Effects of flooding frequencies on soil carbon and nitrogen stocks in river marginal wetlands in a ten-year period

Wetland hydrology can greatly influence the variations in soil carbon and nitrogen stocks. Soil cores were sampled to a depth of 100 cm at 10 cm intervals above 20 cm soils and 20 cm intervals below 20 cm soils in river marginal wetlands with different flooding frequencies (i.e., permanently flooded, one-year, five-year, ten-year, and one-hundred-year floodplains) in 1999 and 2009, respectively. Soil organic carbon and total nitrogen were measured to investigate spatial and temporal variations in soil organic carbon and total nitrogen stocks in five floodplains with different flooding frequencies on a small scale. The results showed that SOCS ranged from 4.62 kg C/m(2) to 13.21 kg C/m(2) and TNS from 0.41 kg N/m(2) to 2.01 kg N/m(2) in the top lm depth in five zones in both sampling years. Higher soil organic carbon and total nitrogen stocks were observed in these floodplain wetlands with higher flooding frequencies (i.e. permanently flooded, one-year, and five-year floodplains) than those in lower-flooding-frequency floodplains (i.e., ten-year and one-hundred floodplains), and the highest soil organic carbon and total nitrogen stocks in top 10 cm appeared in one-year floodplain rather than permanently flooded floodplain in both years. This indicated that higher flooding frequencies could contribute to soil carbon and nitrogen accumulation due to better hydrological conditions compared with lower flooding frequencies. Soil organic carbon and total nitrogen stocks in top lm depth decreased by approximately 8-53% and by 22-55% from 1999 to 2009, respectively, of which the highest change rate occurred in one-hundred fooldplain and the lowest in permanently flooded floodplain. The decline in soil carbon and nitrogen stocks of deeper soils mainly caused by heavy alkalinity, reduced water table, and elevated temperature in a ten-year period possibly contribute to explaining the total carbon and nitrogen losses in soil profiles. Correlation analysis showed that soil organic carbon and total nitrogen levels in this region were significantly correlated with flooding frequencies, soil depth, soil pH value, bulk density, soil texture, and microbial biomass. It is necessary to pay much more attention to carbon and nitrogen stocks in deeper soils and find out the key factors that cause carbon and nitrogen loss in these floodplain wetlands to improve carbon sink function of wetland soils. The findings of this work provide a potential explanation for the "missing" carbon sinks at a larger scale.