Effects of semi-lunar tidal cycling on soil CO2 and CH4 emissions: a case study in the Yangtze River estuary, China

Abstract

Coastal wetlands, commonly inundated by periodic tides, have been recognized as important sources of greenhouse gases. However, little is known of tidal effects on in situ soil CO2 and CH4 emissions in a semi-lunar tidal cycle consisting of neap and spring tide periods (NTP and STP). A field study was conducted in the Yangtze River estuary to investigate temporal variations of soil CO2 and CH4 emissions along with the transition from NTP to STP in a semi-lunar tidal cycle. Soil moisture, salinity and sulfate were significantly greater in STP than in NTP, whereas soil redox potential had an opposite pattern because of frequent tidal inundation. Soil CO2 and CH4 effluxes decreased significantly in STP, being 29–34 and 28–35 %, respectively, compared with those in NTP. The decrease in soil CO2 effluxes was likely attributable to two causes, an anaerobic environment inhibiting CO2 production, and tidal inundation impeding CO2 diffusion from the soil into the atmosphere. The inhibition of methanogenesis by increased soil salinity and sulfate was likely the primary reason of the decrease in CH4 effluxes during STP. Our results suggest that the effects of semi-lunar tidal cycling significantly reduce soil carbon emissions, which may be one of the potential mechanisms underlying strong carbon accumulation in wetlands of the Yangtze River estuary.