Pathways of biogenically excreted organic matter into soil aggregates

Soil organisms are recognized as ecosystem engineers and key for aggregation in soil due to bioturbation, organic matter (OM) decomposition, and excretion of biogenic OM. The activity of soil organisms is beneficial for soil quality, functions, and nutrient cycling. These attributions are based on field-scale observations that link the presence and activity of organisms to spatiotemporal changes in soil properties and can be traced back to the formation of biogenic aggregates. This biogenic formation pathway encompasses a cascade of processes so far not discussed comprehensively. A more general approach needs to consider the activity and feedback loops between soil biota, the active release of biogenic OM by excretion, the interaction of biogenic OM with soil constituents, the formation of organo-mineral associations, and how these become incorporated in aggregated structures. Especially the function of biogenically excreted OM, which is quite complex in composition, is controversial as it permits or inhibits aggregation. This review analyzes the various roles of biogenically excreted OM may take as an aggregation agent. We will show that its function depends on the interplay of numerous factors, including environmental conditions, variety of OM producers, composition and availability of biogenically excreted OM, and type of interacting mineral phase. We consider biogenically excreted OM to affect aggregate formation in three different ways: (I) as a bridging agent which promotes the aggregation due to surface modifications and attraction, (II) as a separation agent which favors the formation, mobility, and transport of organo-mineral associations and inhibits their further inclusion into aggregates, and (III) as a gluing agent which mediates aggregate stability, after an external force provokes a close approach of soil particles. We conclude that biogenically excreted OM takes these functional roles simultaneously and to a varying extent across spatiotemporal scales. Hence, biogenically excreted OM is involved in the surface modification of soil particles, in the enmeshment and gluing of particles into soil aggregates, in the (im-)mobilization, and in facilitating the transport of particles. All that depends on the interplay of a hierarchy of factors comprising the local soil community's composition, the properties of biogenically excreted OM, and the conditions of the immediate environment.