Effects of groundwater fluctuations on nutrient transformation in riparian sediments in a Mediterranean catchment - Column study

A clear need exists to understand the role that water table fluctuations play in mobilizing nutrients in soils and shallow groundwater near streams, particularly in dry Mediterranean watersheds, which experience marked wetting and drying seasonal patterns. As groundwater level varies, so does the supply of inorganic nitrogen and organic carbon from different soil layers, which affects processes such as coupled nitrification-denitrification and the chemistry of groundwater that flows to streams. Along discharging groundwater flow paths, carbon-rich soils release dissolved organic carbon (DOC), which biogeochemically reacts with nitrogen (N) and oxygen (O) as groundwater levels rise and become saturated. For understanding the biogeochemical dynamics in N at the interface between soils and groundwater, a cylindrical, meter-long polyvinyl chloride (PVC) soil column was constructed and filled with heterogeneous soil and aquifer layers of decreasing organic matter with depth. A cyclical water level cycle was imposed for 16 days using influent to the column from local groundwater at the experimental site with < 1 mg nitrate-N/L. Water table dynamics were monitored with a pressure sensor, tensiometer, and two soil moisture sensors. Vertical arrays of redox sensors and pore water samplers were used to observe changes in pore water chemistry. Water samples were analysed for pH, ammonium-N (NH4-N), nitrate-N (NO3-N), nitrite-N (NO2-N), and DOC. Soil samples were taken for microbial activity and solid chemistry analysis. Near the soil-aquifer transition, nitrate accumulates under aerobic conditions and DOC from organic matter is mobilized under anaerobic conditions. Preliminary pore water analysis shows that during wetting cycles, there is an increase in dissolved inorganic N (NO3+NO2) near the surface (57 mg N/L at 40 cm depth) but a decrease in DIN concentrations in deeper layers (0.92 mg N/L at 55-100 cm depth), suggesting that nitrification and denitrification processes stratified with depth. The results illustrate the significance of groundwater level fluctuations on DIN and DOC cycling and mobilization in Mediterranean riparian soils during wetting events.