Iron oxides, enzymes, and nutrients control soil organic matter stability in subtropical forest ecosystems

Although mineral protection plays a pivotal role in maintaining the long-term stability of soil organic matter (SOM), the mechanisms by which mineral protection acts through interactions with nutrients and enzymes remain unclear. Our aim was to explore how Fe and Al oxides, enzymes, and nutrients affect SOM stability. Here, a field sampling was conducted in subtropical forests, examining carbon (C) of SOM, particulate organic matter, and mineral-associated organic matter (SOM-C, POM-C, and MAOM-C), Fe and Al oxide contents, enzyme activities, and nutrient contents from topsoil to the maximal soil depth (60-215 cm). The increase in the content of amorphous and complexed Fe and Al oxides decreased the specific C mineralization rate (Cmin) and increased the SOM-C, POM-C, and MAOM-C. The shared effects of Fe and Al oxides, enzymes, and nutrients explained 31% and 39% more variance in Cmin and SOM-C, respectively, than Fe and Al oxides did individually. Combined with the result that Fe and Al oxides increased enzyme activities and nutrient contents, we suggested that Fe and Al oxides can increase SOM stability by adsorbing enzymes and nutrients. Among these Fe and Al oxides, complexed Fe oxides (Fep) emerged as the dominant factor controlling SOM stability. Furthermore, Fe and Al oxides also indirectly protected POM-C from decomposition by adsorbing available phosphorus, NH4+, and hydrolase and polyphenol oxidase. Notably, the influence of Fe and Al oxides on SOM stability was depth-dependent. Their regulatory effects, mediated through enzymes and nutrients, were weaker in the subsoil than in the topsoil. Overall, our findings highlight the role of Fe and Al oxides in regulating SOM stability by adsorbing extracellular enzymes and nutrients. This insight provides a more comprehensive understanding of the mechanisms underlying SOM preservation in forest ecosystems.