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-1 5 study was conducted to compare the fate of applied legume and fertil izer N in a long-term cropping systems experiment. Nitrogen-15-labeled red clover (Trifolium pratense L.) and (NH4)2SO4 were applied to micr oplots within the low-input and conventional cropping systems of the F arming Systems Trial at the Rodale Institute Research Center in Pennsy lvania. 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 spr ing 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 inp ut), and similar amounts of N from both sources were lost from the cro pping systems (39% of input) over the 2-yr period. More fertilizer tha n legume N was lost during the year of application (38 vs. 18% of inpu t), but more legume than fertilizer N was lost the year after applicat ion (17 vs. 4% of input). Residual fertilizer and legume N-15 was dist ributed similarly among soil fractions. Soil microbial biomass was lar ger in the legume-based system. A larger, but not necessarily more act ive, soil microbial biomass was probably responsible for the greater s oil N supplying capacity in the legume-based compared with fertilizer- based system.