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

Abstract

Agricultural losses of dissolved reactive phosphorus (DRP) emanate from both historic P applications (i.e., “old P”) and recently applied fertilizer (i.e., “new P”). Understanding the relative contributions of these sources is important for mitigating DRP losses from agriculture. This study provides a proof‐of‐concept for resolving new P vs. old P source contributions to DRP losses in subsurface tile drainage using edge‐of‐field water quality data and management records from eight fields in Ohio. Weighted regressions on discharge and season (WRDS) were fitted using data from periods without P fertilizer applications and then used to predict DRP losses in tile drainage during new P loss risk periods (default length, 90 d) after fertilizer applications. Differences between observed and predicted DRP concentrations during the new P loss risk period were attributed to the new P source. Remaining losses were attributed to the old soil P source. The WRDS model performance was modest (modified Kling–Gupta efficiency ranged from −0.074 to 0.484). New P sources contributed between 0 and 17% of overall DRP losses (average, 7%), with old soil P contributing 83–100%. Individual P fertilizer applications were associated with new DRP losses up to 192 g P ha−1. Increasing the length of the risk period for new P losses up to 180 d after fertilizer application marginally increased the estimated contribution of the new P source. The WRDS‐based analysis provides a novel approach for resolving the contributions of new and old sources to edge‐of‐field DRP losses.

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

“…This study is the first to estimate new P losses from tile drainage systems across a large number of fields and P applications. Average new DRP estimates from inorganic fertilizer applications (41 g P ha −1 ) were lower than an earlier analysis of a small subset of the same dataset (86 g P ha −1 ) (W. Osterholz, Shedekar, et al., 2023), likely due to the greater frequency of injected applications in the current study. A separate analysis of two fields included in this study indicated that inorganic fertilizer resulted in new DRP losses of 30 and 410 g P ha −1 (Williams et al., 2022), within the range found in this study.…”

“…Daily water quality data were analyzed with the weighted regressions on discharge and season (WRDS) method, with new P loss risk windows used to define P loss periods and resolve the contributions of new and old sources (W. Osterholz, Shedekar, et al., 2023). The WRDS models are a modified version of the WRTDS model (Hirsch et al., 2010).…”

“…Despite the recent progress in understanding factors that influence subsurface P losses, examples of direct quantification of the contribution of new P sources to edge‐of‐field losses (EoF) are limited. Recently, EoF discharge and P concentration data have been utilized to estimate the contributions of new and old P sources (W. Osterholz, Shedekar, et al., 2023; Williams et al., 2022). These few studies were conducted on a small number of fields and fertilizer applications and are therefore limited in scope.…”

Accumulation of soil phosphorus within closed depressions of a drained agricultural watershed

“…Only recently have P inputs and outputs in these basins aligned (Powers et al., 2016); however, water quality remediation faces unprecedented challenges from legacy P (Sharpley et al., 2013). Legacy soil P has been estimated to account for >80% of dissolved reactive phosphorus (DRP) loss from tile‐drained agricultural fields in the US Midwest (Osterholz et al., 2023) and often results in chemostatic transport behavior at both field (Guan et al., 2011; Williams et al., 2018) and watershed scales (Basu et al., 2010; Jawitz & Mitchell, 2011; King et al., 2017). As a result, legacy soil P can mask the effect of conservation on downstream water quality (Meals et al., 2010).…”

Probability of crop response to phosphorus and potassium fertilizer: Lessons from 45 years of Ohio trials

“…Optimizing P and K inputs is a critical component of increasing profitability for grain producers. Sufficient P and K fertilizer must be supplied to meet crop demand (Bender et al., 2013, 2015; Woli et al., 2017), but indiscriminate fertilizer application can result in loss of applied nutrients (Guo et al., 2021; Kast et al., 2021; King et al., 2015), and excessive soil test values can result in higher nutrient losses (Osterholz et al., 2020, 2022), resulting in subsequent water quality declines. Finding a balance between agronomic nutrient needs and environmental outcomes is conceptually straightforward but much more difficult in practice due to the complex nature of agroecosystems (Withers et al., 2019).…”