Densimetric fractionation of organic matter in an agricultural chronosequence in no-till areas in the Cerrado region, Brazil

Abstract

Mineralization of organic residues deposited on the soil surface in a no-till system (NT) maintains a continuous flow of different forms of carbon (C), which might interfere with densimetric fractions of soil organic matter (SOM). Currently, there are few studies on variations in these fractions in NT areas with different deployment times in the Cerrado region. Thus, the objective of this study was to evaluate the total C, nitrogen (N), and 13C in the soil, as well as to quantify the mass and contents of C, N, and 13C in the light and heavy fractions of SOM extracted using different solutions (water, sodium iodide [NaI], and sodium polytungstate [SPT]) in areas of Cerrado, pastures, and NT with different deployment times with Distroferric Red Latosol soil. The study areas are located in Montividiu, Goiás (GO), Brazil: (1) pasture of Brachiaria decumbens (PA); (2) NT with three years of deployment with soybean in summer and fallow instead of a second crop (NT3); (3) NT with 15 years of deployment with soybean in the summer and maize/sorghum as the second crop (NT15); and (4) NT with 20 years of deployment with soybean in the summer and maize as the second crop (NT20). These areas were compared to an area of native Cerrado sensu stricto (CE). The NT according to the deployment time showed an increase in the contents of C (0-0.05 m) and N (0-0.2 m) in the soil. The origin of C in the soil of the NT areas is associated with C4 plants; however, in up to 0.2 m of the soil profile, the 13C contents reduced according to NT deployment years. Extraction of light-fraction organic matter (LFOM) with SPT better represents this SOM fraction quantitatively and qualitatively than extraction of LFOM with water and NaI. This pattern is evident because of a greater consistency in the mass, C, N, and 13C contents of the LFOM extracted with SPT among the evaluated areas and the lower C content in residual particulate organic matter among the extractants.