Linking soil phosphorus to water quality in the Mask catchment of western Ireland through the analysis of moist soil samples

Greater understanding of the relationship between soil test phosphorus (STP) and loss of soluble phosphorus (P) from soil to water is essential for integrated management strategies attempting to reduce diffuse P losses to aquatic systems. Although previous studies have suggested that the relationship between STP and soluble P loss is soil-type specific [e.g. Pote, D.H., Daniel, T.C., Nichols, D.J., Sharpley, A.N., Moore, P.A., Miller, D.M., Edwards, D.R., 1999b. Relationship between phosphorus levels in three ultisols and phosphorus concentrations in runoff. J. Environ. Qual. 28, 170–175; Daly, K., Mills, P., Coulter, B., McGarrigle, M., 2002. Modeling phosphorus concentrations in Irish rivers using land use, soil-type, and soil phosphorus data. J. Environ. Qual. 31, 590–599]; these conclusions were typically derived from analyses on dried soil samples. Drying soil samples has been found to substantially increase P solubility [e.g. Bartlett, R., James, B., 1980. Studying dried, stored soil samples—some pitfalls. Soil Sci. Soc. Am. J. 44, 721–724; Turner, B.L., Haygarth, P.M., 2001. Phosphorus solubilization in rewetted soils. Nature 411, 258]. In this study, P solubility was compared among air-dried soil samples, and samples maintained at sampled moisture content in order to minimise drying-induced changes. Water-extractable P (Pw) and P desorbed to solutions containing an iron-oxide strip (Pfeo) were used to represent soil P solubility. The soils analysed included a range of soil-types from the Mask catchment in western Ireland. STP (Morgan P) and the degree of P sorption saturation (DPSS) were found to be strongly linearly related with moist soil sample P solubility (e.g. Pfeo with Morgan P r2 = 0.81, p ≤ 0.0001). Soil organic matter content had a significant negative influence on P solubility, and, among moist samples, peat soils contained significantly less soluble P than mineral soils per unit Morgan P content (p ≤ 0.0001) and per unit DPSS (p ≤ 0.05). A solubility factor of 0.27 was derived for peat soils, compared with mineral soils, per unit Morgan P content. This solubility factor was incorporated into a simple P desorption index (PDI). When this PDI was applied to existing Morgan P and soil-type data for the 12 Mask subcatchments of contrasting typology, the consequent subcatchment ranking was correlated weakly but significantly with average subcatchment stream P concentrations and annual molybdate-reactive P loads (rs = 0.54–0.64, p < 0.02 to < 0.05). Drying soil samples resulted in large increases in soil P solubility, obscured the differential peat-mineral soil-type effect, and resulted in the development of an unsuccessful PDI.