Approaches for evaluating subsurface phosphorus loss potential from soil profiles

The importance of subsurface phosphorus (P) transport in deterioration of surface water quality is well documented. Practices and treatments have been identified and modeling approaches have been implemented to decrease the subsurface P load to surface waters. Soil phosphorus storage capacity (SPSC) is a site-assessment tool that predicts the potential for P loss from soils. We examined the use of SPSC to assess soil profile P distributions and loss potentials for two contrasting soil types: manure-impacted Spodosols and Ultisols. Our specific objectives were to (i) validate prediction of the Langmuir P bonding strength, KL, the Freundlich adsorption coefficient, KF, or the linear adsorption coefficient, KD, of Spodosols (Ap, E and Bh horizons) and Ultisols (Ap, E and Bt horizons) using SPSC-based equations developed for surface and sub-surface horizons (Ap, E and Bt) of Ultisols and (ii) identify factors affecting retention and release of P. Results showed that SPSC derived from oxalate-extractions data can effectively predict P isotherm parameters used in P transport models. The SPSC also captured differences between soil horizons within a profile pertinent to P loss. Magnesium (Mg) and calcium (Ca) concentrations related to negative SPSC, likely due to the presence of these components in dairy manure. The initial sorbed P in the solid phase (S0) of both soil types was related to soil components such as Mg, Ca and total P when SPSC was positive. The equations developed from A, E and Bt horizons of Ultisol relating isotherm parameters to the PSR and SPSC made successful predictions for all horizons of Ultisols sampled from locations different from those in the current study and Spodosols suggesting that such equations might be applicable across a wide range of soils.