Fate of Legume and Fertilizer Nitrogen‐15 in a Long‐Term Cropping Systems Experiment

Abstract Relying more on biological N 2 fixation has been suggested as a way to meet one of the major challenges of agricultural sustainability. A 15 N study was conducted to compare the fate of applied legume and fertilizer N in a long‐term cropping systems experiment. Nitrogen‐15–1abeled red clover ( Trifolium pratense L.) and (NH 4 ) 2 SO 4 were applied to microplots within the low‐input and conventional cropping systems of the Farming Systems Trial at the Rodale Institute Research Center in Pennsylvania. The 15 N was applied to soil and traced into corn ( Zea mays L.) in 1987 and 1988. Residual 15 N was also traced into second‐year spring barley ( Hordeum vulgare L.). Legume and fertilizer 15 N remaining in soil was measured and loss of N was calculated by difference. More fertilizer than legume N was recovered by crops (40 vs. 17% of input), more legume than fertilizer N was retained in soil (47 vs. 17% of input), and similar amounts of N from both sources were lost from the cropping systems (39% of input) over the 2‐yr period. More fertilizer than legume N was lost during the year of application (38 vs. 18% of input), but more legume than fertilizer N was lost the year after application (17 vs. 4% of input). Residual fertilizer and legume 15 N was distributed similarly among soil fractions. Soil microbial biomass was larger in the legume‐based system. A larger, but not necessarily more active, soil microbial biomass was probably responsible for the greater soil N supplying capacity in the legume‐based compared with fertilizer‐based system.