Zooming in on surface chemical composition of soil microaggregates – Is there an effect of plants (Festuca)?

Formation of soil microaggregates (SMA) is a surface-driven process and depends on mineral cementing and organic gluing agents. Yet, the role of plants in soil microaggregation by input of fresh organic matter remains little understood. In a mesocosm experiment silty Luvisol topsoil (<250 µm; original soil material) was incubated in absence (bare soil) and presence of plants (Festuca) and water-stable free and occluded SMA were isolated after 4, 12, and 30 weeks and investigated for the surface chemical composition by X-ray photoelectron spectroscopy (XPS) and for wetting properties by contact angle determination.

Compared to the original soil, the surfaces of both free and occluded SMA tended to smaller O and larger C contents, thus a smaller O/C ratio, along with a slight increase in initial contact angle from about 10° (original soil) to about 20° (SMA). The O/C ratio decreased slightly further from 4 to 12 weeks, especially for bare soil without plants. Slightly greater C contents were detected for occluded than for free SMA, probably hinting at higher retention of organic matter on surfaces of microaggregates entrained in larger soil structures. For bare soil, a slightly greater N content was observed for free SMA while in the presence of Festuca free and occluded SMA had same N contents.

Regardless of the presence of Festuca, C speciation indicated a lower proportion (in % of total C) of C=O/O-C-O and a higher proportion of C - C/C -  H species for occluded than for free SMA, probably indicating less altered organic matter at the surfaces of occluded SMA. While the proportion of C=O/O-C-O species slightly decreased, that of C- C/C-H species slightly increased towards the end of the incubation. This may hint at some preferences in microbial respiration with respect to C compounds and formation of microbial metabolites. From N speciation a higher ratio between protonated and non-protonated organic N species (N_p/N_np) was indicated for Festuca than for bare soil after 4 and for 30 weeks of incubation, i.e., the presence of plants seems to impact N compounds present. The N_p/N_np ratio tended to decrease after 30 weeks compared to 4 weeks for both treatments, hinting on changes in N species present.

In summary, aside some effect on N species present, results indicate rather incubation and SMA origin (free, occluded) than the presence of plants (Festuca) to impact surface chemical composition of the tested SMA. This suggests no defined contribution of plants and their products to formation of 250-53 µm-sized SMA.