USDA Forest Service , Northern Research Station , 1831 Highway 169 E , Grand Rapids , Minnesota 55744-3399 , United States.
University of Vermont , Dept. of Plant and Soil Science , Burlington , Vermont 05405-1737 , United States.
Environ Sci Technol. 2019 Apr 2;53(7):3620-3633. doi: 10.1021/acs.est.9b01276. Epub 2019 Mar 15.
Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δO and δN) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.
关于在森林中没有经过生物处理而被输送到溪流中的大气沉积硝酸盐的区域范围和可变性，人们知之甚少。我们测量了美国和加拿大北部森林地区的水质和硝酸盐源的同位素示踪剂（δO 和 δN），并重新分析了其他研究的数据，以确定未经过处理的大气硝酸盐在集水区中何时、何地以及如何输送。这些输入比通常认识到的更为广泛和多样，但具有很高的空间和时间变异性。在基流期间，只有 32 条溪流中有很高比例（>20%）的未处理大气硝酸盐。在暴雨或融雪期间，有 17 条溪流中有很高的比例，这对应于近地表土壤水或地下水的高比例，但不是深地下水。其余 10 条溪流在暴雨或基流期间偶尔会有一些（<20%）未处理的大气硝酸盐。由于暴雨之间的大气沉积变化以及对这些事件几乎完全缺乏监测，大型、零星的事件可能仍然是隐蔽的。普遍缺乏观测可能会影响对事件发生的看法；对受硝酸盐源分配影响的森林进行持续的慢性氮污染影响监测，可以为推进科学以及评估监管和管理计划提供所需的见解。
