Characterization of the Colloidal Properties of Dissolved Organic Matter From Forest Soils

Abstract

Components of dissolved organic matter (DOM) span from sub-nm molecules to colloidal aggregates of several hundred nm. The colloidal fraction is important for the transport of organic matter and associated elements in the environment, and for the stability of DOM constituents with respect to microbial decomposition. This study focuses on the colloidal properties of DOM extracted from spruce forest soils of a chronosequence. The DOM samples were obtained by common water extraction procedures at 21 and 100°C, respectively. We applied an experimental approach combining chemical analysis with light and X-ray scattering techniques that informed on the colloidal size, charge, and structure of DOM. Results showed that two main types of colloids were present: semi-flexible cylinders and fractal aggregates. The cylinders consisted of carbohydrates, presumably hemicelluloses, while the aggregates were a composite material containing a large fraction of carbohydrates together with aliphatics and clay particles. These fractal aggregates dominated the cold-water extracts whereas the strong increase in total organic carbon by hot-water extraction caused a concomitantly strong increase of semi-flexible cylinders, which became the predominant species. Comparison between the chronosequence soils showed that with increasing forest age, the amount of carbon extracted per gram of soil declined and the concentration of the semi-flexible cylinders decreased. Thus, the distribution between the fractal aggregates and cylinders in the forest soil DOM samples depends on the composition of the soil organic matter and the leaching temperature. Changes in this distribution may have important implications for the reactivity and stability of DOM colloids.

Phototrophic Biofilms Transform Soil-Dissolved Organic Matter Similarly Despite Compositional and Environmental Differences

“…Concurrently, consumption of labile DOM left behind persistent compounds that can enhance the storage of SOC. Moreover, due to the colloid traits, persistent DOM can adsorb a large number of ions and thus maintain the fertility . As increasing the bioavailability or persistence of soil DOM enhances fertility, both growth and decomposition of phototrophic biofilms may help promote rice yields and thereby tackle food insecurity.…”

Decomposition of soil organic matter by ectomycorrhizal fungi: Mechanisms and consequences for organic nitrogen uptake and soil carbon stabilization

“…A recent study from our group shows that ca. 40-70% of the total organic C in DOM extracted from boreal forest is present in a colloidal size fraction (Meklesh et al, 2022). Important challenges for future research are to examine whether nutrients present in colloidal fractions are accessible to ECM fungi and whether the decomposing activity of ECM fungi will alter the colloidal fractions of organic matter.…”