Declines of organic nutrient pools in tropical semi-arid soils under subsistence farming

Soils under subsistence farming in semi-arid northeastern Brazil are showing progressive declines in soil fertility. In this study, we assessed the effects of subsistence farming on losses of organic nutrient pools in 10 independent sites that were distributed in five districts of two states. Each site had adjacent areas under dry forest and subsistence farming. Areas under each land use were separated in two groups that had different degrees of land degradation. Group assignment was based on land use history and visual observations of vegetation-soil degradation and was further evaluated using Cs-137 activity to estimate erosion. Four combinations of land use degradation were defined: undisturbed (UDF) and disturbed (DDF) dry forest, and preserved (PC) and degraded (DC) cultivated land. Soils were sampled in the 0- to 7.5- and 7.5- to 15-cm layers. The Cs-137 concentrations declined in the order UDF > DDF approximate to PC > DC. Soil C and N concentrations in DC (8.9 and 0.94 g kg(-1) soil, respectively) were half of those in UDF (17.8 and 1.51 g kg(-1) soil, respectively). The organic P (P-o)/inorganic P (P-i) ratio was 1.47 in UDF and decreased to 0.82 in PC and DC. Carbon losses in DC were attributed to erosion (43%) and to mineralization (57%) processes. Differences between UDF and other land uses were greater in the 0 to 7.5 cm than in the 0- to 15-cm layer. Carbon and N losses in low-P status soils, in addition to limited water availability and unsuitable land management techniques, are likely to restrict the recovery of degraded soils by traditional bush fallow techniques.