Crop management systems that favor soil organic matter can improve the available nutrient content for plants and reduce the use of industrial fertilizer. Despite the importance of organic compounds as a P source, little is known about its dynamics in tropical environments. The objective of this study was to identify organic P species present in humic acids by NMR 31P analysis in a fine clay Fluventic Eutrochrepts in Campos dos Goytacazes, in the north of Rio de Janeiro State, Brazil, under long-term sugarcane cultivation with preservation of trash and stillage addition. NMR 31P showed an accumulation of organic P in more easily mineralizable forms such as P in diester linkages in the areas with organic matter preservation. In the areas where the crop trash was burned prior to harvesting, more stable organic P forms such as orthophosphates in monoester linkages were observed. These results obtained with NMR 31P would suggest that the humic acids constitute an important pool of labile organic P in the areas with higher organic matter content. In the areas with sugarcane burning, however, the recalcitrant organic P is accumulated in humic acids. This suggests that available P in humic substances is an important source of P for plant nutrition.
green cane; humic substances; vinasse; tropical soil
