Dual Consequences of Reclamation: Enhanced Ammonia Oxidation and Carbon Sink Dynamics Versus Escalated Nitrous Oxide Production in Estuarine Wetlands

Abstract

Reclamation activities have dramatically altered the biogeochemical cycling of nitrogen (N) and carbon (C) in estuarine and coastal wetlands. However, how reclamation affects sediment ammonia oxidation processes mediated by ammonia‐oxidizing archaea (AOA) and bacteria (AOB), and associated nitrous oxide (N2O) production remains unclear. Here, different potential ammonia oxidation rates (PAR), N2O production rates, associated functional gene abundances, and driving factors were examined in surface sediment (0–5 cm) from estuarine wetlands (reed) and adjacent agricultural lands (aquaculture pond, paddy field, and vegetable field) on Chongming Island, China. We found that land‐use changes from reed marshes to paddy and vegetable fields significantly promoted PAR by 58% and PARAOA by 119% (p < 0.05), whereas the PARAOB was not significantly affected. Although reclamation activities suppressed the abundances of ammonia oxidizers, the N2O production rates were significantly promoted (p < 0.05). Season and land use type changes jointly influenced spatiotemporal variations in PAR and N2O production rates (p < 0.05); these variations were driven by the complex interaction between environmental factors (temperature, water content, and organic matter) and microbial activities. Overall, reclamation activities enhanced sediment N turnover through ammonia oxidation, and the elevated C pumping flux resulting from nitrification in the Yangtze Estuary (0.95–17.58 × 103 t CO2 year−1) could potentially act as a significant contributor to the C sink in estuarine and coastal wetlands. However, reclamation activities also resulted in a marked increase in N2O production by 363%, which should be carefully considered when estimating global greenhouse gas emissions.