Combined modeling and high-resolution monitoring approach for the assessment of nitrate-related redox processes at an agricultural site

Biogeochemical redox processes control the chemical behavior of many major and trace elements. Nitrogen is particularly sensitive to changes in soil redox conditions and its presence also affects the cycles of other redox-sensitive species, which causes its excessive application through agricultural fertilizers to be a multi-faceted problem.

To assess these processes, we constructed a high-resolution monitoring station at an agricultural site featuring sensors and sampling facilities for analyzing hydraulics and hydrogeochemistry in the vadose zone and shallow groundwater. Monitoring has been performed for over two years during which different types of crops such as dill, spinach, wheat, and sunflower have been grown on the site. Observed variations of the oxidation-reduction potential over time and depth confirm the transient behavior of the redox reactive zone, whose variation is consistent with the fluctuation of the groundwater level. Also, a strong decrease in NO₃^- concentrations could be observed. This corresponds to changes over depthin both the sulfateconcentration and δ³⁴S-SO₄^2- signatures, whichconfirms the presence of autotrophic denitrification using sulfur as an electron donor. Moreover, a hydraulic model coupled with a heat transport model was set up for the estimation over depth of water fluxes, water content, and temperatures. In combination with the monitored concentrations, this allows us to estimate solute fluxes.

Preliminary results indicate an average nitrate input to groundwater of 200 kg·ha^-1·a^-1, which is almost completely reduced in the shallow groundwater. However, at the same time, a production of only 25 kg·ha^-1·a^-1 of sulfate is estimated, which indicates that not only sulfur serves as an electron donor, and thus heterotrophic denitrification must also be taking place. This can be confirmed based on increased bicarbonate concentrations in the reactive zone. Furthermore, other nitrate-triggered redox processes were detected, including selenium accumulation at the redox interface, presumably resulting from seleno-pyrite-driven denitrification and geogenic uranium roll-front mobilization.