Tracing (15)N in cropped and uncropped soil treated with enriched potassium nitrate and labeled clover residues

In developed countries, nitrogen (N) fertilizers are cheap and easily available. In developing countries peasant farmers rely on biological N fixation because fertilizers can be prohibitively expensive. Legumes are sometimes used as green manure in southeastern USA, but crop uptake of legume N is usually low. To investigate how well applied N from an organic and inorganic N source could be accounted for in a cropped and uncropped soil, 15N-labeled KNO₃ (F) and clover residues (Cl) were applied to samples of a Typic Kanhapludult soil. Nitrogen in microbial biomass, inorganic, organic and potentially mineralizable soil N pools (by anaerobic incubation) was monitored for 26 wk. After 26 wk, recovery of applied N in soil inorganic N was F: 66%, Cl: 40%; in soil organic N, F: 18% and Cl: 50%; and in microbial biomass N, F: 0.75% and Cl: 1.5. Ammonia volatilization was negligible during the initial 12 wk following clover application. We assumed that applied N not found in the measured N pools was denitrified: F, 16% and Cl, 10%. The anaerobic incubation-extraction procedure underestimated the size of potentially mineralizable N pool by a factor of 3 in Cl and 4 in C. Then, 15 monolith soil cores were removed from the same soil and 15N-labeled materials applied as before. In 1997 and 1998, 15N movement was monitored in vegetation (corn and weeds), soil, and percolating soil water. Treatments affected corn dry matter accumulation in 1997 only (F > Cl=C). No effect was found in weeds. Treatments did not affect recovery of applied N in vegetation in 1997, but more clover N than fertilizer N was recovered in the organic (86.7% vs. 34.4%) and inorganic soil N pools (3.6% vs. 1.8). In 1998, more clover N than fertilizer N was recovered in vegetation (3.4% vs. 1.0%) and as soil organic N (91.3 vs. 21.3%). More clover N than fertilizer N was found in the surface 25 cm soil as inorganic N: 2.6% vs. 1%, organic N: 65% vs. 18% in 1997 and as organic N: 70% vs. 14% in 1998. Nitrate leaching via percolation was negligible both years.