Effect Of Tan Sheep Rotational Grazing And N Addition On Soil And Vegetation Of Typical Steppe In Eastern Gansu Loess Plateau

Grazing is one of the most important ways of livestock production in the world, and nitrogen is one of the most important nutrient elements to maintain the function of grazing ecosystem. Therefore we take the Long Dong Loess Plateau Tan sheep rotational grazing system as the object to study the influence of stocking rates (enclosure,2.7 sheep/ha,5.3 sheep/ha,8.7 sheep/ha) and nitrogen addition levels (CK,O g/m2,5 g/m2,10 g/m2,20 g/m2)to the soil nitrogen mineralization, the greenhouse gas discharge and the vegetation characteristics. The main results are as follows:(1) The soil ammonium nitrogen and mineral nitrogen content reaches a maximum in August, respectively reached 15.97 mg/kg and 36.95 mg/kg, the nitrate content reaches a maximum of 23.86 mg/kg in September in 2013. The nitrate, ammonium and mineral nitrogen content reached a maximum in July in 2014, which were 57.37 mg/kg,11.19 mg/kg, and 68.56 mg/kg respectively. There is a good positive linear correlation between nitrogen application rates and the soil nitrate, ammonium and mineral nitrogen contents. The nitrogen mineralization rate of the nitrogen addition group reached maximum in August in 2013 and in July in 2014 (p<0.05), and nitrogen addition groups are significantly higher than non-nitrogen addition groups (p<0.05); the minimum mineralization rates of the nitrogen addition groups are in September in 2013 and in August in 2014 (p<0.05), and they are significantly lower than the non-nitrogen addition groups (p<0.05). It indicates that nitrogen addition can promote the nitrogen mineralization, but the promotion will be weaker as the time goes on. Grazing has no significantly influences on the soil nitrogen mineralization. The interaction of the application amounts of nitrogen and stocking rates on the soil nitrogen mineralization has not reached the significant level. It indicates that grazing can weaken the influence of nitrogen addition to the soil nitrogen mineralization.(2) Grazing has significant influence on CO2 emissions (p<0.05), and it has no significant influence on the CH4 and N2O emissions. The diurnal dynamic of the three kinds of greenhouse gases is various under different stocking rates. The 5.3 sheep/ha grazing pasture has the maximum absorption rate on CH4 and the maximum emission rate on CO2, were 0.59×10-3 mg·m-2·min-1and 0.61 mg·m-2·min-1 respectively, while this grazing pasture’s N2O emission rate is 8.1×10-3 mg·m-2·min-1,it’s placed in the middle; 8.7 sheep/ha grazing pasture’s N2O emission rate is 11.53×10-3 mg·m-2·min-1, it’s the highest. Nitrogen addition can accelerate the N2O emission (p<0.05), and it inhabits the CH4 absorption and the CO2 emission. Nitrogen addition has no significant influence on the trend of CH4, CO2, and N2O emissions diurnal dynamic. The interaction of the stocking rates and the application amounts of nitrogen has not reached the significant level.(3) Nitrogen application have no significant influence on the biomass of Artemisia capillari, Stipa bungeana and Lespedeza davurica (there is significantly difference between parts of nitrogen addition group).The stocking rates have significant influence on the vegetation biomass, the biomass of Artemisia capillari, Stipa bungeana, Lespedeza davurica and total biomass reachs the highest in the grazing pasture of 5.3 sheep/ha, respectively attain 49.55 g/m2,12.68 g/m2, 13.67 g/m2 and 82.84 g/m2. Grazing can reduce the forage crude protein, pasture with stocking rate of 8.7 sheep/ha has the minimum crude protein content of 45.16 g/kg, which significantly lower than that with stocking rate of 2.7 sheep/ha and 5.3 sheep/ha (p<0.05). With the increase in stocking rate, the pasture phosphorus content increases accordingly. The application amounts of nitrogen has no significant influence on the number of pasture vegetation species, but significantly increase the forage crude protein content; there is a good linear correlation between forage crude protein and nitrogen addition concentration, y= 0.665x+44.936 (R2=0.9026, p<0.01), the maximum forage crude protein content is in 20 g/m2 group, reaches 57.37 g/kg; Nitrogen addition has no significant influence on the forage phosphorus content. The interaction of nitrogen addition and grazing have non-significant influence on pasture’s vegetation biomass, forage crude protein and phosphorus content, it indicates grazing can counteract the influence of nitrogen addition.The results shows that grazing can weaken the influence of nitrogen addition to the pasture, nitrogen addition also can weaken the influence of grazing to the pasture, the superimposition of grazing and nitrogen addition can be helpful for the sustainable of grassland ecosystem. Furthermore, the feces and urine of livestock, especially the urine has the similar effect of nitrogen addition, indicates that the combinations of grazing livestock intake, trample, feces and urine have positive effect on the sustainable utilization of grassland.