Abstract
Equilibrium models used for predicting phosphorus (P) loss from a site often use the Langmuir strength of P bonding, KL and the P sorption maximum, Smax, or the Freundlich adsorption coefficient, KF, obtained from traditional isotherms, as model input parameters. The overall objective of the study was to develop a protocol to allow estimation of isotherm parameters for soils using simple extraction techniques without generating time-consuming isotherms. A threshold P saturation ratio (PSR; molar ratio of P to [Fe+Al] in an oxalate- or soil test extracting solution) is the PSR value at which P release from a soil increases abruptly. The soil P storage capacity (SPSC) indicates the amount of P a soil can hold before becoming an environmental risk: SPSC = (Threshold PSR-Soil PSR)*(Fe+Al)*31 mg kg-1. Soil samples with varying P-impact levels from four manure-impacted sites were collected by horizon (Ap, E and Bt). The PSR, SPSC and isotherm parameters (KL, KF, Smax) were determined for all soils and regression equations among various parameters evaluated. Equations were validated with soils data from three other sites. Relationship between predicted and determined parameters were significant (R2 = 0.98 for both KL and KF; 0.95 for Smax) suggesting that isotherm parameters can be obtained from P, Fe and Al in an oxalate or soil test solution without generating time- and resource-consuming isotherms.
Highlights
Water-soluble P, an indicator of the amount of P that will be released from the soil when it is in contact with water from sources like rain or irrigation, showed a decreasing trend from Ap and E horizons to subsurface (Bt) horizons
The Freundlich model could be used in cases of negative soil P storage capacity (SPSC); such a relationship cannot be obtained for the Langmuir parameters since KL approaches zero for the A horizons in this study
Extant P loss predictive models use the Langmuir KL or Freundlich KF obtained from P adsorption isotherms
Summary
Eutrophication of water bodies caused by phosphorus (P) loss from manure and fertilizer-impacted sandy soils has become a major concern in several parts of the world including Canada (Chambers et al, 2009), Europe (Withers and Haygarth, 2007), New Zealand (McDowell and Monaghan, 2015), Sweden (Bergström et al, 2015), and the southeastern United States (Greening and Janicki, 2006; Lehrter, 2008; Paerl, 2009; Jarvie et al, 2013; Sharpley et al, 2013; King et al, 2015; Kleinman et al, 2015). Cost-effective tools are needed to identify locations within agricultural systems impacted by excess P that pose a threat to water quality. A threshold PSR, or “change point,” has been identified as the PSR at which P release from a soil through runoff or leaching increases abruptly with further. Based on a threshold PSR value, the soil P storage capacity (SPSC) can be calculated (Nair and Harris, 2004). The SPSC indicates the amount of P a soil can hold before it becomes an environmental risk. The SPSC, unlike the PSR, provides an absolute calculation of remaining P storage (mg kg−1, kg ha−1, etc.) prior to P release at levels posing environmental risks (Nair and Harris, 2004, 2014)
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