NUTRIENT AND SOIL ORGANIC-MATTER DYNAMICS UNDER SHIFTING CULTIVATION IN SEMIARID NORTHEASTERN BRAZIL

In subsistence farming systems using shifting cultivation, crop production depends on the natural fertility of the soil and depletion of fertility during cultivation is the cause for land abandonment. During the subsequent bush fallow, fertility levels are improved and the land may be available for further cultivation cycles. In the present study, the effects of cultivation and the regrowth of bush fallow on soil organic matter and phosphorus fertility were evaluated, using adjacent plots with different histories of cultivation and lengths of bush fallow in semi-arid northeastern Brazil. Six years of cultivation with minimal fertilisation resulted in reductions of C, N and organic P by 30%, or about 10 t ha-1 of C. Available N and P were greatly reduced under these conditions. In a fertilised and limed field, this trend was to some extent avoided, but such inputs are usually not an option for local farmers. Eight to 10 years of bush fallow were sufficient to re-establish fertility levels similar to the original uncultivated site. Phosphorus fractionation indicated that the decline in P ferlility was not a result of net export of P in the crop, but arises from the mineralisation of organic P and subsequent transformation of the surplus inorganic P to unavailable forms.