DYNAMICS OF ORGANIC MATTER OF A SANDY SOIL UNDER CERRADO TURNED INTO CROPLAND CONVERTED TO SOYBEAN AND MILLET SUCCESSION

The use and management of Quartzipsamments can influence soil organic matter (SOM), but the magnitude of these variations for the specific conditions in the Cerrado (savanna-like vegetation) is still poorly studied. The aim of this work was to study the variations in SOM in a Quartzipsamment, after the conversion of native vegetation to cropland (soybean-millet rotation). Soil was sampled from 0-5, 0-10, 0-20 and 0-40 cm depths of a commercial field in Alto Garcas, Mato Grosso, Brazil, of plots under soybean-millet rotation in plots converted to croplands for 1, 2, 6, 8, and 10 years. Additionally, soil samples were taken from an adjacent area under native Cerrado (savanna) as a reference of the original vegetation. Total organic carbon (TOC), total C, total N, C content in humic and fulvic acids, light organic matter (LOM), and the SOM carbon isotope ratio were determined. The conversion of Cerrado to agriculture increased the soil TOC content, mainly due to the contribution of LOM and charcoal fragments. The management of this soil with soybean-millet rotation, despite the use of heavy disking, did not reduce the TOC levels over time. However a variation in the C distribution in the different SOM fractions was observed over time, indicating significant changes in the SOM quality. Over time, an increase was observed in light organic matter and a reduction in C contents in the form of fulvic acids, suggesting a selective loss of the more soluble C-compounds. The isotope ratio of soil C increased with time, indicating a higher contribution of C derived from millet, maintaining the SOM levels. The SOM properties in the 0-20 cm layer were more sensitive to variations as a function of cultivation time than the same properties in the other layers. Soil organic matter fractions were more sensitive to variations in function of cultivation time than the soil TOC content, showing the potential use of these variables as soil quality indicators.