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Abstract: Changes from natural tree species to rapidly growing exotic species as well as intensification of forestry operations with heavy machinery can lead to changes in the quantity and quality of organic matter inputs to soil and to disruption of soil physical structure. These two ecosystem properties are tightly linked to organic matter dynamics. Five adjacent forest stands were selected to study soil organic matter dynamics in soil physical fractions. On one hand, two semi-natural broadleaved forests (Quercus robur, Fagus sylvatica) and an adult radiata pine plantation (40-year-old,) in order to study the effect of species change on these parameters, and on the other, a chronosequence of Pinus radiata plantations (40-year-old; 3-year-old; 16-year-old), to study the effect of mechanization during harvesting and intense site preparation. Samples of intact topsoil (0–5 cm) were collected and aggregate-size distribution, mean weight diameter (MWD), total C and N, particulate organic matter (POM)-C, POM-N and microbial biomass-C were determined in each aggregate size fraction. Microbial respiration and nitrogen mineralization were also assessed in each aggregate size fraction, during a 28 day incubation period. Losses of POM-C and POM-N in the bulk soil due to mechanical site preparation were high relative to total soil C and N, which suggests that POM is a sensitive parameter to the effect of mechanization. The ratio C-POM:SOM was significantly related to MWD (R 2 = 0.75, P < 0.001) reflecting that POM may play a key role in the topsoil aggregate formation in these stands. Semi-natural stands had a higher proportion of macroaggregates (0.25–2 mm) than the cultivated adult one. Megaaggregates (>2 mm) were the most abundant class in mature stands (82–92%), whereas macro- and microaggregates (<2 mm) were the most abundant ones in the intensely soil prepared P. radiata plantation (49%). Indicators for sustainable forest management related to soil organic matter should not only be assessed in terms of total C stocks but also with respect to sensitive organic matter and its degradability in different size classes. [Copyright &y& Elsevier]

Abstract: Carbon stabilization by macroaggregate-occluded microaggregates (Mm) has been proposed as a principal mechanism for long-term soil organic carbon (SOC) sequestration in temperate alternative agricultural and (af)forested systems. The aim of this study was to evaluate the importance of the Mm fraction for long-term C stabilization in Oxisols and to validate its diagnostic properties for total SOC changes upon changes in land use. Soil samples were taken from the 0–5 and 5–20cm soil layers of native forest vegetation (NV), conventional tillage (CT) and no-tillage (NT) systems at an experimental site near Passo Fundo and one near Londrina in Southern Brazil. After aggregate-size separations by wet-sieving, macroaggregate-occluded water-stable microaggregates (53–250μm) (Mm) were isolated from large (>2000μm) and small (>250μm) macroaggregates. Particulate organic matter located inside the Mm (intra-Mm-POM) and the mineral fraction (< 53μm) associated with the Mm (mineral-Mm) were separated from the POM fraction located outside the Mm (inter-Mm-POM) by density flotation followed by mechanical dispersion. Sand-free Mm-C concentrations on a macroaggregate basis were generally greater under NV and NT compared to CT in the 0–5cm depth at both sites. Our findings support the importance of Mm (especially the mineral-Mm fraction) as long-term C-stabilization sites in highly weathered tropical soils under sustainable agricultural and natural systems. At both sites, significant differences in total SOC stocks (gCm−2) among different land use systems were always accompanied by parallel Mm-C stock differences. Though total SOC did not differ among land use systems in the 0–20cm depth at both sites, Mm-C stocks were greater under NT compared to the CT treatment in the 0–20cm depth at the Londrina site. We concluded that in these highly weathered tropical soils the Mm-C fraction is a more responsive fraction to management changes than total SOC and represents a diagnostic fraction for present as well as potential total SOC changes upon land-use change. [Copyright &y& Elsevier]