Productivity in subsistence or low-input agriculture depends on the release of nutrients through mineralization of soil organic matter (SOM). For this reason, SOM quantity and quality are two important variables related to the sustainability of subsistence family farming. The objective of this work was to study the interrelationships among soil use, texture and relief, at basin scale, on the concentration of total soil C (Ct) and N (Nt), the oxidizable C fraction with 16.5 mol L-1 KMnO4 (Cox) and the amino-sugars fraction of N (Namino). These properties were quantified in 260 single soil samples (0-20 cm layer) from the Vaca Brava basin, Paraiba State, Brazil. The region was sampled using a stratified sampling design with an incomplete factorial arrangement of the strata in areas under subsistence farming. Concentrations of Ct, Nt and Namino decreased in the following order: pasture > fodder > crops. In relation to the relief, Ct, Nt and their fractions were greater in lowlands and smaller in foot-hill areas. The latter tended to accumulate sand, while lowlands showed greater accumulation of silt. Sediment redistribution was caused mainly by erosion processes of the back-slope regions, in which finer textures predominated. In relation to textural classes, SOM concentrations and their fractions were greater (p < 0.05) in clayey sand samples and smaller in sandy loam ones. In all strata, Cox and Namino concentrations tended to parallel the variations in Ct and Nt. For this reason, Cox/Ct and Nt/Namino ratios that could be used as SOM quality index varied very little as a function of use, relief or texture.
subsistence farming; soil fertility; pasture; labile fraction of C and N
