Root iron plaque abundance as an indicator of carbon decomposition rates in a tidal freshwater wetland in response to salinity and flooding

Abstract

Sea-level rise is projected to influence soil carbon (C) stocks in tidal wetland systems. Although soil C and iron cycling are considered to be related in tidal wetlands, the empirical link between root ferric iron [Fe(III)] plaque formation and C decomposition in response to salinity and flooding is scarcely known. Here, we established mesocosms loaded with soils from a tidal freshwater wetland and subjected to three salinity treatments (fresh control, oligohaline, mesohaline) plus three flooding (i.e., flooding water-level height) treatments. Root Fe(III) plaque abundance, C-degrading enzyme activities, and potential C mineralization rates (CMRs) and microbial Fe(III) reduction rates (FeRRs) were simultaneously quantified. The oligohaline treatment elevated root Fe(III) plaque abundance relative to fresh control, while the mesohaline treatment suppressed it. Owing to high abundance of root Fe(III) plaque, microbial Fe(III) reduction predominated C mineralization in the oligohaline treatment (56 ± 7%). However, the importance of microbial Fe(III) reduction decreased by up to 25 ± 5% in the mesohaline treatment due to inhibition by salinity. The potential CMRs increased by 17% from fresh control to oligohaline treatment, but declined by 27% from fresh control to mesohaline treatment. The potential CMRs were affected by potential FeRRs and C-degrading enzyme activities. The latter two were associated with root Fe(III) plaque abundance. These results together showed that root Fe(III) plaque abundance was linked to potential CMRs in response to salinity. Flooding did not affect root Fe(III) plaque abundance and had much less of an effect on potential CMRs compared to salinity. Altogether, root Fe(III) plaque abundance could be as an indicator of C decomposition rates in tidal freshwater wetland soils in response to salinity and flooding. Future C decomposition prediction models under sea-level rise could embed root Fe(III) plaque abundance as an indicator for integrating plant-microbe-soil interaction into models.