Characterization of legacy soil P and quantification of subsurface losses under artificial drainage
Date
2022
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Publisher
University of Delaware
Abstract
Phosphorus (P) losses from agriculture can contribute to excess P loading within water bodies and cause eutrophication. Many soils on the Delmarva Peninsula have an increased risk of soil P loss to water bodies due to historical application of poultry litter at N-based rates. The risk for P loss through subsurface pathways is increased due to artificial drainage. However, the mechanisms of soil P loss via subsurface pathways are poorly understood. In addition, there is evidence P solubility in P-enriched soils may decrease with time, especially if P applications are stopped. In this dissertation, we applied methods in chemical and isotope hydrology, soil chemistry and physics, and agronomy to better understand the risk of P mobility and loss through subsurface pathways in soils with a history of P amendment. The objectives of this dissertation project were to: 1) characterize the inorganic P species in agricultural soils amended with various P sources to predict the risk of P mobility 2) determine if soil health practices exacerbate P losses by evaluating soil P stratification and dry aggregate stability in soils with and without a history of soil health practices, and 3) use concentration-discharge (C-Q) relationships, hydrograph separation, and the seasonal origin index (SOI) to identify pathways of subsurface P loss in a ditch-drained field. ☐ Chapters 2 and 3 focused on characterizing the sources and solubility of soil P to predict the risk of P loss. The X-ray absorption near edge structure spectroscopy fitting results identified various aluminum (Al)-, calcium (Ca)-, and iron (Fe) phosphates and P sorbed phases in soils with at least 1% total P and amended with fertilizer, poultry litter, and dairy manure (Objective 1). In drawdown scenarios, we would expect plant P uptake first from semicrystalline Al and Fe phosphates followed by P sorbed phases. Surface P stratification, P accumulation with depth, and soil aggregate stability was not apparent in selected Delaware soils, which was likely due to significant enrichment of soil P from historical applications of manure prior to the implementation of soil health practices (Objective 2). As such, we saw no evidence that adding soil health practices to these P-enriched soils significantly increased the risk for runoff and leachate dissolved soil P losses as these soils were already a significant potential source of dissolved P. ☐ Chapters 4 and 5 focused on the use of C-Q relationships and the SOI to understand subsurface pathways of P transport and loss. In general, findings from the C-Q study showed that groundwater dilution from matrix flow resulted in low P concentrations within the ditch during small storm events (Q <6.4 L s–1). For large storm events (Q >6.4 L s–1), ditch water P concentrations were elevated due to high water tables interacting with high surface soil P concentrations (averaging 425 mg kg-1 in the top 20 cm) and infiltrating precipitation that transported high P soil waters via vertical preferential flow paths (Objective 3). Data from the SOI study revealed that ditch waters were generally synchronized with precipitation at the time of sampling, with wintertime ditch waters exhibiting slight winterlike SOI values (mean daily SOI = -0.20) and summertime ditch waters showing modest summerlike SOI values (mean daily SOI = 0.22). Hence, a relatively equal portion of winter and summer precipitation moved rapidly to the ditch during these seasons, indicating a role for preferential flow (Objective 3). Higher total dissolved P (TDP) concentrations were observed during the summer compared to winter (0.38 vs. 0.13 mg L-1) suggested that antecedent soil moisture may have influenced the amount of P that was lost via preferential flow. ☐ Overall, this dissertation provided insights on how to help land managers make improved nutrient management decisions by predicting P mobility in soils with long-term applications of manure or fertilizers and soil health practices and better understanding pathways of P loss in artificially drained fields.
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Keywords
Agroecosystems, Artificial drainage, Hydrology, Phosphorus, Soil chemistry, Soil health