Soil microaggregate size composition and organic matter distribution as affected by clay content

Aggregation assembles different size mixtures of soil particles into a larger architecture. Such mixtures impede resolving which particles build aggregates and how these control the accumulation of soil organic matter (OM). Here we present an approach to differentiate the size distributions of soil fractions in the size range of microaggregates (< 250 mu m) from their dispersible particle-size distribution using dynamic image analysis. This approach enabled us to differentiate the magnitude and preferential size ranges of aggregates and non-aggregated particles. Wet sieving was used to isolate free microaggregate-sized fractions. Larger soil structures > 250 mu m were sonicated to isolate occluded size fractions < 250 mu m. To investigate the impact of soil texture, we analyzed topsoil samples of an arable site on Cambisol soils with a gradient in clay content of 16-37% and organic carbon concentrations of 10-15 g kg(-1). Our results demonstrate how soil texture governs aggregate size distributions: most water-stable microaggregates were found to be of approximately 30 mu m diameter, independent of the clay content gradient. High-clay soils contain more water-stable macroaggregates (> 250 mu m) and larger microaggregates in the 50-180 mu m size range. The low-clay soils, on the other hand, contained more non-aggregated sand-sized particles > 100 mu m which probably hampered the buildup of larger aggregates. The size distribution of particles < 100 mu m in size fractions < 250 mu m showed a similar prevailing soil texture pattern, with approximately 24% clay, 59% silt, and 17% sand-sized particles at all clay contents. In contrast to the prevailing texture pattern along the clay content gradient, 4% more clay-sized particles helped build up water-stable macroaggregates. In the low-clay soils, the aggregates were smaller and the size fractions < 53 mu m had higher OM concentrations. This indicates that the low-clay soils held most of their OM in smaller microaggregates. Such arrangement of OM in smaller microaggregates demonstrates that soil texture may control OM stabilization mostly indirectly via the distribution of OM in different aggregate fractions. The occlusion of microaggregates in larger structures led to lower alkyl:O/N-alkyl ratios in C-13 nuclear magnetic resonance (NMR) spectroscopy, indicating increased preservation.