Approaching environmental phosphorus limits on a volcanic soil of Southern Chile

Unexpected incidental phosphorus (P) losses from volcanic soils recently threatened water quality in Southern Chile. Such soils are generally thought of as P sinks and no rapid indicators exist that reliably predict the potential for these soils to release P to water supplies. In this study we propose a local buffering capacity indicator (BCp) already in use for agronomic purposes to determine a threshold BCp value to predict the onset of significant P loss. The BCp represents the amount of P required to increase the value of Olsen-P by 1 mg kg(-1) at 0.2 m soil depth per hectare (kg P ha(-1)). Changes in BCp, and soil P variables were evaluated in a volcanic soil from the Osorno series in Southern Chile. The treated soil was incubated for 90 days with increasing P rates (0, 10, 25, 50, 100, 500 and 1000 mg P kg(-1) soil). A segmented linear model was used to identify the BCp threshold value from the relationship between the BCp and water-soluble soil P (P-w). Significant changes in soil P status and P sorption parameters were evident only after the soil was treated with the highest P rates, denoting the high P sorption capacity of the soil. Ammonium oxalate extractable P (P-ox), Olsen-P, P-w, and the degree of P saturation (DPS) increased with P rates higher than 500 mg kg(-1). This was coincident with a significant decrease of the BCp value. For this particular soil, the segmented model gave a BCp threshold of 15.1 kg ha(-1) (R-2 = 0.96, P < 0.05) below which P-w increased abruptly. This BCp was equivalent to an Olsen-P of 53.4 mg P kg(-1), which is two times higher than the optimum agronomic value recommended for most crops and pastures of Southern Chile. Given that the BCp is already in widespread use as an agronomic indicator in this region, we show that it also has promise as an environmental indicator of a soil's susceptibility to soil P loss. (C) 2013 Elsevier B.V. All rights reserved.