我国河流N2O饱和度与释放系数变化及其与河流氮水平的关系研究

通过选择我国3个不同流域的河流,研究了河流N2O饱和度与释放量的时空变化及其与河流氮水平的关系,并评估了IPCC关于河流N2O的释放系数.结果显示,河流硝态氮和氨氮的浓度变化范围分别为0.023～5.24(均值1.29±0.822)mg N·L-1和0.020～40.3(均值2.54±5.47 )mg N·L-1；相应地,河流N2O饱和度和释放量的变化范围分别为90％～8213％(均值407％±1010％)及0.250～1960(均值58.3±221)μg N·m-2·h-1.不同河流N2O饱和度均呈现明显的季节变化特征,N2O饱和度几乎持续处于过饱和状态,表明河流N2O是大气N2O的源.不同类型的河流,其氮浓度水平、N2O饱和度与释放量均有显著差异,城市纳污型河流——南淝河,其氨氮浓度、N2O饱和度和释放量显著高于其他河流,均值分别达(12.5±6.10)mg N·L-1、1760％±2620％及(363±548)μg N m-2·h-1.研究发现,除南淝河外,所有径流主导型的河流,其N2O饱和度与NO3-含量存在显著线性正相关关系,说明高NO3-含量的河流能增加N2O的表观产量.除南淝河以外的河流N2O释放系数变化范围为0.05％～0.87％,均值为0.20％,较为接近IPCC的参考值0.25％.但我们的研究建议采用修正后的河流N2O释放系数(均值为0.10％),该系数更能体现河流释放N2O的实际情况.

Variations of River N2O Saturations and Emission Factors in Relation to Nitrogen Levels in China

Nitrous oxide(N2O)is a potent greenhouse gas that contributes the climate changes and stratospheric ozone depletion.An increase of active nitrogen(N)loading resulting from human activities to watersheds makes river systems a potentially important source of N2O.Here,we present the results of spatial and temporal variations of river N2O saturations and emissions in relation to N levels from rivers among three watersheds with multiple land uses in China,and we also assess the emission factors based on the Intergovernmental Panel on Climate Changes(IPCC)guidelines.The results showed that nitrate and ammonium concentrations ranged from 0.023 to 5.24(average 1.29±0.822) mg.L-1and from 0.020 to 40.3(average 2.54±5.47)mg N.L-1,respectively;Correspondingly,N2O saturations and emissions ranged from 90% to 8213%(average 407%±1010%)and from 0.250 to 1960(average 58.3±221)μg N.m-2.h-1,respectively among the rivers.Our study demonstrated all the rivers were sources of N2O to the atmosphere.There existed significant differences of ammonium concentrations,N2O saturations and emissions for rivers in these watersheds,and the highest mean values of ammonium concentrations,N2O saturations and emissions were(12.5±6.10)mg N.L-1,1760%±2620% and(363±548)μg N.m-2.h-1,respectively,which were observed in Nanfei River receiving urban wastewaters.We found N2O saturations from study rivers(except for the Nanfei River)were positively related to nitrate concentrations,demonstrating that rivers with increased nitrate levels can results in more N2O production.On the basis of the IPCC definition of "emission factor",the measured emission factor ranged from 0.05% to 0.87%,with an average of 0.2%,very close to IPCC given value of 0.25%.However,our study suggests that the revised "actual" emission factor with an average value of 0.1% is more appropriate instead of IPCC value.