Research on biological nitrogen fixation of grass-legume mixtures in a semi-arid area of China

Biological nitrogen fixation(BNF),nitrogen transfer and the contribution of nitrogen derived from air to nitrogen yield were investigated in a semi-arid area of China using four grass-legume mixtures: Leymus chinensis-Astragalus adsurgens,L.chinensis-Lespedeza daurica,L.chinensis-Medicago ruthenica,and L.chinensis-M.sativa cv.Aohan.The nitrogen yield of the L.chinensis-Astragalus adsurgens and of the L.chinensis-M.sativa cv.Aohan were significantly greater than that of L.chinensis mono-culture and they had an improved grassland quality.The BNF of the four legumes was 35%-40%,suggesting that it was an important nitrogen resource for the growth of legumes.The amount of nitrogen transferred from legume to L.chinensis was 2-12 kg N/(ha·year),about 5%-24% of the nitrogen yield of L.chinensis.Legume BNF promoted the nitrogen resources of grassland ecosystems,contributing 20.30%,17.78%,11.29% and 31.77% to total nitrogen yield for A.adsurgens,L.daurica,M.ruthenica,and M.sativa cv.Aohan,respectively.