Soil nitrogen cycling dynamics in a poplar-vegetated land application system under long-term wastewater irrigation

Understanding nitrogen cycling in land application wastewater treatment systems is important for predicting the fate of nitrogen compounds and preventing nitrate leaching. We investigated nitrogen cycling processes in a poplar-planted land application system. Soil samples were collected from synthetic wastewater-irrigated planted (hybrid poplar) and unplanted (bare-soil) reactors, as well as planted controls irrigated with clean water. Key nitrogen cycling functional genes amoA and nirK , as well as bacterial 16S, were quantified and ex-situ nitrate activity tests with soil slurries were conducted to quantify denitrification activity. It was found that amoA gene abundance was elevated in both treatments receiving wastewater compared to controls, suggesting that nitrification gene abundance was correlated with presence of nitrogen in wastewater. nirK gene abundance increased over time but did not vary by treatment, which was unexpected, as previous results demonstrated greater nitrogen removal in planted reactors compared to unplanted reactors. Ex-situ nitrate dynamics tests showed similar denitrification rates in soils from planted and unplanted reactors, revealing that the soil communities had similar denitrification potentials in line with the similar nirK gene abundance. These results suggested that differences in nitrogen removal from wastewater at a system level were not primarily driven by soil microorganisms, but instead were most strongly controlled by the presence of trees and their ability to take up nitrogen from the wastewater and soil. • Nitrogen cycling in land application systems was influenced by nitrogen inputs in wastewater and by presence of vegetation. • amoA abundance increased with ammonia exposure in wastewater while nirK was most strongly influenced by seasonal patterns. • Similar soil denitrification potential in planted and unplanted reactors suggested a key role of trees in nitrogen removal.