Deep soil nitrogen storage decreases nitrate leaching through the vadose zone
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Nitrogen fertilizer applications are important for agricultural yield, yet not all the applied nitrogen is taken up by crops, leading some to either be stored in soil or leached into groundwater. Leaching loss of nitrogen from farmland represents a loss of a valuable resource for farmers and has consequences for human health and the environment, especially in the southern Willamette Valley in the Pacific northwestern region, USA, which has widespread groundwater nitrate contamination. While farmers have adopted management practices to minimize leaching over the past decade, nitrate levels in groundwater continue to increase, prompting questions about controls on leaching rates and nitrogen dynamics within soils. We evaluated nitrate leaching losses over four years by intensively monitoring the transport of water and nitrate through the vadose zone at three depths (0.8, 1.5, and 3.0 m) in a field planted with sweet corn (maize). Leaching losses of nitrate were highly variable between lysimeters at the same depth and across years. Annual losses generally decreased with depth, averaging ~108 kg N ha-1 yr-1 near the surface (0.8 m) versus ~55 kg N ha-1 yr-1 in the deep soil (3.0 m), a 51% reduction in leaching between the surface and deep soil. Greatest leaching occurred during the wet fall and winter, accounting for ~73% of total annual nitrate losses across depths. When all nitrogen inputs and outputs were taken into account, our study revealed that nitrogen was lost in excess of inputs at the 0.8 m depth, but ultimately only ~29% of surface nitrate inputs leached below 3.0 m into the deeper soil and groundwater, indicating nitrogen storage below the rooting zone, as instances of denitrification at the site were sporadic. The accumulation and long-term dynamics of deep soil nitrogen is a legacy of agricultural N management that should be better understood in order to manage and reduce nitrate loss to groundwater.