| Under the background of global sea level rise and aggravated saltwater intrusion, studying the nitrogen biogeochemical cycle, N2O production and emission of estuarine tidal wetlands under different salinity gradients is particularly important. In Min River estuary, the Daoqingzhou and Shanyutan Cyperus malaccensis tidal marshes (freshwater and brackish wetland, respectively) were selected as the research objects. To reveal the N2O flux characteristics and its main environmental factors of tidal marshes under different salinity, the static black chamber-gas chromatography method, combination of in situ field observation and indoor gas sample analysis, was applied. The ATU inhibition and acetylene inhibition indoor cultivation methods were applied for determination of sediment nitrification and denitrification rates, further the effects of nitrification and denitrification on the N2O release were analyzed, and the main ways of N2O production and its main influence factors of tidal wetland sediments under different salinity were revealed. The results showed that:(1) The N2O flux in Daoqingzhou wetland was significantly higher than that in Shanyutan wetland, both functioned as a source of N2O. N2O flux of Daoqingzhou ranged from 27.11 to 80.30 μg·m-2·h-1 and the average value was 48.81±9.01 μg·m-2·h-1, N2O flux of Shanyutan ranged from 13.22 to 41.54μg·m-2·h-1 and the average value was 27.69±4.01 μg·m-2·h-1. There were obvious seasonal variations of N2O fluxes in the two wetlands, the maximum (minimum) value of Daoqingzhou and Shanyutan N2O flux was appeared in summer (winter) and summer (autumn), respectively.(2) Temperature, including air temperature and soil temperature, was an important factor affecting the temporal dynamics of N2O flux, but not the only factor. The main influence factors of N2O flux in the two wetland were different:N2O flux of Daoqingzhou was mainly affected by the air temperature and soil bulk density, while N2O flux of Shanyutan was mainly affected by the tidal water salinity and NH4+-N concentration. Soil electric conductivity, pore water Cl- concentration, tidal water salinity and Cl- concentration had a significantly negative correlation with N2O flux (except for a significantly positive correlation between soil conductivity and N2O flux in Daoqingzhou). Salinity had an inhibitory effect on N2O flux, and was an important cause of N2O flux in Shanyutan significantly lower than that in Daoqingzhou. In Daoqingzhou N2O mainly produced from nitrification, pore water NH4+-N concentration promoted it, and soil water content and pore water DOC concentration inhibit N2O emission by affecting the validity of oxygen. SO42- is an important factor affecting N2O flux in freshwater and brackish wetland, it had an inhibitory effect on N2O emission and was also an important cause of N2O flux significantly lower in Shanyutan than in Daoqingzhou.(3) There was no significant difference between the sediment nitrification rates in Daoqingzhou and Shanyutan, neither among the difference seasons, indicating that the seasonal variation of nitrification rates was not obvious. N2O exchange flux from the interface between sediment and water in Daoqingzhou was significantly higher than that in Shanyutan, but the differences among the different seasons were not significant, indicating N2O seasonal variation of exchange flux was not obvious. Sediment denitrification rate was significantly higher in Daoqingzhou (32.72±19.15 μmol N·m-2·h-1) than in Shanyutan (4.97±2.64 μmol N·m-2·h-1), sediment denitrification rates had an obvious seasonal variation, the maximum (minimum) value of Daoqingzhou and Shanyutan sediment denitrification rate was in summer (autumn) and spring (winter), respectively. In Daoqingzhou and Shanyutan sediment nitrification was the major source of N2O, sediments realized denitrification mainly by N2 emission, and N2O reduction was obvious.(4) Temperature was an important factor affecting the seasonal variations of sediment denitrification rate, and also has co-effects of sediment NH4+-N and organic matter concentration. The main influence factors of sediment denitrification rate in the two wetlands were different, sediment denitrification rate in Daoqingzhou was mainly affected by the overlying water SO42- concentration, while sediment denitrification rate in Shanyutan was mainly affected by the overlying water salinity and NO3--N concentration.(5) Salinity had an inhibitory effect on sediment denitrification rate in freshwater and brackish wetland, and was an important cause of sediment denitrification rate in Shanyutan significantly lower than that in Daoqingzhou. Salinity was not conducive to the NH4+-N absorption of sediment, so nitrification and denitrification coupling effects may affected.(6) The overlying water SO42- concentration had an inhibitory effect on sediment denitrification rate in freshwater and brackish wetland and was also an important cause of sediment denitrification rate in Shanyutan significantly lower than that in Daoqingzhou. Sediments direct denitrification in the two wetlands was weak, it more obviously demonstrated nitrification and denitrification coupling effects. NO3--N produced by sediment nitrification in Daoqingzhou was used by a more direct way, which also a reason for the sediment denitrification rate higher in Daoqingzhou than in Shanyutan. |
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