Efeito dos sistemas de colheita da cana-de-açúcar sobre o estoque de carbono no brasil e qualidade do solo em Alagoas

Abstract

The cultivation of sugarcane is one of the main agricultural activities in Brazil, among the crop harvesting systems, the mechanized one has been gaining space, mainly in substitution to the harvest with prior straw removal by fire. Mechanized harvesting deposits organic material on the soil surface, which may favor the increase of soil organic carbon (SOC). Thus, the objective of this work was to evaluate the impacts promoted by the conversion of sugarcane with burning (CQ) to sugarcane with mechanized harvest (CM) and the conversion of native vegetation (VN) to the systems CQ and CM on SOC stocks in Brazil, by deriving change factors in SOC stocks, and soil organic matter (SOM) compartments in Alagoas. This study included soil sampling and a literature review with COS data in sugarcane areas in Brazil. Soil sampling took place in sugarcane areas (CQ and CM) and VN. Samples were collected in five replicates per study area at depths of 0-10, 10-20, 20-30 and 30-50 cm. COS stocks, physical fractionation of SOM, carbon mineralization, microbial carbon, density, granulometry and soil sorption complex were determined. Briefly, the COS change factors (emission factors) showed that the CQ-CM conversion in Brazil ranged from 1.03 ± 0.15 to 1.21 ± 0.10, that is, gains ranged from 3 % and 21% depending on conversion time, depth and region. The factors of change in the COS stocks indicate that the CQ-CM conversion in the Southeast increases the COS stocks by 17% and 16% in the 0-30 and 0-50 cm layers, respectively, after 10 years of conversion, a value higher than the found in the Northeast (9% and 3%) for the same layer and conversion time. The VNCM conversion promotes reductions in COS stocks, with a factor ranging from 0.65 ± 0.07 to 0.80 ± 0.04. The VN-CQ conversion promotes reductions in COS stocks, with response ratios ranging from 0.65 ± 0.15 to 0.85 ± 0.16. The results for changes in SOM compartments in Alagoas showed that VN conversion in sugarcane cultivation areas, regardless of the harvesting system, reduced COS stocks between 7% and 62%, reduced the quality of the soil due to changes in important soil properties, such as the reduction of particulate organic matter (POM), increase of soil density and reduction of microbial biomass (Cmic), being corroborated by indices, such as, for example, high values of the quotient metabolic (qCO2) and degree of soil compaction (DSC) and lower values of the structural stability index (SSI). The CQ-CM conversion showed an increase in COS in some areas, between 2% and 9%. In general, it showed improvements for soil properties, such as POM, density and Cmic, confirmed by indices that show improvement, for example, lower DSC and higher SSI. However, it was not a consensus, as some areas of CM showed worse values for soil properties compared to CQ.

Keyword: soil organic matter; soil quality; emission of greenhouse gases; mitigation