QUALITY OF DIFFERENT MULCH MATERIALS AND THEIR DECOMPOSITION AND N-RELEASE UNDER LOW MOISTURE REGIMES

During the dry season in the tropics, agriculture which is solely depe ndent upon rainfall as its source of water is frequently affected by s oil moisture stress, resulting in crop failures. Farmers therefore dep end mainly an other sources of limited water supply during this period , such as ground water. Soil moisture conservation measures, especiall y surface mulching with loppings and, occasionally, leaf litter and cr op residues, are practised. Our objective was to study the decompositi on and nitrogen (N) release from these plant materials under continuou sly wet, low moisture regimes, i.e. comparable to those which prevail in the mulches used in the agriculture. A greenhouse experiment was co nducted with fresh, chapped leaves of six leguminous trees, wild sunfl ower and rice, which were spread as a mulch on a layer of soil. They w ere maintained at eight moisture levels (a total of between zero and 4 31 water m(-2) applied over 8 weeks) by spraying water. Different opti mal moisture requirements for the rapid decomposition of these species were observed. These were presumably determined by different physical and chemical properties of the leaves. The amount of water received t o the mulches and their soluble polyphenolic and carbon (C) concentrat ions played an important role in determining the decomposition and the mode of N release under non-limiting conditions of leaf N. Specifical ly, the C concentration governed N release, while the effect of polyph enolics was important when their concentration was low as a result of leaching under relatively high moisture regimes. Leaves with a high po lyphenolic and C content, which were subjected to high leaching losses of these fractions, underwent a change in their N dynamics from net i mmobilization to mineralization. This study indicates that leaves with a fast rate of decomposition should be mixed with other species, leav es which decompose more slowly in order to increase the conservation o f soil moisture and also improve the synchronization between N release from the mulch and its demand by crops.