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Deep soil nitrogen storage slows nitrate leaching through the vadose zone
Citation:
Weitzman, J., J. Renee Brooks, J. Compton, B. Faulkner, P. Mayer, R. Peachey, W. Rugh, R. Coulombe, B. Hatteberg, AND S. Hutchins. Deep soil nitrogen storage slows nitrate leaching through the vadose zone. AGRICULTURE, ECOSYSTEMS & ENVIRONMENT. Elsevier Science Ltd, New York, NY, 332:107949, (2022). https://doi.org/10.1016/j.agee.2022.107949
Impact/Purpose:
Non-point source pollution of nitrogen to aquatic systems has largely been driven by leaching of excess nitrogen (N) fertilizer from agricultural fields. While improved nutrient management and conservation practices have been implemented to minimize leaching, nitrate levels in groundwater continue to increase in many long-term monitoring wells. The disconnect between improved management practices and continued increases in groundwater nitrate comes from legacy N - N that has accumulated over time below the rooting zone of plants. In this study, we follow nitrate leaching to 3 m depth, and find most leached N is being stored between 0.8 and 3 m depth over 4 years of study. Developing field specific N budgets over many years may help farmers to understand if their fields are contributing to N leaching and legacy accumulations within the soil.
Description:
Nitrogen (N) fertilizer applications are important for agricultural yield, yet not all the applied nitrogen is taken up by crops, leading to surplus N storage in soil or leaching to groundwater and surface water. Leaching loss of fertilizer N represents a cost for farmers and has consequences for human health and the environment, especially in the southern Willamette Valley, Oregon, USA, where groundwater nitrate contamination is prevalent. While improved nutrient management and conservation practices have been implemented to minimize leaching, nitrate levels in groundwater continue to increase in many wells. To elucidate controls on leaching rates and N dynamics in agricultural soils, we intensively monitored the transport of water and nitrate every two weeks for four years through the vadose zone at three depths (0.8, 1.5, and 3.0 m) in a sweet corn (maize) field. Nitrate leaching was highly variable among lysimeters at the same depth and across years. Annual leaching 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. Even though crops were irrigated in summer, most leaching (~71% below 3.0 m) occurred during the wet fall and winter. Interestingly, even in years when crop N uptake accounted for all N inputs, we still found considerable leaching at the 0.8 m depth. However, only ~29% of surface nitrogen inputs leached below 3.0 m into the deeper soil and groundwater, indicating considerable nitrogen retention in the soil, as there is little evidence for at the site. The accumulation and long-term dynamics of deep soil N is a legacy of agricultural management that should be better understood to manage and reduce nitrate loss to groundwater.
