CARBON AND NITROGEN STOCKS IN SOIL UNDER CASTOR BEAN IN THE SEMI-ARID CAATINGA OF BRAZIL

CARBON AND NITROGEN STOCKS IN SOIL UNDER CASTOR BEAN IN THE SEMI-ARID CAATINGA OF BRAZIL Much of the Brazilian castor bean (Ricinus communis L.) production is concentrated in the semi-arid region (Caatinga biome) of the Northeast, as an alternative crop. The purpose of this study was to evaluate changes in C and N pools due to land use change in the Caatinga biome by castor bean production. The study was conducted on the Fazenda Floresta in the municipality of Irece (Mid-North region of Bahia, Brazil). The climate is type BSwh (Koppen) - Semi-arid climate. The soil was classified as clayey Oxisol. Three areas under castor bean were evaluated after different cultivation periods i) after 10 years; ii) after 20 years, and iii) after 50 years, and a reference area (native vegetation of the Caatinga) compared with the above situations. C and N stocks were determined in soil samples with five replications (layers 0-5, 5-10, 10-20 and 20-30 cm). The results showed that the constant plant residue input in the native vegetation ensured the maintenance of the C stocks (90 Mg ha(-1)) and N stocks (10 Mg ha(-1)). The land use change to castor bean production resulted in a reduction of about 50% in soil C and N stocks compared to Caatinga, in the first 10 years of crop establishment. The half-life of SOM of the semi-arid region was 4.7 yr. The C emission factor from soil, due to land use change after 20 years as proposed by the IPCC methodology was 2.47 Mg C yr(-1). The results show the fragility of the soil in the Caatinga biome due to SOM loss caused by land use changes.