Processing of organic matter input influences aggregate formation in artificial soils with different texture

Aggregate formation and stabilization depends on the interaction of minerals and soil organic matter (SOM). So far, little is known about the interplay of individual organic matter qualities and soil texture within this process. We developed an experimental set-up to study early soil development and aggregate formation within a controlled lab environment. We designed artificial soil microcosms with different texture, mimicking natural soils, and added organic carbon (OC) derived from particulate organic matter (POM, milled hay litter), dissolved organic matter (DOM, solution derived from hay), and bacterial necromass (Bacillus subtilis). We performed a short-term incubation for 30 days under constant water tension and investigated microbial activity, soil structure development and OC allocation compared to a control that did not receive additional OC input. 

OC input led to the formation of mostly large, water-stable macroaggregates (3000-630 µm) and some small microaggregates (<63 µm) in all microcosms as effect of microbial processing of the added OM. The addition and microbial decay of litter pieces led to physical occlusion of the particles into mainly large (3000-630 µm), OC-rich macroaggregates independent of the texture. The addition of DOM solution also induced the formation of large macroaggregates besides small microaggregates, although the OC input was much lower. Here, the aggregate formation was impaired by higher sand content in the mixtures. The addition of bacterial necromass led to the highest microbial activity, but relatively low aggregate formation, which might be a result of less physically active organic matter nuclei.

The results show that our experimental design allows to specifically investigate selected process complexes of soil structure formation defined by the addition of OM and soil texture.