Resolving new and old phosphorus source contributions to subsurface tile drainage with weighted regressions on discharge and season

Authors: Osterholz, William - Will; SHEDEKAR, VINAYAK - The Ohio State University; SIMPSON, ZACHARY - Iowa State University; King, Kevin

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2022
Publication Date: 10/26/2022
Citation: Osterholz, W.R., Shedekar, V., Simpson, Z., King, K.W. 2022. Resolving new and old phosphorus source contributions to subsurface tile drainage with weighted regressions on discharge and season. Journal of Environmental Quality. 52:100–112. https://doi.org/10.1002/jeq2.20426.
DOI: https://doi.org/10.1002/jeq2.20426

Interpretive Summary: Phosphorus (P) losses from agriculture are a major driver of hypoxia and harmful and nuisance algal blooms in waterbodies worldwide. The sources of these P losses are newly applied P fertilizers (new P) as well as old soil P, accumulated as a legacy of past P applications. Different management strategies are needed to address the two P sources, so estimating their contributions to P losses is important to optimize mitigation efforts. In this study we introduced a new approach to estimating the contributions of new and old P sources to dissolved reactive P losses through subsurface tile drain systems. The approach consists of analyzing water quality data collected at the edge of agricultural fields using a statistical model called weighted regressions on discharge and season (WRDS). We demonstrated the ability of this approach to calculate source contributes in a proof-of-concept analysis using data from 8 fields in Ohio. Results indicated that from -3% (not different from zero) up to 17% of the tile drain DRP losses originated from new P sources, and the remainder originated from old soil P sources. As edge-of-field water quality data is currently being widely collected, the WRDS –based could be extended to assess the contributions of new and old sources to losses of P and other nutrients, thereby providing important insight into the sources of these losses.

Technical Abstract: Agricultural losses of dissolved reactive phosphorus (DRP) emanate from both old soil P and newly applied fertilizer P, and understanding the relative contributions of these sources is important for optimizing DRP loss mitigation efforts. This study provides a proof-of-concept determination of P source contributions to subsurface tile drain DRP losses using edge-of-field water quality data and management records. Weighted regressions on discharge and season (WRDS) were employed to predict tile drain DRP losses following a new fertilizer application, presuming the absence of the fertilizer application; increases in DRP concentration above the prediction for 90 d after application were attributed to the new P source, while all other losses were attributed to the old soil P source. The WRDS models from 8 fields in northwest Ohio produced modest model performance statistics, and source calculations indicated new P sources contributed between -3 and 17% of overall DRP losses, with old soil P contributing 83 to 103%. Individual P fertilizer applications of 11 to 30 kg P ha-1 were associated with new DRP losses of -17 to 192 g P ha-1. Increasing the length of the risk period for new P losses up to 180 d after fertilizer application increased the calculated share of DRP from the new P source. The WRDS based analysis provides an approach to estimating contributions of newly applied and old sources to DRP losses using edge-of-field data, and could be extended to other datasets and nutrients including total phosphorus and nitrogen.