Transformation of inorganic reactive nitrogen species in a simulated vadose zone

Recent research correlates irrigation water inputs with the occurrence of inorganic reactive nitrogen species in the vadose zone; particularly, low water input is linked with a greater occurrence of ammonium in intermediate vadose zone sediments beneath agriculturally intensive areas. We hypothesize that lower volume of water input, observed in rainfed agriculture, increases nitrate residence time within the vadose zone, enhancing transformations into other reactive nitrogen species. To investigate the potential relationship between inorganic nitrogen pools and water inputs, a controlled column experiment (150 cm) with intact vadose zone cores to preserve soil structure, chemical, and biological properties was established. One set of columns (n = 3) received artificial rainwater (495 mm) over 159 days, while another set (n = 3) was irrigated with an additional 272 mm of groundwater in addition to rainwater application (495 mm). These applications mirrored the farmer's irrigation schedule for the growing season and the natural rainfall at the core collection site in York County, Nebraska. A mixture of dissolved organic carbon compounds, simulating root zone exudates, was applied at the beginning of the irrigation. Initial (day 0) soil nitrate concentrations (4.1 ± 0.6 μg-N g^-1) were significantly higher than final (day 159) in rainfed (2.2 ± 0.2 μg-N g^-1) and irrigated systems (2.2 ± 0.3 μg-N g^-1, p < 0.05) in the column experiment. Conversely, the final soil ammonium concentrations under rainfed (13.0 ± 2.8 μg-N g-1) and irrigated systems (12.7 ± 2.5 μg-N g^-1) were significantly higher than day 0 soil (4.6 ± 0.5 μg-N g-1, p < 0.01). Throughout the experiment, rootzone pore water nitrate-N concentration of irrigated systems was significantly (p < 0.05) higher than in the rainfed system. Interestingly, the overall ammonium concentration of irrigated systems' groundwater was higher than that of rainfed systems, whereas the nitrate concentrations were similar. Our study suggests that differences in water volume input influence nitrogen reactivity in the vadose zone and impact reactive nitrogen transformation.