Effects Of Tan Sheep Rotational Grazing And N Addition On Soil N Mineralization And GHG Emissions Of Typical Steppe In Eastern Gansu Loess Plateau

Grazing is one of the most important ways of utilizing grassland,nitrogen?N?is a key element to maintain grassland ecosystem functioning.In this study,I examined the effects of nitrogen fertilizer?NH₄NO3?addition to a grassland that had been rotationally grazed by Tan sheep at a typical steppe in eastern Gansu Loess Plateau.The treatments of the experiment included four levels of stocking rates?0 sheep/ha,2.7 sheep/ha,5.3 sheep/ha,8.7 sheep/ha?and four levels of nitrogen additions?0 g/m2,5 g/m2,10 g/m2,20 g/m2?.The effects of stocking rate and N addition on nitrogen mineralisation and greenhouse gas emissions were measured.The experiment was conducted for four years 2013 to 2016.Below are the main research results:1 In the year 2013-2016,under different stocking rate,soil ammonium nitrogen did not reach a significant difference;There was no significant effect of stocking rate on soil ammonium nitrogen?NH₄+-N?levels in all four years.In contrast,soil nitrate nitrogen?NO3--N?and total mineral nitrogen contents were different at different stocking rates.In 2014,the soil nitrate nitrogen and total mineral nitrogen contents of the 5.3sheep/ha grazing pasture were significantly higher than the grazing pasture of 2.7sheep/ha and 8.7 sheep/ha?p<0.05?,which were 16.87 mg/kg and 22.28 mg/kg respectively.The results of 2014 were generally different other years.In 2014,the precipitation,total mineral nitrogen content and the soil water content was higher than the other years,In 2014,soil water content showed a significant positive relationship with soil nitrogen mineralization.In all four years,nitrogen addition significantly increased the total mineral N,soil NH₄+-N,and NO3--N?p<0.05?.The soil N levels measured monthly followed a similar pattern during all four years except during 2014.All the soil N forms were highest in August.However,in 2014,soil NH₄+-N levels were lower than the other years and the levels were highest in April.The reason for lower soil NH₄+-N levels in 2014 possibly due to the greater litter mass found in the pastures that could have been increased the C/N,resulting inhibition of the soil nitrogen mineralization.In all four years,there was no significant interaction between stocking rate and nitrogen addition on the soil nitrogen mineralization.Grazing diminished the stimulation of nitrogen mineralization by nitrogen addition.Soil nitrogen mineralization was affected by various biotic and abiotic factors as well as human disturbances.2 There were no significant effects of grazing and nitrogen addition on the CH₄ uptake and CO₂ emissions from the grassland.Nitrogen addition inhibited the CH₄ uptake by grassland soil and.there was no significant effects on soil CO₂ emissions.Soil CO₂ emissions were stimulated by the 0,2.7 and 5.3 sheep/ha stocking rates.But under heavy grazing?8.7 sheep/ha?CO₂ emissions were inhibited.There was no significant increase N₂ O emissions from grassland by grazing.As expected,nitrogen addition significantly promoted N₂ O emissions?p<0.05?,but the increased emissions by N additions were inhibited by the overgrazing.3 The effects of grazing and nitrogen addition on soil nitrogen mineralisation and GHG emissions were mainly depend on the soil temperature and soil water content.Plant and litter biomass had direct or indirect effects on soil nitrogen mineralization and GHG emissions.