Climate and soil nutrients control the geographical variations in anaerobic CH4 and CO2 emissions from coastal salt marshes.

As powerful greenhouse gases, CH4 and CO2 are actively produced in coastal wetlands that are vulnerable to climate warming and land use change, yet current CH4 and CO2 estimations, as well as future shifts in CH4 and CO2 predictions, have high uncertainties. Here, we incubated 480 soils from natural wetlands, reclaimed wetlands, invasive wetlands, and bare mudflats along the Chinese coastal line at three temperature levels (15 °C, 25 °C, and 30 °C) to determine the influence of climate and land use on CH4 and CO2 and their temperature sensitivities (Q10). We found that average CH4 and CO2 emissions were significantly higher at 25 °C than those at 15 °C and 30 °C, and Q10 values of CH4 and CO2 were significantly higher at 25-30 °C than those at 15-25 °C and 15-30 °C, and all of these patterns remained consistent with increasing soil depth. Across all coastal wetlands studied, the average of CH4 and CO2 emissions (1.87 ± 0.67 μg·g-1·d-1 and 1.23 ± 0.47 μg·g-1·d-1, respectively) and Q10 values of CH4 (Q10-CH4) and CO2 (Q10-CO2) (1.23 ± 0.47 and 1.09 ± 0.53, respectively) were obviously lower in coastal wetlands than those in freshwater wetlands, indicating a relatively lower risk of coastal ecosystems to global warming. We also observed that CH4 emission and Q10-CH4 increased, but CO2 emission and Q10-CO2 decreased as latitude increased, and these patterns were strongly correlated with mean annual temperature (MAT) and precipitation (MAP). The average CH4 and CO2 first increased and then decreased significantly along the hydrological gradients from bare mudflats to invasive wetlands, natural wetlands, and reclaimed wetlands; In contrast, Q10-CH4 and Q10-CO2 increased slightly, and all of these patterns were strongly affected by the soil C, N, and P conditions. However, these changes and key driving factors varied greatly across climate zones. These findings emphasized the importance of a comprehensive multi-site approach for understanding the effect of land-use and of current and future climate changes on anaerobic CH4 and CO2 emissions in coastal wetlands.