FATE OF LEGUME AND FERTILIZER N-15 IN A LONG-TERM CROPPING SYSTEMS EXPERIMENT

Relying more on biological N2 fixation has been suggested as a way to meet one of the major challenges of agricultural sustainability. A N-15 study was conducted to compare the fate of applied legume and fertilizer N in a long-term cropping systems experiment. Nitrogen-15-labeled red clover (Trifolium pratense L.) and (NH4)2SO4 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 N-15 was applied to soil and traced into corn (Zea mays L.) in 1987 and 1988. Residual N-15 was also traced into second-year spring barley (Hordeum vulgare L.). Legume and fertilizer N-15 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 N-15 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.