Nitrate and Water Isotopes as a Tool to resolve Nitrate Travel Times at a Mixed Land Use Catchment

Jarno Rouhiainen1
1 Climate-Smart Agriculture, Johann Heinrich von Thünen Institut

11.8 in Isotopenmethoden zur Analyse von Stoffumsatz und Fließprozessen

Reduction of the nitrogen loads in surface waters has been on the agenda of the European Union policy-makers in the last decades. However, the relevant time scales between the programmes of measures and their expected impacts on the water quality are not well known, making it difficult to evaluate the applicability of the new management practices. Therefore, it is essential to better understand the nitrate transport through catchments and the age of exported nitrate for effective planning of required measures.

To investigate the relevant times scales of nitrate transport from soil to stream, we studied the nitrate dynamics and water transient times at a mixed land use lowland catchment, located within the TERENO observatory in central Germany. We monitored the water fluxes and isotopes of precipitation and stream water at the catchment in 2017 and 2018. The observational data was used to simulate 18O-NO3 signatures in stream water. As nitrate is moving readily with water in the subsurface, we used the concept of rank Storage Age Selection functions to model the transient time distributions for water. Secondly, we used daily precipitation 18O-H2O time series to reproduce the time variant ambient water signatures, as the 18O-NO3 signature is partly defined by the ambient water where nitrification occurs. Finally, we simulated the stream signature by mixing initial 18O-NO3 nitrification signatures, based on water transient time distributions model, and calculated the nitrate contributions from each age for the nitrate export.

The results of the model indicated that on average half of the exported nitrate is younger than two months old. Thus, catchment scale measures affecting nitrate concentrations in slow water pathways, such as ground water, would be of little importance for the stream water quality.