Mechanisms of Fe and Mn mediated organic carbon fraction transformation in vegetable garden soils from manganese mining areas under extreme drought and flooding conditions

Iron (Fe) and manganese (Mn) play a key role in the soil organic carbon (SOC) fraction transformation. Guizhou Province is an important Mn-enriched region, and frequently experiences extreme rainfall, as well as occasional droughts. The common effect of Fe, Mn and extreme drought and flooding on the SOC fractions transformation in Mn-enriched region remains poorly understood. Here, this study focused on vegetable garden soils adjacent to a manganese mining area in Songtao County, examining the responses of SOC fractions and microbial community structure to extreme drought and flooding, with attention to the role of Fe and Mn in SOC transformation. The results showed that Mn-rich soils contained more particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) than that of soils with low manganese content under both drought and flooding. In contrast, the less dissolved organic carbon (DOC) were detected in Mn-rich soils. DOC is relatively low under flooding, and three-dimensional excitation-emission matrix fluorescence analysis revealed lower fulvic and humic acid components under flooding. Meanwhile, the lower levels of fulvic acid and humic acid were detected in Mn-rich soils than in other soils. MBC was positively related to microbial diversity and richness, with higher microbial activity and MBC under flooding. Mn- and Fe-rich soils reduced microbial activity and MBC, especially under flooding. This study reveals the transformation of SOC fractions in vegetable garden soils adjacent to a manganese mining area, providing a basis for carbon sequestration assessment in mining-affected agricultural soils.