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TECHNICAL REPORTS

Ecological Risk Assessment

Environmental Index for Estimating the Risk of Phosphorus Loss in Calcareous Soils of Manitoba

Department of Soil Science, University of Manitoba, Winnipeg, Canada R3T 2N2

^* Corresponding author (akinremi{at}ms.umanitoba.ca)

Received for publication December 10, 2004. The degree of phosphorus saturation (DPS) has been used in evaluating the risk of P loss from soil to runoff. While techniques are available for calculating DPS for acid soils, no widely used technique exists for neutral to calcareous soils that are typical of the Northern Great Plains, including Manitoba (Canada) soils. This study aimed to develop techniques of calculating the DPS of neutral to alkaline soils. Four measures of soil labile P and ten indices of P sorption capacity were used to calculate the DPS of 115 Manitoba soils. The various DPS calculated were evaluated using water-extractable (_H2O) P as an index of P susceptibility to runoff loss. The DPS obtained using Olsen-extractable (_Ols) P and the Langmuir adsorption maximum (ES_max) ranged from 0.5 to 31.9% while those obtained from P_Ols and the single-point adsorption index (P₁₅₀) ranged from 0.9 to 73.9%. Of all the DPS evaluated, those that included P_Ols and Mehlich 3–extractable (_M3) P as the numerator with either P₁₅₀ or ES_max as the denominator were fairly well correlated with P_H2O (r values ranged between 0.45 and 0.63). Along with ES_max and P₁₅₀, a new method of calculating DPS was formulated as the ratio of P_Ols or P_M3 to Ca_M3 or (Ca + Mg)_M3. We found that the ratio of ammonium oxalate–extractable (_ox) P to (Al + Fe)_ox, which has been widely used to calculate DPS in acid soils, was not suitable for neutral to alkaline soils of Manitoba. In these neutral to alkaline soils, Ca_M3 or (Ca + Mg)_M3 were better indices of P sorption capacity while P_Ols and P_M3 provided better estimates of labile soil P. The DPS calculated using Ca_M3 or (Ca + Mg)_M3 were well correlated with P_H2O; however, they were numerically smaller than those obtained from the Langmuir adsorption maximum. As such, a saturation coefficient () with a value of 0.2 was generated to improve the numerical values of the newly estimated DPS. This new approach can be used to estimate the DPS in neutral and calcareous soils without the need to generate a P adsorption maximum.

Abbreviations: DPS, degree of phosphorus saturation • ES_max, estimated Langmuir adsorption maximum • ex (subscript), exchangeable • H2O (subscript), water extractable • M3 (subscript), Mehlich 3 extractable • Ols (subscript), Olsen extractable • ox (subscript), ammonium oxalate extractable • P₁₅₀, phosphorus sorption index at 150 mg L^–1 of added P

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