The Effects Of Nitrogen Addition And Warming On NPP And Soil Respiration On A Semiarid Alfalfa-grassland In The Loess Plateau Of China

climate warming and atmospheric nitrogen deposition as the main contents of global change effect grassland ecosystem community structure, net primary productivity and biogeochemical cycle. Investigating effects of climatic warming and increasing nitrogen deposition on plant growth and soil respiration on a grassland ecosystem in the semi-arid Loess Plateau in this paper. To this end, field experiments and simulation experiments by Biome-BGC were conducted.In the field experiments,four treatments were used:1)WN:warming and nitrogen addtion; 2)W: warming; 3)N:nitrogen addition; 4)Co:no warming and no nitrogen addtion(control).And open-top chambers were used for warming,mean daily temperature was rised by 1.7℃.Nitrogen was applied as NH4NO3.The total amount of nitrogen was divided equally among five application times spread in every month early from April to August in 2013 and 2014.In the simulation experiments by Biome-BGC,four simulation scenarios were used:1)WN:mean daily temperature and nitrogen deposition were rised by 1.7℃ and 1.75 gN m-2 yr, respectively; 2)W:mean daily temperature was rised by 1.7℃; 3)N:nitrogen deposition was rised by 1.75 gN m-2 yr; 4)Co:ambient simulation(control).The results showed that1) the growing season started 10 days earlier, M.sativa was blooming earlier and significantly higher in 2014 after warming; however, no changes after 1.75 gNm-2 yr-1 nitrogen addition.2)the foliar N:P of M.sativa and H.altaicus decreased significantly by 4.42,2.29 respectively under warming, but had no significant change.3)through curve estimation, three kinds of biomass models were established, were Logistic model beside A. capillaris, allometric growth model and linear mode.4)NPP increased significantly under two warming treatments, but there was no significant difference between two N addition treatments and no N addition treatments.5)the predictions of NPP by Biome-BGC in four scenarios compare favorably to the observations in WN, W, N and Co.6) There was no significant difference in Rt between the warming treatments and the no warming treatments, however, Rh increased significantly under warming.And there was no significant difference in Rt between WN and Co but Rh modeled increased significantly in WN.7)The whole-year Q10 increased with the order:Co, WN, W, and N and the growing season Q10 increased with the order:Co,WN,N, and W.The findings suggest that climate warning and the increase of atmospheric nitrogen deposition affect alfalfa growth by influencing the phenology, extending growing season, changing the soil hydrological conditions and foliar nutrient distribution, and besides, exsisting different thresholds in NPP and Rt responding to the warming or nitrogen addition. And it offered a scientific basis for the future research in this regional ecological system.In addtion, the three biomass models and the parameterized BIOME-BGC model can provide reliable theoretical basis for the future research of the response of the ecosystem to global change in this region.