GRANULOMETRIC AND HUMIC FRACTIONS CARBON STOCKS OF SOIL ORGANIC MATTER UNDER NO-TILLAGE SYSTEM IN UBERABA, BRAZIL

摘要

The use plant cover preceding grain crops in Cerrado soil can increase the carbon stocks of chemical and physical fractions of soil organic matter (SOM). The present study aimed to quantify the carbon stocks of SOM granulometric and humic fractions in a Cerrado area under no-tillage system with different plant cover, and compare the results with those from conventional tillage and fallow areas, in Uberaba, MG, Brazil. The implemented plant cover were: millet, tropical grass and sunn hemp. Furthermore, an area was used in fallow and another as a control area (conventional tillage). After plant cover removal, the areas were subdivided (subplots) for the corn and soybean plantation. Soil samples were collected in the 0.0-0.025, 0.025-0.05, 0.05-0.10 and 0.10-0.20 m depths, with posterior quantification of total organic carbon (TOC) levels and chemical and granulometric fractionation of SOM. Humic acid carbon (C-HAF), fulvic acids (C-FAF) and humin (C-HUM) were quantified through these fractionations. The granulometric fractions consisted in particulate organic matter (POM) and mineral organic matter (MOM). Using the carbon levels for each fraction, the respective stocks for each depth were calculated, including the 0.0-0.20 m layer. In the depth of 0.0 to 0.025 and 0.025 to 0.05 m, the TOC had the highest stock for the plant covers of tropical grass, fallow and sunn, respectively, with the lowest stoks found under conventional management The highest POM stocks were found for the corn plantation over sunn hemp and the fallow and soybean area over millet and tropical grass (0.0-0.20 m). In relation to the MOM stocks, the highest values were observed in the areas with millet, sunn hemp and tropical grass, all superior to those found in the conventional tillage and fallow areas, independent of evaluated culture (0.10-0.20 m). The highest C-HUM stocks were observed in the area with tropical grass when compared to conventional tillage, independent of evaluated culture (corn and soy bean) or the depth (0.0-0.025 and 0.10-0.20 m). The highest C-FAH stocks in the depth of 0.0-0.025 m were found in the areas with plat covers than the conventional tillage and fallow areas. In a general manner, the use of grasses (millet and tropical grass) as plant cover preceding soybean tillage and legumes (sunn hemp) preceding corn tillage, favored a raise in the stocks of humic and granulometric fractions of SOM, especially POM and C-FAH, the fractions most benefitted by this soil management.