Indicadores de qualidade física e química do solo e espectrometria infravermelha em um Latossolo sob sistemas de pecuária extensiva no Cerrado Brasileiro

Abstract

The general objective of this work was to evaluate the chemical changes in the soil after the renovation of degraded pastures, using Fourier Transform Infrared Spectroscopy (FTIR) and physical fractionation of soil organic matter (MOS) techniques. The thesis is arranged in chapters, composing, in addition to the bibliographic review presented in Chapter I, Chapters II and III, which have specific objectives. In Chapter II, we sought to understand the spatial distribution of organic compounds characteristic of two bands related to soil organic matter (C aliphatic C–H and C=C aromatic elongation) as a function of soil disturbance and pasture renewal, as well as their relationships with CO2 flux (CO2-F) and changes in soil pH. In Chapter III, the objective was to evaluate the impact of the renewal of degraded pastures on the physical compartment of soil organic matter and its effect on the mitigation of CO2 emissions. The study that involves the two chapters was carried out in two areas of extensive beef cattle, in a region of the Brazilian Cerrado located in Selvíria, Mato Grosso do Sul. Geostatistical meshes were installed in the areas for CO2-F assessment and soil sampling. A semiquantitative analysis of the C–H and C=C vibrational regions was performed by means of spectral deconvolution, guided by the position of the peaks obtained in the second derivative and adjustment of the Gauss curve. The results from Chapter II indicated a spatial correlation between CO2-F and the relative abundance of symmetrical CH2 and asymmetric CH2 compounds in the C–H band. Pasture renovation, followed by sorghum cultivation intercropped with Urochloa brizantha, increased soil pH up to 5.6 and reduced concentrations of aliphatic compounds related to the C–H band. It also reduced CO2-F emissions to a range of 0.94 and 0.97 μmol m-2 s-1. The aromaticity index followed these changes and can be used as an indicator of soil organic matter decomposition, as well as CO2-F emissions. The results of Chapter III indicated that soil management practices, followed by the cultivation of sorghum intercropped with U. brizantha, increased total C stocks (TOC) through the stable fraction (C associated with the mineral fraction) and consequent reduction in CO2- F of the ground. This highlights the spatial variability in CO2 emissions and how soil attributes affect the flow of CO2 to the atmosphere. The use of multivariate geostatistics allowed the prediction of CO2 emissions from the soil. The results obtained suggest that the adoption of management practices, such as sorghum intercropped with U. brizantha and liming, increases soil pH and reduces the lability of organic compounds with the functional groups CH, CH2 and CH3, consequently, reduces emissions of CO2 from the soil. This small-scale work provides a theoretical basis for large-scale spatial monitoring of the biogeochemical processes that control CO2 emissions in agricultural ecosystems, particularly in degraded pasture areas. These practices can help Brazil achieve the goals of COP26.