Estimation　of　N_2　Fixation　by　Some　Legumes　Using　^（15）N－Labelled　Soil　in　Greenhouse　Experiment

ＥｓｔｉｍａｔｉｏｎｏｆＮ_２ＦｉｘａｔｉｏｎｂｙＳｏｍｅＬｅｇｕｍｅｓＵｓｉｎｇ ^（１５）Ｎ－Ｌａｂｅｌｌｅｄ ＳｏｉｌｉｎＧｒｅｅｎｈｏｕｓｅＥｘｐｅｒｉｍｅｎｔＷＥＮＱＩＸＩＡＯ；ＣＨＥＮＧＬＩＬＩ；ＷＵＳＨＵＮＬＩＮＧａｎｄＤＵＬＩＪＵＡＮＩ...

Pot experimeats were carried out to estimate N2 fixation by vetch, milk vetch, sickle alfalfa and broadbean in pure stand using a 15N-labelled soil. Winter wheat was used as the non-fixing control. The 15N-labelled soil used was prepared by growing corn-wheat-corn successively on a nearly organic-matter-free Xiashu loess supplemented with adequate amounts of (15NH4)aSO4, P, K and micronutrients, then incorporating these 15N-labelled plant msterials into the soil after each harvest, and allowing the plant materials to be decomposed aerobically for 410 d after incorporation of the plant material of the third crop. The 15N enrichment of wheat plant-N varied slightly with organs,with a maximum difference of 9.8%. Based on 15N enrichment of soil N inferred from the mean value of the 15N enrichment in different organs of wheat 79%-91% of total N in the tops and 67%-74% of total N in the roots of legumes studied were derived from atmosphere. Estimate by isotope dilution method was in good agreement with that by the conventional difference method provided values obtained by the latter were corrected for seed N, and also with that from the measurement of N accumulated in the tops of the legumes.