Storage of organic carbon in aggregate and density fractions of silty soils under different types of land use

The type of land use and soil cultivation are important factors controlling organic carbon storage in soils and they may also change the relative importance of different mechanisms of soil organic matter stabilization. Our objectives were: i) to quantify the soil organic carbon (SOC) and nitrogen (N) storage in silty soils under wheat, maize, grassland and spruce, ii) to determine the SOC and N storage in water-stable aggregates of different size (2000 μm) and in density fractions (Mineral-associated soil organic matter >2 g cm−3 (Mineral-SOM), free particulate organic matter 2000 μm) and in density fractions (Mineral-associated soil organic matter >2 g cm−3 (Mineral-SOM), free particulate organic matter 2000 μm) and in density fractions (Mineral-associated soil organic matter >2 g cm−3 (Mineral-SOM), free particulate organic matter 1000 μm were most abundant in the grassland and forest soil. The SOC concentration and the C/N ratio were greater for macroaggregates (>250 μm) than microaggregates (1000 μm were most abundant in the grassland and forest soil. The SOC concentration and the C/N ratio were greater for macroaggregates (>250 μm) than microaggregates (1000 μm. The major part (86–91%) of the SOC was associated with the heavy mineral fraction at the grassland, maize and wheat site. In the A horizon of the spruce stand, the particulate organic matter accounted for 52% of the total SOC content. The C/N ratios of density fractions decreased in the order free POMoccluded POM>Mineral-SOM for all soils and depths. The mean age of organic carbon in the water-stable aggregates in the Ap horizon of the maize site increased with decreasing aggregate size from 35 yr (>1000 μm) to 86 yr (<53 μm). For the density fractions the order was free POM (22 yr)