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

Abstract

Greaterunderstandingoftherelationshipbetweensoiltestphosphorus(STP)andlossofsolublephosphorus(P)fromsoiltowaterisessential forintegratedmanagementstrategiesattemptingtoreducediffusePlossestoaquaticsystems.Althoughpreviousstudieshavesuggestedthatthe relationshipbetweenSTPandsolublePlossissoil-typespecific[e.g.Pote,D.H.,Daniel,T.C.,Nichols,D.J.,Sharpley,A.N.,Moore,P.A.,Miller, D.M.,Edwards,D.R.,1999b.Relationshipbetweenphosphoruslevelsinthreeultisolsandphosphorusconcentrationsinrunoff.J.Environ.Qual. 28,170‐175;Daly,K.,Mills,P.,Coulter,B.,McGarrigle,M.,2002.ModelingphosphorusconcentrationsinIrishriversusinglanduse,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 r 2 = 0.81, p � 0.0001). Soil organic matter content had a significant negative influence on P solubility, and, among moist samples, peat soils contained significantlyless 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 consequentsubcatchmentrankingwas correlatedweaklybutsignificantlywithaveragesubcatchmentstreamPconcentrations 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. # 2005 Elsevier B.V. All rights reserved.