Redox dynamics at a dynamic capillary fringe for nitrogen cycling in a sandy column

Abstract

The capillary fringe (CF) is the last barrier for groundwater protection. Studies investigating the effects of cyclic groundwater fluctuation on nitrogen transformations, a critical biogenic element, in soil systems received less attention relative to near-surface and groundwater processes. We conducted an experiment in a highly instrumented 91-cm sandy soil column under five hydrologic regimes: (1) stagnant system with a static water level, (2) periodic pulsed infiltration with a constant water level, and (3) groundwater fluctuation under three frequencies (6/ 12/ 18-day-cycle). Under groundwater fluctuation regimes, the soil column can be divided into four zones: aerobic zone (10–30 cm), hypoxic zone (40–50 cm), anoxic zone (60–70 cm), and anaerobic zone (80–100 cm). The stable high Eh occurred in the aerobic and hypoxic zones, while the stable low Eh appeared in the anaerobic zone. The Eh at the upper anoxic zone (60 cm) appeared to show a strong cyclic pattern and the Eh at the lower anoxic zone (70 cm) gradually decreased until it reached values similar to the anaerobic zone. Nitrate overwhelmed other nitrogen species in the aerobic and hypoxic zones. The lower boundary of the nitrate-dominant zone was sensitive to the water level, which means that oxygen supply depends on the water level fluctuation. Ammonium dominated the anoxic and anaerobic zones, and the upper boundary of the high ammonium zone was constant, which implies that denitrification reached the maximum capacity in the anaerobic zone. The dominance of a single species in these zones suggests incomplete cycling of nitrogen. In contrast, nitrogen removal was enhanced with increasing frequency of groundwater fluctuation. Low total nitrogen concentration was observed at the hypoxic and anoxic zones boundary, due to the co-occurrence of nitrification and denitrification. The ratio of dissolved organic carbon to dissolved inorganic nitrogen can indicate the relative strength of nitrification–denitrification, subsequently distinguish the zonation of nitrogen status in this soil system. Additionally, ferrous sulfide particles accumulated at the anaerobic zone with intensified reducing condition, which deserves more attention. The observations presented in this study strengthen the understanding that the CF acts as a hot-spot and occasionally as a hot-moment, which is of great importance to chemical cycling, highlighting the role of groundwater fluctuation in mass transfer across the CF.