Ecosystem fertility and fallow function in the humid and subhumid tropics

The regeneration of natural vegetation (fallowing) is a traditional practic e for restoring fertility of agricultural land in many parts in the tropics . As a result of increasing human population and insufficient fertilizer in puts, the ecosystem fertility functions of traditional fallows must now be improved upon via the use of managed fallows. Interactions between vegetati on and soil determine nutrient losses and gains in crop-fallow systems and are influenced by fallow species, patterns and rates of biomass allocation, and crop and fallow management. Nutrient losses occur through offtake in c rop harvests during the cropping phase and through leaching, runoff, and er osion in the cropping phase and the initial stage of fallows - when nutrien t availability exceeds nutrient demand by vegetation. Gains in nutrient sto cks in later stages of fallow are generally more rapid on soils with high t han low base status due to greater quantities of weatherable minerals and l ack of constraints to N-2 fixation, deep rooting, and retrieval of subsoil nutrients by fallow vegetation. On low base status soils (exchangeable Ca < 1 cmol(c) kg(-1)), N-2 fixation and atmospheric inputs are likely to be th e main sources of nutrient additions. On high base status soils limited by N, gains in N stocks by inputs from N-2 fixation and retrieval of subsoil n itrate can occur relatively rapidly; hence short-term fallows can often imp rove crop performance. Large losses of Ca associated with soil organic matt er (SOM) mineralization and soil acidification during cropping and fallow e stablishment, combined with chemical barriers to root penetration, suggest that long-duration fallows (> 5 yr) are needed for recovery of cation stock s and crop performance on low base status soils. On both soils, however, re sidual benefits of fallows on crop yields usually last less than three crop s.