Soil Organic Matter Quality and Quantity in Sugarcane Production Systems in Brazil.

The high demand for biofuels as an alternative energy source has generated a wide expansion of ethanol industry in Brazil, being considered the largest sugarcane producer worldwide. Legal and environmental issues promote changes in sugarcane production, with the conversion of burned to unburned harvesting system and the increase of sugarcane organic production as an important trade. Soil organic matter (SOM) is considered one of the most important indicators of soil quality. However, there is still little information available on a qualitative and quantitative assessment basis in sugarcane production systems. The present study compared soil C and N stocks and soil carbon in the microbial biomass (C-MB) in conventional and organic sugarcane systems. Additionally, as a reference, SOM in an adjacent native vegetation area was also evaluated. Higher values of C stock were observed for native vegetation (NV) (27.11 Mg C ha^-1), unburned sugarcane (US) (23.03 Mg C ha^-1), 12-year organic sugarcane (OS12) (21.80 Mg C ha^-1) in 0-10 cm layer. In sugarcane areas, C stocks varied from 10.87 to 23.03 Mg C ha^-1 in conventional and from 13.62 to 21.80 Mg C ha^-1 in organic system. In relation to N stocks, higher values were observed for OS12 and NV in 0-10 cm layer; 1.78 and 2.06 Mg N ha^-1, respectively. However, N stocks in 0-100 cm were higher in OS12 when compared to NV; 5.24 and 4.56 Mg N ha^-1, respectively. The highest MB-C was observed for NV (608.51 μg C g^-1 soil). In sugarcane areas, the highest C-MB was observed for OS12 (506.61 μg C g^-1 soil) and the lowest for burned sugarcane (BS) (144.21 μg C g^-1soil). The unburned harvesting and organic management provided an increase in soil C and N stocks on soil surface, which is confirmed till 1 m depth. Regardless of the management practices, the conversion of natural to agricultural systems promotes a decrease in soil microbial biomass. However, conservation farming systems can provide an increase of those values, being considered as important indicators in relation to soil quality and management.