| Legumes have important influence on improving nitrogen nutrition balance ofgrassland ecosystem and promoting output of animal protein in pasture. The majorobjectives of this research, which service for selecting suitable species forgrass-legume mixture in semi-arid area in Northeastern of China, were exploring twospecies mixture productivity and interspecific relationship between grass Leymuschinensis and five legumes: Astragulus adsurgens, Lespedeza davurica, Medicagoruthenica, Medicao sativa and Vicia amoena, respectively.According to replacement series design, the mono-grass, mono-legume andmixture of one grass and five legumes were established and researched at FieldStation in Changling county of Jilin province (E123?51′02″,N44?12′11″) in 2004 and2005. Dry matter yield and nitrogen yield were measured at September in 2004, Juneand August in 2005. Relative yield (RY), relative density (RD) and relative yield total(RYT) were calculated to determine the interspecific relationship and resourceutilization. The amount of nitrate and ammonium in 0-40, 40-80, 80-100cm soil layerwere measured before and after growing season in 2005. Furthermore soil availablenitrogen was calculated. Biological nitrogen fixation and nitrogen transfer wasdetected by 15N dilution technology. The percentage of BNF, fertilizer nitrogen andsoil nitrogen to nitrogen yield of grass and legume in monoculture and mixture wereanalyzed, respectively. Also the concentrations of BNF to mixture nitrogen yield werecompared among legumes. Influences of aboveground competition and belowgroundcompetition on growth of legumes, interaction of above-and belowgroundcompetition were investigated using target-neighbor technology.Results indicated that DM yield of all mixture were similar to mono-grass,however the nitrogen yield in mixtures with A. adsurgens, M. sativa and M. rethenicawere higher than mono-grass.L. chinensis was suppressed, when growing with A. adsurgens and M. sativa,and the clonal growth had positive effects on development of L. chinensis population.Competition, especially belowground competition from L. chinensis with higherproportion in mixture restrained growth of M. ruthenica. L. davurica and V. amoenapopulation also decreased growing with L. chinensis. Except L. chinensis-M. sativacombine, other mixtures acquired additional resources at different period.The influences of aboveground competition and belowground competition ongrowth of legumes were affected by the soil water content, which belowgroundcompetition was more important in 2004 when precipitation was lower andaboveground competition was more important in 2005 when precipitation was higher.The interaction of above-and belowground competition, mostly positive interaction,was occurred in 2004.The amounts of BNF in mixture were: M. sativa (47.82 kg N/ha/yr) > A.adsurgens (20.42 kg N/ha/yr) > M. ruthenica (12.16 kg N/ha/yr) > L. davurica (11.98kg N/ha/yr), and the amounts of N transfer to grass were: A. adsurgens (11.79 kgN/ha/yr) > L. davurica (8.15 kg N/ha/yr) > M. ruthenica (5.47 kg N/ha/yr) >M. sativa(2.37kg N/ha/yr).Mixtures utilize similar amount of fertilizer nitrogen and soil nitrogen tomono-grass, however the percentage of fertilizer nitrogen and soil nitrogen to totalnitrogen yield decreased. The soil available nitrogen decreased from surface to depthin mixture, furthermore the deeper legume root was, the lower the available nitrogenwas. The consume of soil available nitrogen, mainly nitrate, around 120-300 kgN/ha/yr in mixture. Both the consume of soil available nitrogen and utilizationefficiency in A. adsurgens and M. sativa mixture were all higher than mono-grass,while other mixtures were similar to mono-grass. Legume, such as A. adsurgens andM. sativa, absorbed soil nitrogen from surface to depth, nevertheless L. davurica andV. amoena use less in deep soil.
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