城市小型景观水体溶存N2O浓度及排放通量特征

Abstract

淡水生态系统是大气中N₂O的重要排放源，受到国内外广泛关注。城市小型景观水体作为区域淡水系统的重要组成，具有环境容量小，受人类活动干扰强烈，其N₂O排放特征及影响机制并不清楚。选择重庆大学城8个典型景观水体和2个城市外围的自然水体（对照）作为研究对象，利用顶空法和漂浮箱法对水体溶存N₂O浓度及排放通量进行季节性监测，并通过分析生境特征及水环境特征，探究城市小型景观水体N₂O排放特征及关键影响因素。结果表明：1）小型景观水体TN、NO₃^--N、NH₄⁺-N、NO₂^--N含量总体偏低但变异性极强（变化范围分别为0.31-1.47 mg/L、0.05-0.79 mg/L、0.03-0.14 mg/L、0.00-0.04 mg/L），硝态氮是主要的氮形态；景观水体氮丰度远高于外围的自然水体；2）10个小型水体N₂O浓度范围为16.51-158.96 nmol/L，平均为（47.60±21.47） nmol/L，均处于过饱和状态；漂浮箱法实测8个景观水体N₂O排放通量均值为（0.13±0.05）mmol m^-2 d^-1，是对照水体的1.3-5.2倍，高于大部分已有研究结果，是大气N₂O的排放热源；3）景观水体N₂O排放通量与水体各形态氮含量呈显著的正相关关系，较高的N负荷和强烈的氮转化过程是导致景观水体成为N₂O排放热源的主要因子，水体N含量可以作为景观水体N₂O排放强度的有效指示因子；同时水生植物分布对水体N₂O排放影响显著，有植物分布的水域比开敞水域高1.4倍；4）漂浮箱法和边界层模型法对小型景观水体N₂O排放通量的监测结果呈较好的线性关系，但不同季节仍存在着一定差异，需要进一步优化模型估算方法；5）水体N₂O排放通量对温度的季节性变化较为敏感，呈夏季最高，春、秋季次之，冬季最低的季节模式。本研究强调，城市小型景观水体具有较高的N₂O排放速率，在区域氮循环及全球淡水系统温室气体排放清单中具有不可忽视的作用，在未来研究中应得到更多关注。;Nitrous oxide (N₂O) is an important greenhouse gas that contributes to climate change and stratospheric ozone destruction. Freshwater ecosystem, acting as a sink of the terrestrial and anthropogenic nitrogen, continuously processes nitrogen loading and emits N₂O to atmosphere. Human perturbation of the global nitrogen cycle has seriously increased nitrogen loading and N₂O emission from aquatic ecosystems and gained widespread attention. Urban small-scale landscape water bodies, as a significant component of the regional freshwater systems but small environmental capacity, are strongly disturbed by human activities and have an unclear nitrogen processing efficiency. As result of small area, the N₂O emission from urban small landscape waters has not been concerned world widely. In this study, we selected eight typical landscape water bodies and two natural water bodies in Chongqing, and carried out four seasonal investigations of the dissolved N₂O concentrations and N₂O fluxes from the surface water by floating chamber method and headspace method in order to clarify the spatiotemporal variations and the key influencing factors of N₂O emission from small landscape waters. The results showed that: 1) TN, NO₃^--N, NH₄⁺-N, and NO₂^--N concentrations in the sampling water bodies were generally low but significantly different from each other with range of 0.31-1.47 mg/L, 0.046-0.789 mg/L, 0.031-0.141 mg/L, 0.003-0.041 mg/L, respectively. NO₃^--N was the main nitrogen form in landscape waters. Besides, the nitrogen abundance of landscape waters was significantly higher than that of the natural waters. 2) The dissolved N₂O concentrations in all sampling waters ranged from 16.51-158.96 nmol/L,average of (47.60±21.47)nmol/L, and were all observably oversaturated with N₂O relative to atmospheric equilibrium, acting as net N₂O sources to the atmosphere. The N₂O concentrations in the eight landscape waters were two times higher than those in the tow natural water bodies. The average N₂O fluxes measured by floating chamber method from eight landscape water bodies were (0.13±0.05) mmol m^-2 d^-1, approximately 1.3-5.2 times higher than those from the natural waters, and also higher than most of the other reports. Our results highlighted that the small urban landscape waters would act as important and nonnegligible sources of N₂O to atmosphere. 3) There was a significantly positive correlation between N₂O flux and nitrogen contents in landscape waters, indicating that nitrogen content could be used as effective indicators of N₂O emission from landscape waters. High nitrogen loading and strong nitrogen biogeochemical processes were key drivers for the high N₂O flux in landscape water bodies. In addition, the distribution of aquatic plants in the small landscape water bodies could enhance the N₂O emissions and create N₂O emission hotspot in the same water body. 4) We also evaluated the availability of floating chamber method and boundary layer model for the monitoring of N₂O emissions from small landscape water bodies. It was found that the N₂O fluxes derived from boundary layer model had a good linear relationship with that derived from floating chamber method, despite there were still some discordance in different seasons. 5) The N₂O fluxes from urban small landscape water bodies were sensitive to the seasonal variation of temperature, and presented obviously seasonal patterns with the highest values in summer, intermediate values in spring and autumn, and the lowest in winter. This study emphasized that urban small landscape waters had high N₂O emission rate and played a crucial part in the regional nitrogen cycle and N₂O budget of global freshwater system, thus should be paid more attention in future research.