Characterization by solid-state CPMAS 13C NMR spectroscopy of decomposing plant residues in conventional and no-tillage systems in Central Brazil

The Savanna region of Central Brazil is currently the most important area for grain production in the country but intensive agricultural activities are related to high losses of soil organic carbon. No-tillage systems were introduced in the mid 1980’s but the use of cover plants in no-tillage systems is poorly studied and there is a demand for selection of suitable species to improve soil organic carbon. This study characterizes the chemical composition of decomposing plant residues of different cover plants (Crotalaria juncea, Canavalia brasiliensis, Cajanus cajan, Mucuna pruriens, Helianthus annuus, Pennisetum glaucum, Raphanus sativus and natural fallow, as a control). Cover plants were used in rotation with maize, under conventional and no-tillage systems. Decomposition rates were estimated using litter bags and residues of C. juncea, C. brasiliensis, M. pruriens and R. sativus were analyzed by CPMAS 13C NMR. The highest decomposition rates were found for C. brasiliensis and C. juncea, while the lowest for M. pruriens, C. cajan and P. glaucum. C. cajan presented the lowest content of polysaccharides and along with M. pruriens, the highest percentage of aromatic C, reflecting the slow decomposition of highly lignified material. The residues of these two species also presented high hydrophobicity, as a consequence of the presence of aromatic groups. Incorporation of plant residues accelerated the decomposition in comparison to no-tillage system. C. cajan, P. glaucum and M. pruriens are more appropriate to increase soil cover due to lower decomposition rates while C. brasiliensis, R. sativus and H. annus, which presented higher decomposition rates, are indicated for an improvement of nutrient availability.