Abstract
Eucalyptus grandis and other clonal plantations cover about 3.5 million ha in Brazil. The impacts of intensively-managed short-rotation forestry on soil aggregate structure and Carbon (C) dynamics are largely undocumented in tropical ecosystems. Long-term sustainability of these systems is probably in part linked to maintenance of soil organic matter and good soil structure and aggregation, especially in areas with low-fertility soils. This study investigated soil aggregate dynamics on a clay soil and a sandy soil, each with a Eucalyptus plantation and an adjacent primary forest. Silvicultural management did not reduce total C stocks, and did not change soil bulk density. Aggregates of the managed soils did not decrease in mass as hypothesized, which indicates that soil cultivation in 6 year cycles did not cause large decreases in soil aggregation in either soil texture. Silt, clay, and C of the sandy plantation soil shifted to greater aggregate protection, which may represent a decrease in C availability. The organic matter in the clay plantation soil increased in the fractions considered less protected while this shift from C to structural forms considered more protected was not observed.
Highlights
Soil organic matter (SOM) is important for the maintenance of soil structure and fertility (Shang & Tiessen, 1998; Fisher & Binkley, 2000)
Soil organic matter is protected within soil aggregates in undisturbed soils; upon disturbance of aggregates by cultivation, the SOM may become vulnerable to accelerated decomposition or redistribution to less labile soil particle-size fractions (Tiessen & Stewart, 1983; Cambardella & Elliot, 1994; Hassink et al, 1997; Six et al, 2000)
Contrast tests for total C in each depth yielded pvalues of 0.92 for forests on the sandy soil, and 0.75 for forests on the clay soil indicating that each forest within a soil texture had an equal amount of C, and that total soil C did not diminish under intensive plantation management (Table 3)
Summary
Soil organic matter (SOM) is important for the maintenance of soil structure and fertility (Shang & Tiessen, 1998; Fisher & Binkley, 2000). Soil organic matter is protected within soil aggregates in undisturbed soils; upon disturbance of aggregates by cultivation, the SOM may become vulnerable to accelerated decomposition or redistribution to less labile soil particle-size fractions (Tiessen & Stewart, 1983; Cambardella & Elliot, 1994; Hassink et al, 1997; Six et al, 2000). The incorporation of SOM into a more protected micro-environment is characterized by a depletion of labile fractions and a shift towards fine silt, coarse clay, and microaggregate-associated materials with low nutrient availability (Tiessen & Stewart, 1983; Sollins et al, 1996), possibly reducing the long-term sustainability of intensive soil management. Besides the greater degree of physical protection, SOM contained within micro-aggregates is generally considered to be more decomposed or more recalcitrant than SOM contained in larger aggregates (Lützow et al, 2007)
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