Microbial mechanisms regulate soil carbon dynamics under nitrogen addition in mangrove wetlands of Dongzhai Harbor, Southern China.

Soil is an important medium for horizontal and vertical carbon (C) exchange in coastal wetlands. Understanding the response mechanism of soil C dynamic changes is of great significance for regulating the C cycle and mitigating climate change. This study explored the C dynamics and microbial mechanisms in mangrove wetlands under nitrogen (N) input conditions through established a simulation experiments. Results showed that the wetland soil served as a source of CO2 and a sink of CH4. C loss originates from vertical soil CO2 emissions (-25.25 to 275.38 mg C m-2 h-1) and horizontal dissolved organic carbon (DOC) loss. The significant loss of DOC caused by simulated tidal action was main factor contributing to the horizontal C loss. N addition promoted plant growth and substrate decomposition, increased DOC content and mitigated DOC loss (DOC loss rate of the CK was 65.78%, while the N1, N2, and N3 treatments were 45.87%, 54.37%, and 45.98% respectively). N input increased the abundance of carbon-degrading genes (amyA and pgu), promoted C degradation, and generated more substrates, leading to increased CO2 emissions. The Interaction between aerobic conditions and inorganic N addition resulted in soil CH4 uptake in this study (-0.74 to -0.0019 mg C m-2 h-1). Changes in salinity, ammonium nitrogen (NH4+-N), and DOC concentrations caused by N input are key drives that altered the abundance of functional genes. These results provide a foundation for assessing soil C cycling under future increases in N inputs.