| To investigate the diffusion characteristics of greenhouse gas(GHGs: CH4 and N2O)water and its influence factors, an one-year(from April 2014 to April 2015) monitoring was conducted in Tuojia watershed of Xiangjiang River, locating in the red soil hilly area of subtropical China. The double-layer-diffusion model was used to measure the diffusion of CH4 and N2 O flux from river water and the environmental factors of water were monitored by a portable multi-parameter meter. Four reaches of Tuojia watershed with the upstream,middle stream and downstream were employed in this study. The results are as follows:1. There was significant spatial difference in the N2 O flux between each reach. The annual dissolved N2 O concentration and the diffusion flux of N2 O from four reaches of Tuojia River varied from 0.006 to 1.38 μmol/L, average: 0.15±0.26 μmol/L) and from-0.88 to 337.94 μg N/(m2·h)(average: 32.50±56.41 μg N/(m2·h)), respectively. The corresponding range of the concentration of ammonia nitrogen, nitrate nitrogen and dissolved organic carbon varied from 0.004, 0.01, and 0.92 to 8.32, 3.05, and 6.72 mg/L, respectively. The annual conductivity from four reaches of Tuojia River varied from 50.36 to 248.43 μS/cm(average: 138.37±47.56 μS/cm). Generally speaking, the totally transfer of N2 O increased with the pollution loading of the river water. However, the temporal variation of N2 O flux between four reaches was not significant(2 >4 >3 >1 for the 4 reaches). Except for several sites in headwater region during winter period where we found negative N2 O fluxes, others expressed the continuous positive fluxes. Winter period(from January to April in 2015)showed the highest N2 O flux and summer period(from July to October in 2014) outputted the smallest N2 O flux. The correlation showed that there were significant positive correlations between dissolved N2 O concentration and NH4+-N concentration(r=0.44,p<0.05), NO3--N concentration(r=0.52, p<0.05), dissolved organic carbon(r=0.49, p<0.05)and conductivity(r=0.45, p<0.05). Besides, there were significant positive correlations between N2 O flux and NH4+-N concentration(r=0.50, p<0.05), NO3--N concentration(r=0.58, p<0.05), dissolved organic carbon(r=0.46, p<0.05) and conductivity(r=0.50,p<0.05), whereas water temperature(r=-0.10, p<0.05) showed negative correlation with N2 O flux in some cases.2. The annual dissolved CH4 concentration of Tuojia River varied from 0.03 to 2.23μmol/L, and the mean was 0.61±0.43 μmol/L. The diffusion flux of CH4 varied from 1.71 to290.08 μg C/(m2·h), and the mean was 63.36±50.76 μg C/(m2·h), using atmospheric CH4 as the net source. There was significant spatial and temporal difference between the CH4 concentration and flux. Generally speaking, the CH4 flux from down stream greater than that from the up stream, and the same as the CH4 concentration(S4>S3>S2>S1). Besides, the temporal variation of CH4 concentration and flux between four reaches were also significant.Spring period(from April 2014 to Jun 2014) showed the highest CH4 concentration(0.74±0.41) μmol/L and the highest flux(93.58±65.24) μg C/(m2·h), and winter period(fromDec 2014 to April 2015) outputted the smallest CH4 concentration(0.53±0.38) μmol/L and the smallest flux(50.79±33.03) μg C/(m2·h). The correlation analysis showed that the water dissolved oxygen(DO:3.49~12.79 mg/L appeared significant negative correlation with CH4concentration(r=-0.39, p<0.001) and CH4 flux(r=-0.36, p<0.001), while the dissolved organic carbon(DOC: 0.92~7.38 mg/L) showed positive correlation with CH4 concentration(r=0.50,p<0.001) and CH4 flux(r=0.44, p<0.001). Both of them were the dominant factors that influenced the CH4 concentration and CH4 flux of Tuojia River. Besides, dissolved inorganic N(NH4+-N: 0.02~4.37 mg/L, average :1.26±1.03) mg/L; NO3--N: 0.24~2.66 mg/L,average: 1.43±0.55 mg/L) concentration, salinity(represented by electrical conductivity EC:50.36~248.43 μS/cm, average: 138.37±47.54 μS/cm) appeared positive correlation with CH4 concentration and CH4 flux. Additionally, water p H value(5.89~8.54, average: 6.82±0.31)showed positive relationships with stream CH4 concentration(r=0.20, p<0.05), but not significant correlation with CH4 flux. The dissolved CH4 in river water was produced from river sediment by methanogenesis, and then diffused from water to atmospheric.3. The results of this study indicated that waste and sewage produced by agricultural non-point source pollution, livestock breeding and human activities could lead to the increase of river pollution loading and the decrease of DO, which increased the generation and emission of CH4 and N2 O.Our findings may provide important reference for further studying the diffusion characteristics and influence factors for CH4 and N2 O of rivers in subtropical region. |
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