Moderate nitrogen enrichment increases CO2 sink strength in a coastal wetland

Coastal wetlands remarkably influence terrestrial carbon (C) stock by serving as natural reservoirs for ‘blue carbon’. Anthropogenic nitrogen (N) enrichment shapes the dynamics of soil and plant communities, consequently affecting the C balance and ecosystem functions. The impacts of various levels of N enrichment on CO2 sequestration in coastal wetlands, however, remain elusive. Here we conducted a long-term field study of N fertilization in a coastal wetland in the Yellow River Delta, China, to investigate N effects on soil properties, indicators of plant dynamics, and fluxes of ecosystem CO2. The results indicated that moderate N enrichment (5 g N m−2 y−1) stimulated C fluxes with increases in gross primary productivity (+26.4%), ecosystem respiration (+23.3%), and net ecosystem exchange (NEE, +31.5%) relative to the control. High (10 g N m−2 y−1) and extreme (20 g N m−2 y−1) amounts of N enrichment, however, had relatively minor impacts on these CO2 fluxes. Overall, we observed a decrease in soil electrical conductivity (−24.6%) and increases in soil organic C (+25.2%) and microbial biomass C (+369.3%) for N enrichment. N enrichment also altered the composition of plant species, with a higher proportion of a local dominant species (Phragmites australis), and affected root biomass distribution, with more biomass near the soil surface. Structural equation modeling explained 65.2% of the variance of NEE and supported the assumption that N enrichment could alter the dynamics of soil properties and plant conditions and accelerate ecosystem CO2 sequestration. These findings have important implications for forecasting the C cycle with increasing N deposition in coastal wetlands, contributing to the projections of the global C budget.