Effects of Different Nitrogen Fertilizer Rates on Nitrogen Absorption by Ryegrass(Lolium muliiflorum) under Salt Stress Estimation of carbon sink function of forests in China Effects of Elevated Air Temperature and Carbon Dioxide Concentration on the Nitrogen Use of Double Rice (Oryza sativa L.) in Open-top Chambers Differences of Nitrogen Status between Different N-uptake-efficiency Rapeseed (Brassica napus L.) Cultivars Characteristics of N Accumulation and Translocation in Rice Genotypes with Different N Use Efficiencies