Abstract
The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension (<53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.
Highlights
Studies providing evidence of the impact of conservation management systems on the improvement of carbon (C) stocks in tropical and subtropical soils have been conducted mostly in soil depths down to 30 cm
We investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate
Considering that the annual biomass addition under both management systems is similar in the two experimental areas, (Table 1), this behavior may be attributed to the maintenance of crop residues on the surface and to the lack of disturbance of the soil under NT, practices that slow down organic matter mineralization (Dieckow et al, 2009)
Summary
Studies providing evidence of the impact of conservation management systems on the improvement of carbon (C) stocks in tropical and subtropical soils have been conducted mostly in soil depths down to 30 cm. As regards the C storage capacity, the results obtained with surface layers of temperate soils indicated that saturation of the C retention capacity may occur in some soils, as shown by a discrete and asymptotic increase in C stocks with an increase in photosynthesized C inputs to the soil (Six et al, 2002; Stewart et al, 2009) This behavior was observed mainly in soils that had a high pre-existing soil C concentration (Carrington et al, 2012; Stewart et al, 2007, 2008, 2009). Fine-textured soils usually present a higher value for maximum C retention capacity than sandy soils (Angers et al, 2011) This behavior is attributed to a greater available surface area for the organo-mineral interactions shown by the clay and silt fractions (Stewart et al, 2008)
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