Evolution CO2-C induced by plant-derived carbon soil input: Evaluation of the priming effect promoted by Meliaceae by-products

The application of organic fertilizer to soils may affect the rate of native soil organic matter mineralisation, inducing a positive priming effect (PE), with an increase of evolution of CO2 and a release of nutrient that may be in excess of crop demand. The objective of this study was to evaluate the effect of different Meliaceae derivatives, characterised by different C:N ratios, on the total soil CO2-C evolution, CO2-C fractions from the derivatives, PE magnitude, and to understand the mechanism that regulates PE. In a laboratory incubation experiment, the effects of three Meliaceae derivatives, named neem 5, neem 6, and melia leaves, were evaluated using the natural 13C abundance method, employing a naturally C4-plant labelled soil. Total CO2-C losses were proportional to the plant-derived C inputs. Soil CO2-C produced by the addition of the derivatives ranged between 22% (neem 6) and 40% (neem 5 and melia leaves) of the total C incorporated into the soil. All the derivatives studied induced a positive PE that increased with the amount of C added, but was not related to their chemical structure. Priming effect was also unaffected by soil mineral nitrogen, showing a response with little or no correlation with the ability of microbial biomass to locate nutrients in the soil (N mining). At the end of the experiment, the proportion of C primed corresponded to approximately 24%–28% of the added C, with no difference among the derivatives, and contributed to total CO2-C loss by approximately 31%, 30%, and 27% for neem 5, melia leaves, and neem 6, respectively.