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Summary The availability and turnover in different soil fractions of residual N from legumi-, nous plant material and inorganic fertilizer was studied in a pot culture experiment using wheat as a test crop. Plants utilized 64% of the residual fertilizer N and 20% of the residual legume N. 50-60% of the N taken up by plants was recovered in grain and 4-8% in roots. After harvesting wheat up to 35% and 38% of the residual legume N and fertilizer N, respectively was found in humic compounds. A loss of humus N derived from legume and fertilizer was found during wheat growth but the unlabelled N increased in this fraction. Biomass contained 6% and 8% of the residual legume and fertilizer N, respectively when both were available. The mineralizable component contained upto 28% of both the residual legume and residual fertilizer N. Only a small percentage of the soil N (3-4%) was observed in biomass whereas the mineralizable component accounted for 7-14% of the soil N. In this fraction legume derived N increased during wheat growth whereas unlabelled N increased in both the mineralizable component and microbial biomass.Some loss of N occurred from residual legume and fertilizer N. Nevertheless, a positive total N balance was observed and was attributed to the addition of unlabeiled N in the soilplant system by N: fixation. The gain in N was equivalent to about 38% of the plant available N in the soil amended with leguminous material. The additional N was concentrated mainly in the mineralizable fraction and microbial biomass, although some addition was also noted in humus fractions.

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