AUTHOR=Tyson Jessica , Corkrey Ross , Burkitt Lucy , Dougherty Warwick TITLE=Modelling Changes in Soil Phosphorus When Phosphorus Fertiliser Is Reduced or Ceases JOURNAL=Frontiers in Environmental Science VOLUME=8 YEAR=2020 URL=https://www.frontiersin.org/journals/environmental-science/articles/10.3389/fenvs.2020.00093 DOI=10.3389/fenvs.2020.00093 ISSN=2296-665X 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 (P_init), each of which received four on-going rates of P fertiliser (P_fert). The model predicts the final P concentration (P_final) by taking into account the P concentration previously measured (CaCl₂-P, Olsen-P or Colwell-P), P_fert applied since measurement, and time since previous measurement: Final P concentration = (previously measured P concentration + e_p x P fertiliser applied) exp (-d_p x years since previous P concentration measurement). Where e_p is the increase in soil P for each unit of applied P and d_p is the decay constant representing how quickly the soil P decreased. The greatest decreases in proportion to P_init 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 P_final concentration greater than actual P_final) of the P_final 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 P_final CaCl₂ of 3.1 mg/kg (26%). The overestimation of P_final 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.