Humus formation in tropical climate is less since temperature is high and at high temperature mulching materials oxidise and lost to atmosphere. When we grow cover crop in rainy season and by the time mulch is formed and by the time it is useful for next cropping cycle, much of it will be lost in the summer coming after the rains.
Recently while reading the book 'Restoring the Soil' by Roland Bunch came across the following text.
A combination of two or three gm/ccs may provide the best results. For example, using trees and another type of low growing gm/cc could be a practical answer to this problem. By reducing the ambient temperature at least 10°C, dispersed trees can cool the fields enough so that the gm/ccs’ organic matter will not be burned off, and the soil’s fertility can be maintained. The temperature can be lowered even more if the trees are not pruned until the months right before the next rainy season, as is normally the case. Thus, dispersed shade can largely eliminate the problem of dry season burn-off of the gm/ccs’ organic matter and nitrogen.