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

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Abstract
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. Soil samples undeformed were collected at the layer of 0-10 cm in second-growth forests in different successional stages and a pasture area. To identify the pathways of aggregation were used morphological patterns proposed by Bullock et al. (1985), and established three clusters: fisiogenic, biogenic and intermediates. The aggregates were analyzed for aggregate stability, exchangeable cations, distribution of total organic carbon (TOC) and oxidizable fractions of total organic carbon. In all areas evaluated the percentage of physiogenic aggregates was higher than biogenic and intermediate aggregates. The biogenic aggregates were found at a smaller amount, with the highest means of MWD (4.520 mm and 4.896 mm) and GMS (3.678 mm and 4.479 mm) under SFAS and MMP areas. The biogenic aggregates presented higher levels of K and P between the morphological types in all areas studied, with the higher phosphorus 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 SFMS, 24.74 g kg-1 in SFIS 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 intermediate and physiogenic aggregates. The biological agregation 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.