AMMONIA VOLATILIZATION FROM TROPICAL LEGUME MULCHES AND GREEN MANURESON UNLIMED AND LIMED SOILS

In the humid tropics, legumes are harvested and surface applied as mul ch or incorporated as green manure. Studies of N dynamics and budgets from these systems report unaccounted losses of N. Ammonia volatilizat ion may account for a significant percentage of these unexplained N de ficits. The main objectives of this study were to: 1) determine the ra te and amount of ammonia volatilization from organic amendments, both incorporated (green manure) and unincorporated (mulch), 2) compare amm onia volatilization of organic amendments on both acid (unlimed) and l imed soils, and 3) correlate quality, i.e. polyphenolic and lignin con centration and carbon-to-nitrogen ratio, of the organic amendments wit h ammonia volatilization and net N mineralization. In an incubation ex periment, ammonia volatilization losses and net N mineralization were measured from fresh leaflets of 10 legumes over a three-week period. A mmonia volatilization losses for the 10 species ranged from 3.4 to 11. 8% of the total N applied in the organic amendment. Lignin content was negatively correlated to ammonia volatilization. Ammonia volatilized from mulches but not green manures, on both unlimed and limed soils, s uggesting that ammonia volatilization is a surface phenomenon and not affected by soil pH. Net N mineralization was affected by species and soil pH, but was unaffected by placement (green manure or mulch). For the farmer in low-input agriculture where N tends to be limiting, vola tilization losses of N from legume mulch systems could be on the same order of magnitude as crop removal.