Dinâmica da matéria orgânica do solo em área de soja cultivada sobre palhada de braquiária e sorgo

Abstract

The organic matter (OM) influence in soil attributes justifies the growing interest in identifying management systems which contribute to increasing and/or maintenance of carbon stocks in soils. The aim of this study was to evaluate the dynamics of soil organic matter in a soybean area cultivated on top of brachiaria (Brachiaria ruziziensis) and sorghum (Sorghum bicolor L. Moench) variety DKB 599 straw. Soil samples from a Rhodic Udox were collected in the municipality of Montividiu, Goiás State (Brazil). The samples were taken at five depths in the following agricultural systems: soybean after brachiaria planted in winter (SB), and soybean after sorghum planted in winter (SS). The physical properties analyzed were: bulk density (BD), soil granulometry, and granulometric fractioning of soil organic matter (SOM). The chemical analyses consisted of: Total Organic Carbon (TOC), Total Nitrogen (TN), and chemical fractioning of soil organic matter, determining the organic carbon in humin (CHUM), humic acids (C-HAF) and fulvic acids (C-FAF) fractions. The carbon stocks in the chemical and physical fractions of the soil organic matter and the N stocks were calculated. To evaluate the performance of the agriculture systems, the Carbon Management Index (CMI) also was calculated. The TOC results only showed statistical differences between the systems for the samples taken at 10-20 cm depth. The TN results were always higher in the SB than in the SS system. The highest carbon stock values were observed in the SS system, for all sampling depths. The particulate organic matter fractions values differed statistically between the systems, at all depths, showing to be an effective parameter to substantiate differences between the management systems. It was verified a predominance of the C-HUM, followed by C-FAF and C-HAF in the soil samples and for both systems. The carbon management index showed negative effect at 0-5 cm profundity when compared to the natural condition of Cerrado. However, for the sections of 5-10 and 10-20 cm, the systems had a positive effect on the increasing of soil carbon, with higher values of CMI compared to the natural condition of Cerrado.