Inundation counteracts the promoting effect of nitrogen enrichment on soil organic carbon mineralization in a tidal marsh

Increased nitrogen (N) loading and sea-level rise (SLR) are two dominant drivers of global change that threaten tidal marshes and the ecosystem services they provide, including the sequestration of organic carbon. Nevertheless, the mechanisms through which N loading enrichment, SLR inundation increase, and their combined effects impact the rates and pathways of soil organic carbon (SOC) mineralization in tidal marshes remain poorly understood. We utilized a factorial design in an oligohaline tidal marsh, utilizing in situ weirs to simulate SLR inundation increase by manipulating the duration of flooding with or without nitrogen enrichment as NaNO3 plus NH4Cl or with a combination of increased flood duration and nitrogen. After nearly 2 years, the addition of N increased total SOC mineralization (CMR), soil microbial Fe(III) reduction (FeRR), NO3– reduction (NRR), and SO42– reduction (SRR) but decreased methanogenesis (MGR). The abiotic factor Fe(III)/Fe(II) ratio and dissolved organic carbon (DOC), and the biotic factors, β-glucosidase (BG), and phenol oxidase (PHO) activity explained the increased SOC mineralization rates following N enrichment. Increased flood duration did not change CMR, but increased flooding offset the stimulatory effects of N addition on CMR, FeRR, SRR, NRR and MGR. The contributions of Fe(III) reduction and SO42– reduction pathways to SOC mineralization increased in all experimental treatments, FeRR, SRR, NRR, and MGR were significantly positively correlated with the abundance of Geobacter, dsrA, nrfA, and mcrA. SLR inundation increase did not increase soil carbon loss in this oligohaline marsh and may counteract the simulation of soil C loss due to N enrichment.