Changes in the microbial necromass carbon and iron-bound organic carbon following land use and salinity in estuary soils

Natural estuary soils are widely recognized as important carbon sinks for climate change mitigation, however, the soil organic carbon (SOC) accumulation and its underlying mechanism regulated by microbial necromass production and mineral preservation in estuarine soils in relation to land use and salinity still remain unclear. Here, we sampled four representative land uses [e.g., Natural Wetland (NW), Aquaculture Pond (AP), Banana Plantation (BP), and Cultivated Land (CL)] with varying salinities in the Pearl River Estuary, and then the microbial necromass carbon (MNC) and iron-bound organic carbon (Fe-OC) in soils by physical fractionation were analyzed. The results showed that the estuarine soil was dominated by mineral-associated fraction, with increasing proportions as converted by NW and increased by salinity. The MNC content in NW was significantly lower than that in CL, but was higher than those of BP and AP. However, NW had the highest Fe-OC content, followed by CL, BP, and AP. As soil salinity increased, the MNC content decreased, accompanied by an increase in Fe-OC content; however, the particulate Fe-OC exhibited a downward trend, which was opposite to that of bulk and mineral-associated Fe-OC. Increased salinity induced an reduction of MNC contribution and an increase of Fe-OC contribution to SOC pool, respectively; however, the NW conversion increased the contribution of MNC to SOC. The MNC was regulated by iron oxides (Fe_d) and microbial biomass, while SOC and MNC had significantly positive impacts on Fe-OC. Overall, the MNC contents and its contribution to SOC were generally greater than Fe-OC in soils of the land use. Nevertheless, the importance of Fe-OC became increasingly prominent in SOC accumulation as soil salinity increased. Our findings emphasize the role of microbial necromass production and mineral preservation in SOM sequestration of estuarine soils depending on salinity and land use.