Soil organic matter and nutrient pools under long-term non-burning management of sugar cane

We employed CP/MAS C-13 NMR spectroscopy and classical humus fractionation to evaluate the behaviour of soil organic matter (SOM) resulting from long-term sugar cane production when harvested without burning. Soil samples were collected at two depths (0-0.2 and 0.2-0.4 m) from a tropical Entisol cropped with sugar cane for 55 years at Campos dos Goytacazes (Brazil), either with or without burning crop residues when harvesting. Continuous incorporation of sugar cane residues increased carbon (C) content and promoted changes in SOM, including increases in humins, humic acids and aromatic C moieties. Sequential extraction methods showed enhanced amounts of available forms of nitrogen (N), sulphur (S) and phosphorus (P). Moreover, P-31 NMR analysis revealed an increase in organic P as diester phosphates of plant origin in the humic acids fraction. The increased macronutrient bioavailability of labile forms of N, P and S could be partially related to the more hydrophobic nature of SOM observed in soil samples collected from areas that were not burnt. These results show that the long-term non-burning management of sugar cane harvest promotes the hydrophobic character of SOM and a more conservative and sustainable management of soil fertility.