Aggregate formation and soil organic matter under different vegetation types in Atlantic Forest from Southeastern Brazil

Changes in soil use and management can affect the soil aggregation, including aggregate formation by biogenic and physicogenic processes. The aim of this study was to analyze physical and biological influences on the genesis of soil aggregates in areas with different vegetations in the Atlantic Forest, as well as to compare physical and chemical attributes of the aggregates formed in different ways. Undeformed soil samples were collected from a depth of 0-10 cm from second-growth forests at different successional stages and a pasture area. To identify the pathways of aggregation we used morphological patterns proposed by Bullock et al. (1985), and established three clusters: physicogenic, biogenic, and intermediates. The aggregates were analyzed for stability, exchangeable cations, distribution of total organic carbon (TOC), and oxidizable fractions of total organic carbon. In all areas evaluated, the percentage of physicogenic aggregates was higher than that of biogenic and intermediate aggregates. The biogenic aggregates with the highest mean weight diameter (MWD) and geometric mean diameter (GMD) were recorded for samples from the second-growth forest at an advanced stage of succession (SFAS) and from the mixed managed pasture (MMP) sites (MWD: 4.520 mm and 4.896 mm; GMD: 3.678 mm and 4.479 mm, respectively). The biogenic aggregates presented higher levels of K and P compared to the other morphological types in all areas studied, with higher P levels in the SFAS area. The TOC content was also higher in the biogenic aggregates in all study areas, with 22.33 g kg(-1) in SFAS, 25.60 g kg(-1) in the site with second-growth forest at the middle stage (SFMS) of succession, 24.74 g kg(-1) in the site with second-growth forest at the initial stage (SFIS) of succession, and 20.28 g kg(-1) in MMP. The highest content of the fractions F1 (6.93 g kg(-1)) and F2 (7.43 g kg(-1)) were found in the biogenic class compared to the intermediate and physicogenic aggregates. The biological aggregation process was the most efficient process in terms of soil structural stability and carbon sequestration, and biogenic aggregates may be considered indicators of soil quality.