Fig. 6: Conceptual framework for the key compounds and processes concerning dissolved organic matter (DOM) from surface soil in grassland ecosystems.

DOM (mainly in the form of dissolved organic carbon flow) in surface soil (0–20 cm) of grassland was mainly derived from plant sources and undergoes transport and transformation⁷⁷. First, as a portion of soil organic matter, DOM compounds would be processed and mineralized by microbes. Plant-derived humic-like compounds with large molecular mass are more persistent, they are degraded much slower and contribute more to carbon sink⁷⁸. Proteinaceous compounds with low or very high molecular weight, mainly derived from microbial activities (e.g., processing of humic-like compounds), are readily accessible to microbes and can lead to greater carbon emissions. Second, in addition to migration downwards, DOM can be delivered into aquatic ecosystems via hydrological processes (e.g., run-off or erosion events)⁷⁷. The intruded DOM can alter water chemistry and increase CO₂ outgassing⁷. Moreover, soil DOM in source water supplying drinking water can be ingested by humans directly or in the form of harmful byproducts (e.g., disinfection byproducts), and thus is potentially linked to human health.