Organic matter in density fractions of water-stable aggregates in silty soils: Effect of land use

The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm-3 (free POM<1.6), occluded particulate organic matter with densities <1.6 g cm-3 (occluded POM<1.6) and 1.6-2.0 g cm-3 (occluded POM1.6-2.0) and mineral-associated SOM (>2.0 g cm-3) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 micrometer) and in the sieve fraction <53 micrometer from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the 13C/12C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM<1.6>occluded POM<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 micrometer mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 micrometer, 76 yr in the 53-250 micrometer aggregate class, and 102 yr in the sieve fraction <53 micrometer. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 micrometer to 66 yr in aggregates <53 micrometer. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 micrometer to 42 yr in the fraction 53-250 micrometer. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.