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

Abstract

The cover plant use 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 cover plant, and compare the results with those from conventional tillage and fallow areas, in Uberaba, MG, Brazil. The implemented cover crops were: millet, tropical grass and sunn hemp. Furthermore, an area was used in fallow and another as a control area (conventional tillage). After cover crop removal, the areas were subdivided 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 0.0-0.20 m layer, TOC had the highest stocks for the millet area. 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 (palisade) 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 (0.025-0.05 m) and areas with tropical grass and sunn hemp (0.10-0.20 m), when compared to conventional tillage, independent of evaluated culture (corn and soybean). The highest C-FAH stocks in the depth of 0.0-0.025 m were found in the areas with millet and sunn hemp rather than the conventional tillage and fallow areas. In a general manner, the use of grass (millet and tropical grass) as cover plant 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.