Effects of Reclamation on Soil Carbon and Nitrogen in Coastal Wetlands of Liaohe River Delta, China

To evaluate the influence of wetland reclamation on vertical distribution of carbon and nitrogen in coastal wetland soils, we measured the soil organic carbon (SOC), soil total nitrogen (STN) and selected soil properties at five sampling plots (reed marsh, paddy field, corn field, forest land and oil-polluted wetland) in the Liaohe River estuary in September 2013. The results showed that reclamation significantly changed the contents of SOC and STN in the Liaohe River estuary (P < 0.001). The SOC concentrations were in the order: oil-polluted wetland > corn field > paddy field > forest land > reed marsh, with mean values of 52.17, 13.14, 11.46, 6.44 and 6.16 g/kg, respectively. STN followed a similar order as SOC, with mean values of 1351.14, 741.04, 632.32, 496.17 and 390.90 mg/kg, respectively. Interaction of reclamation types and soil depth had significant effects on SOC and STN, while soil depth had significant effects on SOC, but not on STN. The contents of SOC and STN were negatively correlated with pH and redox potential (Eh) in reed marsh and corn field, while the SOC and STN in paddy field had positive correlations with electrical conductivity (EC). Dissolved organic carbon (DOC), ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) were also significantly changed by human activities. NH4+-N and NO3--N increased to different degrees, and forest land had the highest NO3--N concentration and lowest DOC concentration, which could have been caused by differences in soil aeration and fertilization. Overall, the results indicate that reed harvest increased soil carbon and nitrogen release in the Liaohe River Estuary, while oil pollution significantly increased the SOC and STN; however, these cannot be used as indicators of soil fertility and quality because of the serious oil pollution.