Organossolos: funções de pedotransferência para densidade do solo, avaliação do grau de subsidência, e estoques de carbono

Abstract

The Histosols are a major source of carbon storage in terrestrial environments and have high susceptibility to carbon losses when disturbed. The objective of this study was to generate pedotransfer functions (FPT) to assess the accuracy of previously published equations, applying them to predict soil bulk density (Bd) in organic soils from Brazil; to evaluate subsidence rate, and the variation of Bd and carbon of humic substances in Histosols over a period of one year; and to estimate stock and potential loss of carbon in Histosols from Rio de Janeiro State. For the first part of the study, there were used organic horizons, i.e., soil materials with total organic carbon (TOC) equal to or greater than 80 g kg-1 soil, totaling 280 horizons from different regions of Brazil. It was used the multiple linear regression technique and the equations were validated on independent data. There were tested 9 equations already published in the literature. The equations with better performance were the Hollis and FPT2, with R2 validation parameters of 0.48 and 0.49. When the clay fraction data is availlable, it is recommended to use the FPT1 equation; if there is no data on clay it is recommended the FPT2 and Hollis equations, which have only the TOC as a predictor variable. For the second part, a greenhouse experiment was carried out. For that, undisturbed samples were collected using PVC pipes, of two profiles of Organossolos Tiomórficos, one in the neighborhood of Santa Cruz (SC Profile), city of Rio de Janeiro, and one in Magé (Profile MG) municipality. There were evaluated 3 drainage levels, at 30, 60, and 100 cm deep, with 5 measuremts over time, being 0, 90, 180, 270, and 360 days (4 replicates). The MG profile presented the highest rates of subsidence, reaching 1.30 cm year-1 for the 100 cm drainage. Bd increased over time for both profiles, and the deeper drainage increased values in the SC profile. The deeper drainage favored the reduction of pH for both profiles. The TOC showed a decreasing trend over time. The values of carbon in the fulvic acid fraction (FAF), humic acid fraction (HAF), and humin fraction (HUM) showed no differences according to drainage levels. However, over time they showed high sensitivity to temperature changes, showing high levels of FAF and HAF at the end period and a consequent reduction in humin fraction. For the third and final section, there were used 43 soil profiles, where 18 of them did not have bulk density data, which were estimated by the FPTs. Comparisons between measured and estimated data groups was performed using the Wilcoxon test. The spatial distribution of the variables was performed using the IDW interpolation method. The average values of TOC was 228.0 g kg-1, Bd was 0.48 Mg m-3, the thickness of the profiles was 86 cm, the depth was 90 cm and the average stock TOC was 73.51 kg m-2. The spatialization showed qualitative differences for the variables in the different profile locations. The estimated TOC stock for the profiles form RJ State was 27,178,631.8 Mg. The estimated rates of loss were 10.87 kg m-2 for Histosols more recently drained and high TOC; of 7.16 kg m-2 for intermediate drained Histosols; and 1.46 kg m-2 for Histosols with longer time after drainage and low TOC. Soils with high TOC levels are more likely to have high losses TOC when disturbed.