Lincoln Agritech Ltd, Private Bag 3062, Hamilton, New Zealand.
GNS Science, PO Box 30368, Lower Hutt, New Zealand.
Sci Total Environ. 2018 Oct 15;639:1205-1219. doi: 10.1016/j.scitotenv.2018.05.122. Epub 2018 May 26.
Natural denitrification in groundwater systems has been recognised as an ecosystem service that reduces the impact of agriculturally-derived nitrate inputs to surface waters. Identification of this ecosystem service within the landscape would permit spatially differentiated land management and legislation. However, spatial variation in groundwater redox conditions poses a significant challenge to such a concept. To gain understanding of the small-scale mosaic of biogeochemical and hydrological controls on denitrification, we established a well field consisting of 11 multilevel well (MLW) clusters on a hillslope containing relict organic matter buried by volcanic deposits 1.8 ka before present. Based on site-specific redox classification thresholds, vertical redox gradients and denitrification potentials were detected at 7 of the 11 sites. Palaeosols or woody debris, which had previously been identified in laboratory experiments as resident electron donors fuelling denitrification, were visually recognisable at 4 of the 7 MLW sites with vertical redox gradients. Moderately enhanced groundwater dissolved organic carbon (DOC) concentrations occurred where resident electron donors were evident. DOC concentrations were lower where anoxic and nitrate-depleted groundwater was found but with an absence of resident electron donors. In these instances, it was assumed that nitrate reduction had occurred somewhere upgradient of the sampled well screen along the lateral groundwater flow path, with the proximate electron donor (DOC) largely consumed in the process, since no evidence was found for denitrification being fuelled by inorganic electron donors. Due to high variability in the isotopic signature of nitrate in oxidised groundwater, the nitrate dual isotope method did not yield firm evidence for denitrification. However, realistic vertical patterns were obtained using the excess N method. Tritium-based age dating revealed that oxic conditions were restricted to young groundwater (mean residence time ≤ 3 y), while anoxic conditions were observed across a wider age range (3-25 y).
地下水系统中的自然反硝化作用已被认为是一种生态系统服务，可减少农业来源硝酸盐输入对地表水的影响。在景观中识别这种生态系统服务，可以实现土地管理和立法的空间差异化。然而，地下水氧化还原条件的空间变化对这一概念构成了重大挑战。为了深入了解控制反硝化作用的生物地球化学和水文小尺度镶嵌体，我们在一个含有 1.8 千年前被火山沉积物掩埋的遗留有机物的山坡上建立了一个由 11 个多层井（MLW）群组成的井场。根据特定地点的氧化还原分类阈值，在 11 个地点中的 7 个地点检测到垂直氧化还原梯度和反硝化潜力。在具有垂直氧化还原梯度的 7 个 MLW 站点中的 4 个站点中，可以看到先前在实验室实验中被确定为支持反硝化作用的驻留电子供体的古土壤或木质碎片。在驻留电子供体明显的地方，地下水溶解有机碳（DOC）浓度适度增加。在缺氧和硝酸盐耗尽的地下水存在但没有驻留电子供体的地方，DOC 浓度较低。在这些情况下，假定硝酸盐还原发生在采样井筛网的上游侧向地下水流路径上的某个位置，由于没有发现证据表明反硝化作用是由无机电子供体驱动的，因此附近的电子供体（DOC）在该过程中大部分被消耗。由于氧化地下水硝酸盐的同位素特征存在高度变异性，硝酸盐双同位素方法并未为反硝化作用提供确凿证据。然而，使用过量 N 方法获得了现实的垂直模式。基于氚的年龄测定表明，好氧条件仅限于年轻地下水（平均停留时间≤3 年），而缺氧条件则在较宽的年龄范围内观察到（3-25 年）。
