decomposition rate at moderate grazing intensity (6.0sheep·hm-2) was increased by about 35% in the mixed grazing system (flat block). In contrast to the mixed systems, the variations of relative carbon and nitrogen loss in litter were similar among traditional systems. On the whole, grazing could accelerate carbon and nitrogen release in grassland ecosystem. At high grazing intensity, the decomposition, weight loss and relative carbon loss rates of litter in mixed grazing systems were significantly higher than that in the traditional grazing systems. Our results were partially in agreement with the grazing optimization hypothesis.
|
PDF Full Text Request