Closed depressions and soil phosphorus influence subsurface phosphorus losses in a tile‐drained field in Illinois

• Published in: Journal of environmental quality Vol. 49; no. 5; pp. 1273 - 1285
• Main Authors: Andino, Luis F., Gentry, Lowell E., Fraterrigo, Jennifer M.
• Format: Journal Article
• Language: English
• Published: 01.09.2020
• ISSN: 0047-2425; 1537-2537
• DOI: 10.1002/jeq2.20120

Artificial subsurface (tile) drainage systems can convey phosphorus (P) from agricultural fields to surface waters; however, controls of subsurface dissolved reactive P (DRP) losses at the sub‐field scale are not fully understood. We characterized subsurface DRP loads and flow‐weighted mean concentration (FWMC) from January 2015 through September 2017 to determine seasonal (growing vs. non‐growing) patterns from 36 individually monitored plots across a farm under a corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation in east‐central Illinois. Using linear mixed models, we investigated the effects of soil test P (STP), depression depth, and their interaction with precipitation and P fertilization on subsurface DRP losses. Dissolved reactive P loads in drainage tiles increased with precipitation and were greatest during the non‐growing season (NGS) in 2016 and 2017. Annual subsurface DRP loads were positively related to STP, and during the NGS, there was a positive relationship between depression depth quantified at the plot‐scale and subsurface DRP loads and FWMC. Along a depression‐depth gradient, piecewise regression displayed a threshold at a depth of 0.38 m at which STP increased, indicating soil P accumulation in deeper closed depressions. Our study highlights the need to identify areas with the greatest risk of subsurface P losses to implement sub‐field scale nutrient management practices.