Soil organic matter distribution governed by aggregation and decoupled from clay content

Mr. Steffen Schweizer1, Ms. Franziska Bucka1, Dr. Markus Graf-Rosenfellner2, Prof. Ingrid Kögel-Knabner1,3

1Soil Science, TU Munich, Freising, Germany, 2Soil Ecology, University of Freiburg, Freiburg, Germany, 3Institute for Advanced Study, TU Munich, Garching, Germany

The accumulation of soil organic matter (OM) through organo-mineral interactions is anticipated to be stimulated by the soil clay content. The mixing of soil particles into larger aggregate structures impedes the identification of which particles comprise the aggregates and how these control the OM distribution. Here we show how the influence of clay content can be resolved based on the underlying impact of size-specific aggregation on OM sequestration. We used dynamic image analysis to differentiate the size distributions of free water-stable microaggregate size fractions (<250 µm) and those occluded in larger soil structures from their dispersible particle-size composition. Differentiating aggregated from dispersed size distributions also enabled to identify the preferential size ranges of aggregates that break down to particles and non-aggregated particles that remain. To investigate the impact of soil texture, we analyzed topsoil samples of an arable site on Cambisol soils with a gradient in clay content within a range of 16–37 %. Our results demonstrate that soil texture governs aggregate distributions and sizes. High-clay soils contain more water-stable macroaggregates (>250 µm) and larger microaggregates in the 50–180 µm size range. Non-aggregated sand-sized particles >100 µm probably impede the buildup of larger water-stable aggregates in low-clay soils. The size distribution of particles <100 µm in size fractions showed a similar prevailing pattern for all clay contents, whereas 4 % more clay-sized particles helped build up water-stable macroaggregates. In the low-clay soils, the aggregates were smaller and had higher OM concentrations. This explains the fact that higher amounts of OM could be held in aggregates of the low-clay soils despite containing coarser texture. This interaction reveals that OM sequestration is decoupled from the particle-size distribution. Instead, the occlusion of aggregate size fractions led to lower alkyl:O/N-alkyl ratios in 13C NMR spectroscopy indicating increased preservation.


Almost-finished PhD student (since December 2015) working on ‘Soil microaggregation and microspatial patterns of organic matter accrual’. Previous to his PhD, he has worked on the impact of organic farming on soil aggregates in Indian Vertisols. Therefore, he’s excited to present his work in the surrounding where the dynamic nature of soil aggregation has been systematically described for the first time.

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