ABSTRACT Especially in the years 2018 to 2020, a severe drought was observed in central Germany. To explore the potential impact of this drought on the catchment‐scale nitrogen cycling, we investigated the ground‐ and surface‐water compartments of the Holtemme watershed, a mesoscale river catchment in the Harz Mountains, Germany. Analysis of nitrate concentrations and corresponding isotope signatures for groundwater and surface water samples was conducted throughout the drought period and continued until discharge conditions returned to the long‐term mean in early 2021. The study revealed a pronounced decline in nitrate concentrations in both compartments during the drought, with groundwater showing an average reduction of ~60%, and surface water exhibiting a wide range of changes from +32% to −59% (mid‐ and downstream decreasing from 19.4 to 2.3 mg L −1 ). In the post‐drought period, surface‐water nitrate concentrations increased sharply, reaching values up to 18.3 mg L −1 . Isotope investigations allowed us to distinguish between different nitrate sources and microbial turnover processes. Time series analysis of δ 15 N–NO 3 showed regular oscillations within the year ranging from 2‰ to 20‰ (AIR), illustrating a periodic organic fertiliser applications. Corresponding δ 18 O–NO 3 signatures showed higher, seasonally independent variations, with a mean of 6‰ (VSMOW), which can be explained by the expected isotope variability of the ambient water during nitrification. However, flow paths for nitrate mobilisation into the surface water appear to be unaffected by the drought, as the contributions of each nitrate source decreased equally during the dry period. Nitrate concentrations increased markedly after the drought, regardless of the recent nitrate supply, reaching values of up to 20 mg L −1 in 2022.
Mueller et al. (Thu,) studied this question.
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