Study on potential of biological nitrogen fixation of soybean in China

【Objectives】Biological nitrogen fixation (BNF) is one of the important sources of nitrogen for soybean.ExcessiveN fertilizerinhibitesthe efficiencies of BNF and increases the cost of soybean production in China. Understanding the N-fixtion capacityof BNF without N fertilizer input in different soybean planting regions in China is the key for reasonable N fertilization. For the reason weinvestigated potential of BNF and its distribution characteristics in this paper.【Methods】The method of 15Nnatural abundance was employed to determine the BNF efficiency in 2011-2012, the amounts of BNF and their contributions to the grain yield under no nitrogen fertilization conditions in 4 soybean producing regions of China, includeding Heihe, Daqing, Changchun, Tieling, Jining, Yanan and Nanning city. This technique is based on principle that the 15N natural abundance of the reference plant which only obtains N from soil is higher than that of N2-fixing legume, then the difference can be interpreted quantitatively to assess contribution of BNF. The reference plant must have same growth season with soybean, and it is also the same plant in different experimental sites to ensure comparability of data. Maize which met above requirements was selected as the reference plant in this study.【Results】 In normal rainfall years, the variation of the soybean BNF efficiency of different sites is observed between 47%-70%. Thelowest BNF efficiency is at Heihe and highest at Tieling. The amounts of BNF are in the range of N 92-150 kg/ha with the lowest in Yanan and the highest in Changchun, and their fixed N amounts in grain account for 65%-81%. The contribution rates of BNF to grain yields are 1039-1867 kg/ha with the lowest in Yanan and the highest in Changchun. In the Yanan site, serious drought appeared before the flowering stage in 2011, the number of nodule and the nitrogen-fixzation activity were intensely inhibited, causing the decreases of the BNF efficiency. The efficiency, amount of BNF and the contribution rate to grain yieldwere decreased to 15%, N 24 kg/ha and 245 kg/ha respectively. 【Conclusions】The findings above indicate that there are differences in the potential of BNF among the soybean planting regions, and also have obvious distribution characteristics. The BNF efficiencies gradually decrease from Tieling (temperate zone) northwards to Heihe(cold temperate zone) and southwards to Nanning (subtropical zone). Due to various planting densities and other factors in different sites, the variation of the amounts of BNF and their contribution to grain yields are the highest in Northeast China, middle in Jining and Nanning and the lowest in Yanan.