Responses of soil respiration to simulated groundwater table and salinity fluctuations in tidal freshwater, brackish and salt marshes

Hydrological and salinity fluctuations in coastal wetlands induced by climatic change (e.g., drought) and anthropogenic activities can significantly affect the blue carbon sink via soil respiration (CO2 emissions). However, little information is available on the interactive effects of the groundwater table and salinity on soil respiration in coastal wetlands. Here, in situ soil columns were collected from tidal freshwater wetlands, brackish wetlands, and salt marshes to conduct a simulated assessment of the effects of fluctuations in the groundwater table (-30 cm, -20 cm and -10 cm) and salinity (0 ppt, 12ppt, and 26 ppt) on soil respiration in the field. The results showed that soil respiration in tidal freshwater wetlands (0.56-2.1 mu mol/m(2)/s) and salt marshes (0.19-2.02 mu mol/m(2)/s) was higher than that in brackish wetlands (0.17-0.9 mu mol/m(2)/s) in all groundwater table and salinity treatments. The diurnal variation in soil respiration in the three simulated wetlands showed a unimodal curve, while the peak time was influenced by the groundwater table and salinity, with the maximum occurring at 11 a.m.-2 p.m. Soil respiration in tidal freshwater wetlands and brackish wetlands was significantly higher in the deep (-30 cm) and medium (-20 cm) groundwater table treatments than in the shallow (-10 cm) groundwater table treatment (p < 0.05) and significantly higher in the low (0 ppt) and medium (12ppt) salinity treatments than in the high (26 ppt) salinity treatment (p < 0.05). However, soil respiration in salt marshes decreased significantly as the groundwater table and salinity increased (p < 0.05). The groundwater table and salinity exhibited significant interactive effects on soil respiration in brackish wetlands (p < 0.05) and salt marshes (p < 0.01). Additionally, significantly negative correlations were observed between soil respiration and groundwater table and salinity in three wetlands (p < 0.05). Soil respiration in the three wetlands was significantly and positively correlated with microbial biomass carbon (p < 0.05) and significantly and positively correlated with soil organic carbon and total nitrogen (p < 0.01). The findings of this work can help reduce the uncertainty of model to accurately estimate carbon cycling of coastal wetlands at local, regional and global scales, and improve the blue carbon sink of coastal wetlands by regulating the groundwater table and salinity levels in wetland restoration projects.

Automated summary

This study conducted a simulated assessment to investigate the effects of groundwater table and salinity fluctuations on soil respiration in coastal wetlands. The results showed that tidal freshwater wetlands and salt marshes had higher soil respiration than brackish wetlands. The timing and magnitude of soil respiration were affected by groundwater table and salinity, with peak time occurring between 11 a.m. and 2 p.m. Soil respiration in tidal freshwater wetlands and brackish wetlands was significantly higher in deep and medium groundwater table treatments, as well as low and medium salinity treatments. However, soil respiration in salt marshes decreased significantly with increased groundwater table and salinity. Groundwater table and salinity had significant interactive effects on soil respiration in brackish wetlands and salt marshes. Soil respiration was negatively correlated with groundwater table and salinity in all three wetlands and positively correlated with microbial biomass carbon, soil organic carbon, and total nitrogen.