Environmental Science, School of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand; Lincoln Agritech Ltd, Private Bag 3062, Waikato Mail Centre, Hamilton 3240, New Zealand.
Environmental Science, School of Agriculture and Environment, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand.
J Environ Manage. 2020 Oct 15;272:111067. doi: 10.1016/j.jenvman.2020.111067. Epub 2020 Jul 28.
Subsurface denitrification plays a key role in the reduction or 'attenuation' of nitrate contamination of groundwater and surface waters. We investigated subsurface denitrification characteristics in the vadose zone and shallow groundwater at four sites under pastoral farming in the Manawatū River catchment, located in the lower part of North Island, New Zealand. The denitrification potential of the vadose zone was determined by the laboratory incubation assays measuring the denitrifying enzyme activity (DEA) in soil samples collected from different soil horizons (up to 2.1 m below ground surface), whereas denitrification rates in shallow groundwaters were measured in situ by single-well, push-pull tests conducted in piezometers installed at multiple depths at the study sites. Soils and underlying geology, defining hydrogeologic settings, appear to influence the spatial variability of subsurface denitrification characteristics at the study sites. Where the vadose zone is thin and composed of coarse-textured soils, percolation of nitrate was evident in observed high nitrate-nitrogen concentrations (>5 mg L) in oxic and young shallow groundwaters, but low nitrate-nitrogen concentrations (<0.05 mg L) were observed in the reduced shallow groundwater found underneath the fine textured soils and/or a thick vadose zone. This was confirmed by the push-pull tests measuring denitrification rates from 0.08 to 1.07 mg N L h in the reduced shallow groundwaters (dissolved oxygen or DO < 0.5 mg L), while negligible in the oxic groundwaters (DO > 5 mg L) found at the study sites. These contrasting subsurface denitrification characteristics determine the ultimate delivery of nitrate losses from agricultural soils to receiving waters, where the fine textured thick vadose zone and reducing groundwater conditions offer nitrate reduction in the subsurface environment.
地下反硝化作用在减少或“衰减”地下水和地表水硝酸盐污染方面起着关键作用。我们在新西兰北岛下游的马纳瓦图河流域的四个牧场农业区调查了包气带和浅层地下水中的地下反硝化特征。通过实验室培养试验测定了不同土壤层（距地面以下 2.1 m ）采集的土壤样本中的反硝化酶活性（DEA），从而确定了包气带的反硝化潜力，而在研究点安装的测压计中进行的单井、推-拉测试则原位测定了浅层地下水中的反硝化速率。土壤和基础地质（定义水文地质条件）似乎影响了研究点地下反硝化特征的空间变异性。在包气带较薄且由粗质地土壤组成的地方，硝酸盐的渗滤作用明显，在观测到的好氧和年轻浅层地下水中硝酸盐氮浓度较高（>5 mg L），但在细质地土壤和/或厚包气带下方发现的还原浅层地下水中硝酸盐氮浓度较低（<0.05 mg L）。这通过推-拉测试得到了证实，该测试从 0.08 到 1.07 mg N L h 的还原浅层地下水中测量了反硝化速率（溶解氧或 DO <0.5 mg L），而在研究点发现的好氧地下水中则可以忽略不计（DO >5 mg L）。这些相反的地下反硝化特征决定了农业土壤中硝酸盐损失最终输送到受纳水体的情况，其中细质地厚包气带和还原地下水条件为地下环境中的硝酸盐还原提供了条件。
