Poorly drained depressions can be hotspots of nutrient leaching from agricultural soils

• Published in: Journal of environmental quality Vol. 52; no. 3; pp. 678 - 690
• Main Authors: Hall, Steven J., Tenesaca, Carlos G., Lawrence, Nathaniel C., Green, David I. S., Helmers, Matthew J., Crumpton, William G., Heaton, Emily A., VanLoocke, Andy
• Format: Journal Article
• Language: English
• Published: United States 01.05.2023
• Subjects: Agriculture; Crops, Agricultural; Depression; Glycine max; Nitrates - analysis; Nitrogen - analysis; Secale; Soil; Zea mays
• ISSN: 0047-2425; 1537-2537
• DOI: 10.1002/jeq2.20461

Much of the US Corn Belt has been drained with subsurface tile to improve crop production, yet poorly drained depressions often still flood intermittently, suppressing crop growth. Impacts of depressions on field‐scale nutrient leaching are unclear. Poor drainage might promote denitrification and physicochemical retention of phosphorus (P), but ample availability of water and nutrients might exacerbate nutrient leaching from cropped depressions. We monitored nitrate, ammonium, and reactive P leaching across multiple depression‐to‐upland transects in north‐central Iowa, using resin lysimeters buried and retrieved on an annual basis. Crops included conventional corn/soybean (Zea mays/Glycine max) rotations measured at fields with and without a winter rye (Secale cereale) cover crop, as well as juvenile miscanthus (Miscanthus × giganteus), a perennial grass. Leaching of nitrogen (N) and P was greater in depressions than in uplands for most transects and years. The median difference in nutrient leaching between paired depressions and uplands was 56 kg N ha−1 year−1 for nitrate (p = 0.0008), 0.6 kg N ha−1 year−1 for ammonium (p = 0.03), and 2.4 kg P ha−1 year−1 for reactive P (p = 0.006). Transects managed with a cover crop or miscanthus tended to have a smaller median difference in nitrate (but not ammonium or P) leaching between depressions and uplands. Cropped depressions may be disproportionate sources of N and P to downstream waters despite their generally poor drainage characteristics, and targeted management with cover crops or perennials might partially mitigate these impacts for N, but not necessarily for P. Core Ideas Poorly drained depressions had greater nitrogen and phosphorus leaching than uplands within several Iowa corn and soybean fields. Depressions with cover crops or perennials had smaller differences in nitrate leaching compared to uplands. Targeted management of cropped depressions could disproportionately benefit water quality.