Dissimilatory iron reduction contributes to anaerobic mineralization of sediment in a shallow transboundary lake

Dissimilatory iron reduction(DIR)coupled with carbon cycling is increasingly being recognized as an influential process in freshwater wetland soils and sediments.The role of DIR in organic matter(OM)mineralization,however,is still largely unknown in lake sediment environments.In this study,we clarified rates and pathways of OM mineralization in two shallow lakes with seasonal hydrological connectivity and different eutrophic situations.We found that in comparison with the domination of DIR(55%)for OM mineralization in Lake Xiaoxingkai,the contribution of methanogenesis was much higher(68%)in its connected lake(Lake Xingkai).The differences in rates and pathways of sediment OM mineralization between the two lakes were attributed to higher concentrations of carbonate associated iron oxides(Fecarb)in Lake Xiaoxingkai compared to Lake Xingkai(P=0.002),due to better deposition mixing,more contributions of terrigenous detrital materials,and higher OM content in Lake Xiaoxingkai.Results of structural equation modeling showed that Fecarb and total iron content(TFe)regulated 25%of DIR in Lake Xiaoxingkai and 76%in Lake Xingkai,accompanied by a negative effect of TFe on methanogenesis in Lake Xingkai.The relative abundance and diversity of Fe-reducing bacteria were significantly different between the two lakes,and showed a weak effect on sediment OM mineralization.Our findings emphasize the role of iron minerals and geochemical characterizations in regulating rates and pathways of OM mineralization,and deepen the understanding of carbon cycling in lake sediments.

Microbial residues as the nexus transforming inorganic carbon to organic carbon in coastal saline soils

Soil inorganic carbon(SIC),including mainly carbonate,is a key component of terrestrial soil C pool.Autotrophic microorganisms can assimilate carbonate as the main or unique C source,how microorganisms convert SIC to soil organic carbon(SOC)remains unclear.A systematic field survey(n=94)was performed to evaluate the shift in soil C components(i.e.,SIC,SOC,and microbial residues)along a natural salinity gradient(ranging from 0.5‰to 19‰),and further to explore how microbial necromass as an indicator converting SIC into SOC in the Yellow River delta.We observed that SIC levels linearly decreased with increasing salinity,ranging from~12 g kg^(-1)(salinity<6‰)to~10 g kg^(-1)(salinity>6‰).Additionally,the concentrations of SOC and microbial residues exponentially decreased from salinity<6‰ to salinity>6‰,with the decline of 39%and 70%,respectively.Microbial residues and SOC was tightly related to the variations in SIC.The structural equation model showed the causality on explanation of SOC variations with SIC through microbial residues,which can contribute 89% of the variance in SOC storage combined with SIC.Taken together,these two statistical analyses can support that microbial residues can serve as an indicator of SIC transition to SOC.This study highlights the regulation of microbial residues in SIC cycling,enhancing the role of SIC playing in C biogeochemical cycles and enriching organic C reservoirs in coastal saline soils.