Abstract
In order for land managers and policy makers to manage excessive soil phosphorus (P) concentrations and reduce the risk of this particular source of P from impacting water bodies, models of soil P decline under various scenarios are needed. We modelled the decrease in calcium chloride-extractable P (CaCl2-P), and sodium bicarbonate-extractable P (Olsen-P and Colwell-P) using data from six Australian grazed pasture soils with contrasting P sorption properties, over a period of 4.5 years. Each soil had four initial soil P concentrations (Pinit), each of which received four on-going rates of P fertiliser (Pfert). The model predicts the final P concentration (Pfinal) by taking into account the P concentration previously measured (CaCl2-P, Olsen-P or Colwell-P), Pfert applied since measurement, and time since previous measurement: Final P concentration = (previously measured P concentration + ep x P fertiliser applied) exp (-dp x years since previous P concentration measurement). Where ep is the increase in soil P for each unit of applied P and dp is the decay constant representing how quickly the soil P decreased. The greatest decreases in proportion to Pinit occurred for CaCl2-P, followed by Olsen-P, and then Colwell-P. The model tended to fit the dataset well for Olsen-P and Colwell-P, with mean overestimation (modelled Pfinal concentration greater than actual Pfinal) of the Pfinal concentrations of 6.1 (32%) and 4.3 mg/kg (10%), respectively. Although there was less CaCl2-P data, the model successfully described it, with a mean overestimation of Pfinal CaCl2 of 3.1 mg/kg (26%). The overestimation of Pfinal CaCl2 was possibly due to the high CaCl2-P concentrations of the low P buffering index soils. The model predicted an average of 32 years (ranging from 26 to 49 years) for Olsen-P concentrations of between 55 and 96 mg/kg to decrease to an agronomic optimum of 17 mg/kg. Agronomic optimum was not a reliable indicator of environmental risk as some soils did not exceed the CaCl2-P environmental threshold until Olsen-P concentrations were twice the agronomic optimum, whereas low P sorbing soils tended to exceed the threshold before reaching agronomic optimum. Further work with more soils is required to examine the influence of soil properties – such as P sorption – on decreases in soil P.
Highlights
Excessive soil phosphorus (P) concentrations are a concern in parts of Europe, the United States of America and Australia (Paulter and Sims, 2000; Sharpley et al, 2001; Gourley et al, 2015)
The precision of the model for P concentrations which were initially near agronomic optimum should be reassuring to land managers and policy makers as it limits the chances of soil P concentrations decreasing below agronomic optimum faster than predicted
This divergence meant that the model overestimated the Pfinal concentration i.e., the greater the Pinit concentration, the greater the predicted Pfinal concentration was compared to the actual Pfinal concentration (Figure 1)
Summary
Excessive soil phosphorus (P) concentrations are a concern in parts of Europe, the United States of America and Australia (Paulter and Sims, 2000; Sharpley et al, 2001; Gourley et al, 2015). Soils with more P than is needed for agronomic purposes can result in unnecessarily high concentrations of P in runoff (Sharpley and Rekolainen, 1997; Burkitt et al, 2010) which can, in addition to other landscape and land management factors, contribute to eutrophication of waterways (Carpenter et al, 1998) These excessive soil P concentrations can be the result of P inputs (fertiliser, livestock feeds and manures) exceeding P removal in produce. Some land managers still apply P fertiliser when soil P concentrations are above agronomic optimum, despite being aware of the environmental risks, especially in intensive pasture industries (Nicon Rural Services, 2010) This practice is used as a form of risk management to ensure that soil P does not suddenly decrease below the agronomic optimum and result in a subsequent decrease in pasture production and consequent economic loss. This approach results in inefficient use of P fertiliser, a finite resource which is in increasing world demand (Smil, 2000; Condron et al, 2005)
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