Abstract
Abstract The objective of this work was to evaluate total soil carbon and nitrogen, as well as their contents in particulate and mineral-associated C fractions; to determine C stock and sequestration rates in the soil; and to verify the effect of C and N contents on soil aggregation, using different crop rotations and crop sequences under no-tillage. The study was carried out for nine years in a clayey Oxisol. The treatments consisted of different cropping systems formed by the combination of three summer crops (cropped until March) - corn (Zea mays) monocropping, soybean (Glycine max) monocropping, and soybean/corn rotation - and seven second crops (crop successions). Soil samples were taken at the 0.00-0.10-m layer for physical fractionation of C and N, and to determine soil aggregation by the wet method. Soybean monocropping increased C and N in particulate C fraction, while the crop systems with corn monocropping x pigeon pea (Cajanus cajan), corn monocropping x sun hemp (Crotalaria juncea), and soybean monocropping x corn as a crop succession increased total C in the soil. Greater rates of soil C sequestration were observed with soybean/corn rotation and with soybean monocropping, as well as with sun hemp as a second crop. The increase in total N increases soil C stock. Soil aggregation was most affected at particulate C fraction. Increases in soil N promote C addition to particulate fraction and enhance soil aggregation.
Highlights
Agricultural soils of the tropical regions contain less soil carbon than their capacity to store, because of the high soil organic matter mineralization (Lal, 2005)
Neither summer crops nor second crops affected the contents of carbon and nitrogen in fine particulate and mineral-associated fractions
Carbon and nitrogen contents showed the lowest values with the second crops corn, sorghum, and sunflower (Table 2)
Summary
Agricultural soils of the tropical regions contain less soil carbon than their capacity to store, because of the high soil organic matter mineralization (Lal, 2005). Legumes are often used in conservationist soil management systems due to their capacity to biologically fix high quantities of nitrogen and can contribute to soil carbon addition at rates of 0.88 Mg ha-1 per year (Diekow et al, 2005) These additions are commonly linked to increases in soil aggregation, which, in turn, protect soil carbon against microbial decomposition (Tisdall & Oades, 1982; Conceição et al, 2013). Grasses are mainly used in conservationist systems due to their high aboveground biomass yield, with greater C:N ratio (Marcelo et al, 2012a), and to their dense root system, which is associated with intense microbial activity These croptype features can increase soil carbon at a rate of 0.71 Mg ha-1 per year (Calegari et al, 2008; Martins et al, 2012a)
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