Long-term nitrogen enrichment accelerates soil respiration by boosting microbial biomass in coastal wetlands

Coastal wetlands are essential in terrestrial carbon balance because they act as natural “blue carbon” sinks, influenced by anthropogenic nitrogen (N) enrichment. N enrichment alters soil environment and plant growth, impacting carbon loss through soil respiration (Rsoil). However, the responses of Rsoil to N enrichment remain elusive in coastal wetlands, hindering the estimation of carbon fluxes. To bridge this knowledge gap, we used an 8-year field N fertilization platform in the coastal wetlands of the Yellow River Delta, China, to measure Rsoil composed of heterotrophic respiration and autotrophic respiration, and multiple indicators of soil properties, microbial activities, and plant growth. We found long-term N enrichment increased Rsoil by 26.6 ± 1.2% (mean ± standard deviation) and quadrupled microbial biomass carbon, accounting for 26.9 ± 1.2% of the increase in heterotrophic respiration. In addition, N enrichment boosted plant growth, increasing the above-ground biomass by 28.7 ± 6.9% while inducing a cooling effect that partly offsets the increase in autotrophic respiration. In particular, N enrichment elevated soil temperature sensitivity of Rsoil with the increase of N levels, suggesting that the nutrient-related control of Rsoil responds to warming. The study indicates that N enrichment stimulates Rsoil in coastal wetlands by boosting microbial biomass carbon through interactions between soil environmental conditions and plant growth. These results have implications for predicting the carbon cycle with anthropogenic N enrichment.