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

Dissolved organic matter (DOM), the most active functional component of soil organic matter (SOM), plays a vital role in regulating soil biogeochemical processes and accumulation and decomposition of SOM and their responses to global change. Although the quantity and fluxes of soil DOM has been inventoried across diverse spatio-temporal scales, the underlying mechanisms accounting for the variability in DOM dynamics remain unclear especially in upland ecosystems.

Mollisols is famous for its high fertility and is vital for global crop production. Northeast China is one of the three Mollisols regions in the world, contributing to 1/4 of Chinese grain production. However, a loss of SOM has occurred in this area over the past decades, both in its content and activity. Understanding the processes involved in DOM transformations is of critical importance for SOM management. Here, a gradient of SOM storage across twelve Mollisols uplands with various cultivation years in northeast China were 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, and iron (Fe), Fe-bound organic matter and nutrient concentrations were quantified to clarify the drivers of DOM quality.

Changes in concentrations of DOM and SOM were tightly coupled across various croplands. The proportion of biodegradable DOM increased exponentially with decreasing DOM concentration. Spectral analyses showed that larger fractions of small-molecular phenols and proteinaceous components mainly contributed to the greater biodegradability of DOM. Unexpectedly, the composition of DOM was decoupled from that of SOM or MAOM, but significantly related to enzymatic properties. Further analyses indicated that soil oxygen availability exhibited a dominant role in DOM generation. As DOM concentration declined, increased soil oxygen availability regulated DOM composition and enhanced its biodegradability mainly through three ways: 1) stimulated oxidase-catalyzed depolymerization of humic substances into small aromatic molecules; 2) promoted production of protein-like DOM components due to lower enzymatic C/N acquisition ratio; and 3) oxygen-induced oxidation of Fe(II) to Fe(III) removed complex DOM compounds with large molecular weight. Therefore, along with aggregates fragmentation during the decline of SOM in Mollisols with longer cultivation history, the increased oxygen availability improved the biodegradability of DOM and accelerated the turnover and loss of active SOM pool.

Overall, this study demonstrated the cascading effects of oxygen on soil Fe oxidation-reduction, microbial metabolism and the dynamics of DOM, which would help understand the processes of labile SOM transformations and interactions among various drivers and improve the SOM managements in upland ecosystems and the predictions of responses of soil carbon to climate change.