Oxygen availability regulates the quality of soil dissolved organic matter by mediating microbial metabolism and iron oxidation

Dissolved organic matter (DOM) plays a vital role in biogeochemical
processes and in determining the responses of soil organic matter (SOM) to
global change. Although the quantity of soil DOM has been inventoried
across diverse spatio-temporal scales, the underlying mechanisms
accounting for variability in DOM dynamics remain unclear, especially in
upland ecosystems. Here, a gradient of SOM storage across twelve croplands
in northeast China was used to understand links between DOM dynamics,
microbial metabolism, and abiotic conditions. We assessed the composition,
biodegradability and key biodegradable components of DOM. In addition, SOM
and mineral-associated organic matter (MAOM) composition, soil enzyme
activities, oxygen availability, soil texture, iron (Fe), Fe-bound organic
matter and nutrient concentrations were quantified to clarify the drivers
of DOM quality (composition and biodegradability). The proportion of
biodegradable DOM increased exponentially with decreasing initial DOM
concentration due to larger fractions of depolymerized DOM that was rich
in small-molecular phenols and proteinaceous components. Unexpectedly, the
composition of DOM was decoupled from that of SOM or MAOM, but
significantly related to enzymatic properties. These results indicate that
microbial metabolism exhibited a dominant role in DOM generation. As DOM
concentration declined, increased soil oxygen availability regulated DOM
composition and enhanced its biodegradability mainly through mediating
microbial metabolism and Fe oxidation. The oxygen-induced oxidation of
Fe(II) to Fe(III) removed complex DOM compounds with large molecular
weight. Moreover, increased oxygen availability stimulated
oxidase-catalyzed depolymerization of aromatic substances, and promoted
production of protein-like DOM components due to lower enzymatic C/N
acquisition ratio. As global changes in temperature and moisture will have
large impacts on soil oxygen availability, the role of oxygen in
regulating DOM dynamics highlights the importance of integrating soil
oxygen supply with microbial metabolism and Fe redox status to improve
model predictions of soil carbon under climate change.