Mineral-associated and particulate organic matter in aggregates as a proxy for soil C changes in pasturesugarcane land use transitions

To meet the growing demand for bioenergy such as ethanol, the area cultivated with sugarcane has expanded, especially in areas currently occupied by extensive pastures with low productivity. However, land-use change (LUC) from pasture to sugarcane implies changes in soil structure and variations in organic matter (SOM) stored in the soil. This study aimed to quantify the impact of LUC on organic matter fractions – particulate organic matter (POM) and mineral-associated organic matter (MAOM) – in soils with contrasting textures, and to explore the correlations between possible alterations in soil aggregation and the effects on carbon (C) stocks and SOM fractions. The study was conducted in two areas in Central-Southern Brazil, one with clayey soil and the other with sandy soil. In each area, a LUC chronosequence was evaluated: native vegetation (NV), pasture (PA), short-term sugarcane (SC1), the area analyzed during the sugarcane plant cycle; long-term sugarcane (SC2), area analyzed during the ratoon sugarcane cycle. In each use, undisturbed and disturbed samples were collected and macroaggregates and microaggregates were obtained by wet sieving and soil samples. In these samples, the physical fractionation of the SOM and the calculation of the C contained in each fraction of the SOM and C total stock of each use were performed. The conversion of NV to PA increased C stocks by more than 50 %, mainly in the MAOM fraction, and maintenance of macroaggregates (more than 80 %) in sandy site; and reduction of C stocks by more than 30 %, mainly MAOM in the clayey area. These benefits acquired from grazing on sandy area were lost with the expansion of sugarcane, a reduction of more than 20 % in macroagregation and C stocks. The sugarcane expansion into pasture with clayey site resulted in C accumulation (more than 2 Mg C ha-1 yr-1) and recovery of stocks on a SC2 basis. Expanding sugarcane areas into low-productivity pasture areas can be considered a sustainable strategy, especially in clayey soils, in which multiple gains occur through C sequestration and improved soil quality, as well as the reduction of CO2 emissions through the diversification of the energy matrix with the production of ethanol.