Experimental analysis of natural organic matter controls on nitrogen reduction during bank storage.

Particulate organic carbon (POC) significantly influences nitrogen processes in riparian zones. However, the role of different types of natural organic matter (e.g., plant leaves and mud deposits) in nitrate reduction during the hydraulically driven mixing between rivers and groundwater within bank storage (BS) is not well known. Here, we used laboratory columns filled with 30 cm of riparian soil and buried with POC of varying quantity and quality (leaf litter, mud deposit, a mixture of both and a sediment control) on the soil surface in the column to investigate the effects of POC addition on nitrate reduction in low-permeable media. Pore waters were collected and measured periodically to compare the physicochemical differences among these treatments over time during scenarios of downward and upward flow. The leaf litter treatment had a larger amount of POC and higher reactivity, driving pore water to be in suboxic conditions with lower Eh and pH values. Nitrate reduction occurred immediately with downward surface water, and NO3- was removed in the POC buried layer. Due to the low POC content and low reactivity of the carbon source in the mud deposit, denitrification primarily occurred in the deeper sediment during the downwelling stage, as well as when groundwater returned to the POC buried layer with longer travel times. Both POC quantity and POC quality had strong effects on nitrate reduction. Our results suggested that the leaf litter treatment was preferential for nitrate reduction over the mud deposit treatment, with a higher NO3- reduction rate and less NH4+ accumulation during the complete BS process.