Department of Geosciences, University of Wisconsin-Madison, Madison, WI, USA.
Water Science Center, US Geological Survey Wisconsin, Middleton, WI, USA.
Ground Water. 2023 Jan;61(1):21-34. doi: 10.1111/gwat.13217. Epub 2022 Jun 23.
Increased nitrate concentrations in groundwater and surface waters represent one of the most widespread and acute impacts of modern agriculture on the environment. However, there is often a fundamental gap in understanding how individual agricultural fields and practices contribute to this broad-scale issue. To practically address nutrient dynamics at individual agricultural sites, methods for assessing nitrogen loss to groundwater that are minimally invasive and thus can encourage farmer "buy in" are necessary. We present an approach that uses edge-of-field monitoring at multilevel samplers along with a once-per-year tracer application (bromide) to calculate nitrogen loss on an annual basis. Using appropriate spatio-temporal integrals of measured concentrations, a net loss of nitrogen to groundwater (per field area) can be calculated. This approach directly measures impacts of nitrogen leaching below the water table, while avoiding permanent in-field installations that can interfere with farm operations. We present an application of this technique to assess nitrogen loss to groundwater over 5 years for a commercial agricultural field in Sauk County, WI. Results from Field 19 indicate that nitrogen losses are similar to (or slightly below) previously reported values for corn and potato crops. In all years, however, we estimate that more than 25% (>60 kg/ha) of nitrogen applied leached as nitrate to groundwater. Use of this mass flux estimation method was most reliable when: (1) tracer is injected directly at the water table, limiting "smearing" within the vadose zone; and (2) nitrate concentrations from laboratory analysis were obtained, rather than using ion-selective electrodes or nitrate test strips.
地下水和地表水中硝酸盐浓度的增加是现代农业对环境造成的最广泛和最严重的影响之一。然而，人们对于个别农业领域和实践如何导致这一广泛问题，往往存在根本的理解差距。为了实际解决个别农业地点的养分动态问题，有必要采用对地下水氮素损失的评估方法，这种方法的侵入性最小，从而可以鼓励农民“接受”。我们提出了一种方法，该方法使用沿多水平取样器的田间边缘监测以及每年一次的示踪剂（溴化物）应用，以便按年度计算氮素损失。通过适当的时空积分测量浓度，可以计算出地下水（每田间面积）的净氮损失。该方法直接测量地下水位以下氮淋失的影响，同时避免了可能干扰农场作业的永久性田间安装。我们应用该技术评估了威斯康星州索克县一个商业农业领域 5 年来地下水的氮素损失。结果表明，19 号田的氮素损失与玉米和马铃薯作物的先前报道值相似（或略低）。然而，在所有年份中，我们估计超过 25%（>60kg/ha）的施氮以硝酸盐形式淋洗到地下水中。当（1）示踪剂直接注入地下水位，限制了在包气带中的“扩散”；以及（2）从实验室分析中获得硝酸盐浓度，而不是使用离子选择性电极或硝酸盐测试条时，这种质量通量估计方法的使用最为可靠。
