Hydroclimatic extremes reveal shifting nitrogen export behavior from catchments

Elevated nitrate concentrations remain a persistent problem for European inland water bodies. Recent unprecedented multi-year droughts have challenged our understanding of nitrate exports from catchments. While both increasing and decreasing concentrations have been observed in response to droughts (Saavedra et al. 2024), summer droughts and subsequent re-wetting were found to decrease the nitrate retention capacity of catchments and increase exported loads. Furthermore, drought-induced forest dieback has created additional nitrate sources, further elevating instream concentrations and fluxes (Musolff et al. 2024). To understand and robustly predict future concentration trajectories in response to climate extremes, we need to first understand the reactive transport processes shaping these observed responses. To this end, we analyze long-term (>40 years) time series of nitrate concentrations in two catchments in Central Germany with diverse land uses and nitrate sources, spanning the recent drought years. Despite relatively constant nutrient inputs over the last 20 years, we found diverging trajectories for annual maximum and minimum concentrations. Drought years amplified intra-annual concentration ranges by increasing high-flow and decreasing low-flow concentrations. Annual maximum concentrations were sensitive to temporal changes in hydroclimatic conditions, with exceptionally high winter concentrations, following low summer drought concentrations. We attribute these high winter concentrations to the rapid mobilization of strong nutrient sources in shallow, hydrologically well-connected agricultural and riparian forest soils. Conversely, annual minimum concentrations responded to slowly reacting groundwater heads of deeper aquifers; lower groundwater levels corresponded to lower summer concentrations. These changes in low-flow concentrations are therefore a function of hydraulic heads controlling the influx of nitrate-rich deeper groundwater to the stream. Thus, we observe a strong effect of hydrological states in shallow and deep storages on the flow paths connecting nutrient sources to streams making export dynamics highly sensitive to hydroclimatic extremes. This data-driven indication raises questions about whether travel-time-based water quality models adequately capture the complexity of flow-paths and connected water ages and nitrate concentration dynamics providing a basis for future model development.

References:

Musolff, A., Tarasova, L., Rinke, K., & Ledesma, J. L. J. (2024). Forest Dieback Alters Nutrient Pathways in a Temperate Headwater Catchment. Hydrological Processes, 38(10). https://doi.org/10.1002/hyp.15308

Saavedra, F., Musolff, A., Von Freyberg, J., Merz, R., Knöller, K., Müller, C., Brunner, M., & Tarasova, L. (2024). Winter post-droughts amplify extreme nitrate concentrations in German rivers. Environmental Research Letters, 19(2). https://doi.org/10.1088/1748-9326/ad19ed

Winter, C., Nguyen, T. V., Musolff, A., Lutz, S. R., Rode, M., Kumar, R., & Fleckenstein, J. H. (2023). Droughts can reduce the nitrogen retention capacity of catchments. Hydrology and Earth System Sciences, 27(1), 303-318. https://doi.org/10.5194/hess-27-303-2023