EFFECT OF RAINFALL PATTERN ON NITROGEN MINERALIZATION AND LEACHING INA GREEN MANURE EXPERIMENT IN SOUTH RWANDA

The effects of green manures, sorghum residues and farmyard manure on N dynamics and crop yields were studied during three dry and wet seaso ns on a Typic Sombriudox in South Rwanda. In addition, a resin core st udy was conducted within a 4-year green manure field experiment to fol low the seasonal pattern of N mineralization and leaching after applic ation of residues from Tephrosia vogelii, Sorghum bicolor, a mixture o f both materials, and farmyard manure. During the dry season, topsoil (0-20 cm) mineral N remained constant. At the beginning of the wet sea son, the rainfall pattern determined N availability. With low rainfall intensities a mineralization flush occurred, doubling topsoil mineral N concentrations within 5 days after wetting. In contrast, under heav y rains at the onset of the rainy season, topsoil mineral N decreased by 50-70% within the first two weeks. The application of organic ferti lizers has a strong influence on N availability, but the effects can b e negated by heavy rainfall. Incorporation of leaves from Tephrosia vo gelii (2.7 t dm ha(-1)) and farmyard manure (7 t dm ha(-1)) doubled th e mineralization flush after the first rains. During the rest of the w et season, N release by the green manure was small, whereas the farmya rd manure was found to mobilize N after a period of N immobilization. Incorporation of sorghum residues had only a small effect, while mixin g the straw with green and farmyard manure immobilized N temporarily. Nitrogen leaching, measured by exchange resins at a depth of 20 cm, wa s increased up to 50% by the incorporation of green and farmyard manur e. This points to rapid N translocation of easily mineralizable N. The additional incorporation of sorghum residues reduced N leaching of bo th materials significantly. Since rainfall is often unpredictable, the synchronization of N released from crop residues with crop N demand m ay require additional management practices.