亚热带农业小流域水系N_2O扩散通量及其影响因素

Diffusion flux of N_2O and its influencing factor in agricultural watershed of subtropics

为研究亚热带丘陵地区农业小流域水系溶存N2O的扩散传输特性,利用双层扩散模型法,研究了一年周期内(2014年4月-2015年4月)脱甲河小流域4级河段N2O浓度和扩散通量的时空变化规律及其与环境变量的相关关系。结果显示:1)脱甲河水体氨态氮(NH4+-N)、硝态氮(NO3--N)、溶解性有机碳(dissolved organic carbon,DOC)和电导率(electrical conductivity,EC)的年变化范围分别是0.004~8.32(均值1.29±1.49)mg/L、0.01~3.05(均值1.43±0.63)mg/L、0.92~6.72(均值2.99±1.25)mg/L和50.36~248.43(均值138.37±47.56)μS/cm,相应的河流N2O浓度和扩散通量的年变化范围分别是0.006~1.38(均值0.15±0.26)μmol/L和-0.88~337.94(均值32.50±56.41)μg/(m2·h);2)除在冬季河流源头区域观测到个别的负通量外,N2O扩散通量在一年时间内几乎持续处于正值,呈现明显的季节变化特征。其季节变化规律为:冬高(70.93±90.89)μg/(m2·h),夏低(12.04±9.02)μg/(m2·h);空间上呈随河流污染负荷梯度的增加通量逐步增加的模式;3)影响脱甲河水体溶存N2O浓度的显著性因子有EC(r=0.45,P<0.05)、NH4+-N(r=0.44,P<0.05)、NO3--N(r=0.52,P<0.05)和DOC(r=0.49,P<0.05);水体N2O扩散通量与NH4+-N(r=0.50,p<0.05)、NO3--N(r=0.58,P<0.05)、DOC(r=0.46,P<0.05)和EC(r=0.50,P<0.05)呈显著正相关,与水体温度T(r=-0.24,P<0.05)呈显著负相关。研究表明,脱甲小流域内,农业面源污染、畜禽养殖以及居民生活废水和污水的排入造成的河流水体污染负荷增大是导致脱甲河水体溶存N2O扩散通量急剧增加的主要原因。该研究可为研究亚热带丘陵地区水系或类似河流N2O扩散特征及影响因素响应机制提供参考。

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 1.38μmol/L (0.15±0.26μmol/L) and from -0.88 to 337.94μg/(m2·h) (32.60±56.41μg/(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 248.43μS/cm (138.37±47.56μS/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.