New phosphorus losses via tile drainage depend on fertilizer form, placement, and timing

Authors: Osterholz, William - Will; Simpson, Zachary; Williams, Mark; SHEDEKAR, VINAYAK - The Ohio State University; Penn, Chad; King, Kevin

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2024
Publication Date: 2/26/2024
Citation: Osterholz, W.R., Simpson, Z.P., Williams, M.R., Shedekar, V., Penn, C.J., King, K.W. 2024. New phosphorus losses via tile drainage depend on fertilizer form, placement, and timing. Journal of Environmental Quality. 53:241-252. https://DOI.org/10.1002/jeq2.20549.
DOI: https://doi.org/10.1002/jeq2.20549

Interpretive Summary: Reducing the loss of phosphorus (P) from agricultural fields is vital for achieving water quality goals, and artificial subsurface drainage (tile drainage) is an important pathway for P loss from some agricultural regions including Ohio and Indiana. These P losses can come from two sources: new applications of fertilizer or manure (new P), or old phosphorus stored in soils (old P or legacy P). However, uncertainty about the effectiveness of fertilizer management strategies for addressing P losses is is a challenge to developing effective mitigation strategies. In this study, P loss data from tile drainage systems in Ohio and Indiana were analyzed to estimate the new P lost from >150 P applications. Results showed that the amount of new P lost was small relative to the amount applied. In terms of fertilizer management strategies, applications of fertilizer or manure to the soil surface were found to have higher risk of new P loss than applications that were placed below the surface. Additionally, manure was found to have greater risk of new P loss than commercial P fertilizer, and several of the late fall applications were associated with the greatest new P losses. Therefore, mitigation of new P losses should focus on reducing surface applications, particularly of manure in late fall. Finally, it was noted that new P losses accounted for <15% of the overall P lost from these tile drains, meaning that old P was the dominant source. Therefore, these results emphasize the importance of addressing the old P source for achieving meaningful P loss reductions, particularly for tile drainage.

Technical Abstract: Agricultural phosphorus (P) losses are harmful to water quality, but knowledge gaps about the importance of fertilizer management practices on new (recently applied) sources of P may limit P loss mitigation efforts. Weighted regression models applied to subsurface tile drainage water quality data enabled estimating the new P losses associated with >150 P applications to 38 fields in Ohio and Indiana, USA. Daily subsurface discharge and dissolved reactive P (DRP) and total T (TP) loads were used to estimate the new P losses during a window of time following each application. Results showed that the magnitude of new P losses was small relative to fertilizer application rates, averaging 79.3 g DRP ha-1 and 96.1 g TP ha-1. The 8 largest new P losses surpassed 330 g DRP ha-1 or 575 g TP ha-1. On average manure applications caused greater new P losses than chemical fertilizers. Surface broadcast applications were associated with greater new P losses than injected or incorporated applications, and November and December applications were associated with the largest new P losses. On an annual basis, new P contributed an average of 14% of DRP and 5% of TP losses from tile drains, which is much less than previous studies that included surface runoff, suggesting that tile drainage is relatively buffered with regard to new P losses. New P loss mitigation strategies should focus on the problematic late fall broadcast applications, but mitigation of old (preexisting soil P) P sources that dominate P losses could have a greater impact.