Impact of agriculture and water paths on organic nitrogen loss to Danish headwater streams

Worldwide, farming activities exert strong impacts on the amount and molecular composition of dissolved organic matter (DOM), which constitutes an important vector of organic nitrogen (ON) transport from soils to the aquatic environment (Graeber et al., 2015). However, there are major knowledge gaps on the drivers of ON loss to water courses. In Denmark, stream data from the Danish national monitoring program (NOVANA) shows that total ON currently accounts for nearly 20 % of the annual total N loading to Danish coastal waters. In a recently initiated research project ‘orgANiC’ we are investigating the loss and fate of ON forms in five smaller agricultural catchments across Denmark (Petersen et al., 2021).

We are measuring dissolved ON (DON) and particulate ON as well as dissolved organic matter (DOM) and particulate organic matter (POM) in various source waters (soil water and groundwater), pathways (tile drains and surface runoff), and receiving streams using a comprehensive array of sampling technologies. In soil water we utilize suction cups taking weekly composite water samples, in groundwater we sample from near-surface (app. 1-5 m below surface) screens in boreholes using the Montejus principle, and in tile drains, surface runoff from fields and streams we are taking both grab samples and automated ISCO samples. These are activated when the hydrograph levels and hydrograph gradients exceed certain thresholds, determined from analysis of the long-term hydrograph data.

We are performing both indirect (total N minus inorganic N) and direct analysis of DON (size exclusion chromatography) on water samples from the different hydrological compartments. The loss of particulate ON (PON) is also monitored in tile drainage water, surface runoff and streams as these three hydrological paths are believed to be of increasing importance with the observed increase in extreme weather conditions. In the presentation we will share our current insights into the challenges of indirect DON measurements across different hydrological pathways by comparing it with direct measurements of DON and PON. We will also demonstrate how the concentrations and composition of ON fractions vary across the agricultural catchments under investigation as they represent different soil types, climate conditions and agricultural management (crops, fertilization, etc.).