Changes in nitrogen concentrations in an upland Mediterranean riparian aquifer during water table fluctuations: Insights from soil column experiments

Abstract

Water table fluctuations regulate the transformation and fate of nitrogen (N) species in groundwater near streams, particularly in Mediterranean catchments with dry conditions interrupted by wet seasons. To understand N dynamics at the interface between soils and groundwater, a 1-m-long soil column was constructed and filled with riparian soil with elevated organic matter content, retrieved from an experimental site in Font del Regàs, Spain. The column underwent 16 days of water-level cycling using natural groundwater with low nitrate (0.48 mg/l NO 3 –N). Hydrologic and chemical dynamics were tracked with pressure, tensiometer, and moisture sensors, vertical redox arrays, and pore-water samples. Water was analyzed for dissolved inorganic nitrogen (DIN) species and dissolved organic carbon (DOC), and pre- and post-experiment soils were sequenced for 16S rDNA. Microbial composition remained stable and was predominantly controlled by soil type. The porous nature of upper riparian soils ensured aerobic conditions, allowing the oxidation of ammonium (NH 4 ) from soil organic matter (SOM) and the accumulation of NO 3 in the variably saturated zone (average 30.7 mg/l NO 3 –N at 40 cm versus 6.42 mg/l NO 3 –N at 55–100 cm). During water-table rise, NO 3 was rapidly consumed under oxygen-limited, saturated conditions in the presence of topsoil-derived DOC. As the water table fell, this DOC flowed downward, promoting denitrification. Surface flooding delivered pulses of NO 3 and DOC to groundwater, explaining elevated post-storm NO 3 and DOC concentrations in headwater streams. These findings show that groundwater level fluctuations are critical for understanding DIN and DOC mobilization from riparian soils.

Bibliographic Information