Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO₃⁻) contamination, yet nitrogen (N) turnover processes within the recharge area are still poorly understood. This study investigated the role of the karstified recharge area in NO₃⁻ transfer and turnover by combining isotopic analysis of NO₃⁻ and nitrite (NO₂⁻) with time series data of hydraulic heads and specific electrical conductivity from groundwater monitoring wells and a karstic spring in Germany. A large spatial variability of groundwater NO₃⁻ concentrations (0.1–0.8 mM) was observed, which cannot be explained solely by agricultural land use. Natural-abundance N and O isotope measurements of NO₃⁻ (δ¹⁵N and δ¹⁸O) confirm that NO₃⁻ derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ¹⁵N- and δ¹⁸O-NO₃⁻ values, elevated NO₂⁻ concentrations (0.05–0.14 mM), and δ¹⁵N-NO₂⁻ values that were systematically lower than the corresponding values of δ¹⁵N-NO₃⁻. Hydraulic and chemical response patterns of groundwater wells suggest that rain events result in the displacement of water from transient storage compartments such as the epikarst or the fissure network of the phreatic zone. Although O₂ levels of the investigated groundwaters were close to saturation, local denitrification might be promoted in microoxic or anoxic niches formed in the ferrous iron-bearing carbonate rock formations. The results revealed that (temporarily) saturated fissure networks in the phreatic zone and the epikarst may play an important role in N turnover during the recharge of fractured aquifers.

在过去的几十年中，压裂和喀斯特地下水系统已经由于它们的高脆弱性硝酸盐深入研究（NO ₃ ^- ）的补给区内的污染，但氮（N）工艺周转仍然知之甚少。该研究调查了喀斯特补给区的作用，NO ₃ ^-转移和周转通过组合NO的同位素分析₃ ^-和亚硝酸盐（NO ₂ ^- ），用液压头和从地下水监控井和喀斯特比电导率的时间序列数据春天在德国。地下水的大空间变异NO ₃ ^-观察到浓度（0.1-0.8 mM），不能仅通过农业土地用途来解释。NO的自然丰度N和O的同位素测量₃ ^-（δ ¹⁵ N和δ ¹⁸ O）确认无₃ ^-导出主要来自粪肥或肥料的应用程序。分数N消除经硝化由相对高的指示δ ¹⁵ N-和δ ¹⁸ O型NO ₃ ^-的值，升高NO ₂ ^-浓度（0.05-0.14毫摩尔），和δ ¹⁵ N-NO ₂ ^-的值即进行了系统的低于δ的值对应的¹⁵ N-NO₃ ^- 。地下水井的水力和化学响应模式表明，降雨事件导致水从瞬时储层（如表岩溶或潜水区的裂隙网络）中排出。尽管被调查地下水的O ₂水平接近饱和，但是在含铁的碳酸盐岩层中形成的微氧或缺氧生态位中可能会促进局部反硝化作用。结果表明，在裂隙含水层的补给过程中，潜水区和表层岩溶的（暂时）饱和裂隙网络可能在氮周转中起重要作用。