亚热带农业小流域水系 N2 O 扩散通量及其影响因素.

To investigate the rule of dissolved N2O concentration and N2O flux diffused from river water and its influential factors, a one-year (from April 2014 to April 2015) monitoring was conducted in Tuojia watershed of Xiangjiang River, which is located in the red soil hilly area of subtropical China. The double-layer-diffusion model was used to measure the diffusion of N2O 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 indicated that there was significant spatial difference in the N2O flux between each reach. The annual dissolved N2O concentration and the diffusion flux of N2O from four reaches of Tuojia River varied from 0.006 to 1.38 μmol/L (0.15±0.26 μmol/L) and from -0.88 to 337.94 μg/(m2·h) (32.60±56.41 μg/(m2·h)), respectively. And the corresponding range of the concentration of ammonia nitrogen, nitrate nitrogen and dissolved organic carbon varied from 0.004 to 8.32, from 0.01 to 3.05, and from 0.92 to 6.72 mg/L, respectively, ((1.29±1.49), (1.43±0.63), and (2.99±1.25) mg/L respectively). And the annual conductivity from 4 reaches of Tuojia River varied from 50.36 to 248.43 μS/cm (138.37±47.56 μS/cm). Generally speaking, the totally transfer of N2O was increased with the pollution loading of the river water. However, the temporal variation of N2O flux between 4 reaches was not significant (2 >4 >3 >1 for the 4 reaches). Except several sites in headwater region during winter period where we found negative N2O fluxes, others expressed the continuous positive fluxes. Winter period (from January to April in 2015) showed the highest N2O flux and summer period (from July to October in 2014) outputted the smallest N2O flux. By the correlation analysis, we found there were significant and positive correlations between dissolved N2O concentration and inorganic N (NH4 +-N and NO3 --N) concentration (r=0.44, P<0.05; r=0.52, P<0.05), dissolved organic carbon (r=0.49, P<0.05) and conductivity (r=0.45, P<0.05) respectively, while there were no significant relationships between stream dissolved N2O concentration and water temperature (r=-0.10), dissolved oxygen (r=0.03), water pH value (r=0.08) and redox potential (r=-0.09). And at the same time, there were significant and positive correlations between N2O flux and inorganic N (NH4 +-N and NO3 --N) concentration (r=0.50, P<0.05; r=0.58, P<0.05), dissolved organic carbon (r=0.46, P<0.05) and conductivity (r=0.50, P<0.05), while water temperature (r=-0.10, P<0.05) showed negative correlation with N2O flux in some cases, and additionally, dissolved oxygen (r=0.10), water pH value (r=0.11) and redox potential (r=-0.08) showed no significant relationships with stream N2O flux. For the N2O production from river water, the nitrification process controlled the fashion. The results of our study indicated that waste and sewage produced by agricultural non-point source pollution, livestock breeding and human activities were the main reasons leading to the increase of river pollution loading, which gave rise to more stream N2O transportation. Our findings may provide important reference for further understanding and research on the assessment of global N2O budget of rivers in subtropical region. [ABSTRACT FROM AUTHOR]