Responses of soil nutrients and biological characteristics to nitrogen deposition in Hulun Buir Grassland, China.

Studies on effects of nitrogen deposition were mainly focused on temperate grasslands in Inner Mongolia of China. In addition, there are substantial differences between the present simulation methods and the natural nitrogen deposition. A three-year experiment was carried out to compare the effects of simulation methods (common urea and slow-released urea) and nitrogen deposition rates (0, 25, 50, 75, 100, 150, 200 and 300 kg N·hm-2 ·a-1 ) on soil nutrients and biological characteristics in Hulun Buir Grassland. We found that simulated nitrogen deposition had significant influences on soil chemical properties, biological properties and enzyme activities. With the increases of nitrogen deposition, soil pH declined with the greatest extent of 0.2 units, while the highest concentrations of total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) increased by 5-7 times and 12%-36%, respectively. There was a decline trend for soil total phosphorus (TP) and organic phosphorus (TOP). Microbial biomass and metabolic activity increased firstly and then decreased. Moderate simulated nitrogen deposition rates significantly increased soil carbon, nitrogen and phosphorus related enzyme activities. Compared to common urea, using slow-released urea to simulate nitrogen deposition decelerate the decline of soil pH and the increase of dissolved nutrients, and smoothed the change of microbial biomass, metabolic activity, and nitrogen hydrolyzed enzyme activities. Overall, the results confirmed that continuous nitrogen input caused the decline of soil pH and the increase of bioavailable carbon and nitrogen, and then changed microbial biomass and activity.