Nitrate losses from Midwest US agroecosystems: Impacts of varied management and precipitation

Abstract

Nitrate (NO₃⁻) losses from agricultural fields to groundwater and surface waterways are a major concern that could be further exacerbated by a changing climate. Although individual field-scale studies provide critical information, investigation on the interactive effect of various management practices across different soil types experiencing wide variations in precipitation is necessary to extend our understanding of what approaches may mitigate NO₃⁻ losses to the environment. Synthesizing and analyzing large data sets from multiple studies provides an opportunity to investigate the interactive impact of multiple management practices, soil texture, and rainfall. We assembled peer-reviewed field studies from the Midwest United States and analyzed their associated field data to (1) quantify the range of NO₃⁻ leaching associated with different agroecosystems and (2) determine the individual and interactive effect of management practices (tillage and amount of nitrogen [N] fertilizer added), cropping systems (crop type and rotation), and precipitation across multiple soil types on NO₃⁻ leaching. Our results showed that fertilized potatoes (Solanum tuberosum L.) had the highest NO₃⁻ leaching rate among all systems studied (59.3 ± 8.4 kg N ha⁻¹ y⁻¹) while unfertilized perennial systems exhibited the lowest NO₃⁻ leaching (6.1 ± 0.9 kg N ha⁻¹ y⁻¹). Our results suggested that corn (Zea mays L.)–soybean (Glycine max [L.] Merr.) rotations can reduce NO₃⁻ leaching compared to continuous corn by 25% in clay soils and also reduce the impact of high rainfall on NO₃⁻ leaching compared to continuous corn management. Nitrate leaching in sandy soils exhibited a greater sensitivity and amplified response to increasing N fertilizer amount and annual precipitation compared to other soil types. Compared to conventional tillage, no-tillage soil management significantly reduced NO₃⁻ leaching in sandy and silty loam soils. While some management practices can curb NO₃⁻ leaching losses, more drastic land management change from row crops to perennial systems offered the most benefit. We conclude that a changing climate will make it more challenging for farmers to increase N use efficiency and reduce NO₃⁻ leaching, especially on coarse textured soils.