Effects Of Nitrogen Adding On Plant Community And Soil Nitrogen Supply Ability Of Alpine Meadow In Qinghai-Tibet Plateau

Grassland on Qinghai-Tibet Plateau accounted for one third of the overall grassland area in China, which plays a very important role in the national unity and the stability of border region. Alpine meadow is the dominant grassland type in Qinghai-Tibet Plateau, which is of great heterogeneity in ecological environment and fluctuation in biology stability. Improving grassland production is essential to develop animal husbandry in this area. Although soil organic carbon is very high in alpine meadow, the production of alpine ecosystem is usually limited by soil nitrogen availability. However, the effects of nitrogen addition and utilization pattern on soil nitrogen supply ability and nitrogen validity in alpine meadow is still known very little. To find out the characteristics of soil nitrogen mineralization ability, the response of fenced and grazed grassland to nitrogen addition was tested in an alpine meadow in Xiahe county of Gansu province in combination with the long-term aerobic intermittent leaching method and green house pot-test. The main results are as follows:1. The effects of nitrogen adding on vegetation and soil nitrogen availability in alpine meadow(1) Nitrogen addition improved grassland biomass. The biomass of the gramineae grass was 1.2%~85.9% higher(P<0.05) than that of the control,proportion of poisonous weeds was 10% lower than that of control in the current year. In the second year, biomass of the gramineae grass began to decline while poisonous weeds and forbs biomass increased in fenced grassland.(2) Nitrogen addition can significantly increase soil mineral nitrogen in the current year and nitrate content under fenced plots in following year. The effect of nitrogen addition on soil nitrogen mineralization was mainly found in the topsoil(0~10 cm). Mineral nitrogen in fenced grassland soil was more sensitive to nitrogen addition than that in grazed grassland.2. Effect of temperature and organic fertilizer on soil nitrogen mineralization potential in alpine meadow(1) After the long-term leaching aerobic cultivation, the rate of soil nitrogen ammonium and nitrification showed a single peak trend with the increase ofnitrogen addittion. Organic nitrogen addition has excitation effect on nitrogen mineralization and increased the rates of soil nitrogen mineralization in the early period of cultivation.(2) Mineral nitrogen content was significantly higher at 25℃ than those of 15℃and 5℃. Ammonium nitrogen dominates mineralized nitrogen in alpine soil.Grazing can increase soi nitrate mineralization. Ammonium and nitrate mineralization were inter-restricted.(3) Nitrogen mineralization capacity can be fitted well by first-order exponential equation under different temperatures and organic nitrogen levels.Temperature determines soil nitrogen mineralization potential of alpine meadow, but has little effect on mineralization rate. Soil nitrogen mineralization rate was increased under 15~45 kg/hm2 nitrogen addition in the fenced grassland.3. The effects of organic nitrogen addition on soils nitrogen supply ability in alpine meadow by biological testing method(1) Under greenhouse condition, organic nitrogen addtion had little positive effect on the biomass accumulation of oat planted in alpine meadow soils.Oat biomass accumulation, leaf area index and total root length were greatly reduced with the increae of plant times.(2) Organic nitrogen addition reduced oat plant nitrogen uptake and increased the residual nitrogen in soil. Under greenhouse condition, soil nitrogen mineralization content reached the peak under N45 and N75 treatment and was 1.7 times and 3.5 times of the control in fenced and grazed treatment,respectively. Organic fertilizer nitrogen mineralization rate was significantly negatively related to the amount of fertilizer, reaching the maximum value of49.6% under N15 treatment.In this study, field measurements, aerobic culture and biological testing methods were combined to analyze the soil nitrogen mineralization characteristics under different temperatures and nitrogen additions. Conclusions obtained by three methods are consistent. Effects of once nitrogen addition on grassland can lasted more than two years. Nitrogen mineralization and its responce to the external environment are various in different utilization of grassland. Fenced grassland has lower heterogeneity than grazed grassland and is more sensitive to nitrogen addition,with the suitable nitrogen addition rate of 45 kg/hm2.