Microaggregate stability and storage of organic carbon is affected by clay content in arable Luvisols

As soil microaggregates ( < 250 mu m, SMA) usually withstand long-term tillage, we hypothesized that i) elevated clay contents in arable soil support aggregation already at microaggregate level, leading to ii) increasing organic carbon (SOC) enrichment in smaller SMA size fractions. To test these hypotheses we sampled the topsoil (5-20 cm) of Luvisols with a long history of agricultural management at the Scheyern experiment station (Germany) in quintuplicates from each of five subsites with different clay contents (19-34%). The field-fresh topsoil was fractionated into macroaggregates (8000-250 mu m), large SMA (250-20 mu m), and small SMA ( < 20 mu m) and the mass distribution was recorded. In addition, the water stable macroaggregates ( > 250 mu m) were dispersed ultrasonically to study occluded SMA and single building units. Finally, we analyzed the size distribution of the small SMA by laser diffraction analysis. The total mass distribution of free and occluded SMA grouped soils into those with small (19, 22, and 24%) and large (32 and 34%) clay contents. The finer textured soils exhibited larger portions of occluded SMA, with a gamma size distribution of small SMA peaking at 6 mu m. Yet the occluded small SMA in the finer textured soils showed an additional enrichment of colloids < 1 mu m. The SOC was indeed enriched in finer fractions, but more in the small SMA of the coarse textured sites than in the finer textured ones, whereas the opposite was true for the large SMA. We conclude, therefore, that elevated contents of clay-sized particles promote SMA formation and stabilization, therewith shifting SOC enrichment from small to larger SMA.