Reassessing Denitrification in the drainage zone below agricultural soils

Nitrate (NO₃⁻) leaching from the rooting zone of agricultural land generally leads to increased NO₃⁻ concentrations in groundwater, thereby significantly contributing to the eutrophication of aquatic ecosystems. Denitrification—the biogeochemical reduction of NO₃⁻ and nitrite (NO₂⁻) to dinitrogen (N₂) and/or nitrous oxide (N₂O)—can mitigate NO₃⁻ inputs to groundwater. However, most research to date has predominantly focused on the root zone. Consequently, substantial uncertainties remain regarding the quantification of nitrate reduction in the unsaturated percolation zone below the root zone (referred to as the drainage zone). The extent to which this zone can reduce NO₃⁻ inputs into groundwater remains contentious, and its contribution to soil N₂O emissions has been scarcely studied.

To address these gaps, we assessed denitrification potential by determining the maximum denitrification capacity (D_cap) using the acetylene inhibition technique for the upper (1.5–2.0 m depth) and deeper (down to 7 m) drainage zone at multiple agricultural sites with contrasting soil textures. Experiments included the addition of seepage water with varying dissolved organic carbon (DOC) concentrations to evaluate the influence of key factors on denitrification in the drainage zone.

Additionally, a mesoscale laboratory incubation experiment was conducted to measure the denitrification rates under oxic and anoxic conditions. Substrates with differing textures were used, and the measurements were performed using the ¹⁵N gas flow method. Key factors influencing denitrification—such as the availability of NO₃⁻ and oxygen, water content, and DOC concentration—were systematically varied in a full factorial experimental design.

Preliminary results revealed very low denitrification emissions in both experiments; however, emissions were higher in the D_cap experiment. Contrary to expectations, initial results suggest less effect of water-filled pore space, NO₃⁻ concentration, or DOC content on denitrification-related emissions. So far, emissions were significantly higher in clayey sediments compared to sandy sediments, highlighting the role of soil texture in influencing denitrification within the drainage zone. These findings emphasize the importance of further research to better understand how the specific characteristics of the drainage zone regulate denitrification processes.