Assessing phosphorus ‘Change‐Points’ and leaching potential by isotopic exchange and sequential fractionation

. There is increasing evidence that phosphorus (P) can be transferred to surface waters by leaching as well as by erosion and surface runoff. Recently it has been suggested that P soluble in 0.01 m CaCl 2 may be a good indicator of the specific Olsen‐P concentration (usually termed the ‘Change Point’) at which the rate of P leaching from soil suddenly increases and poses a greater threat of eutrophication to standing waters. We know that these ‘Change Points’ vary from soil to soil but, so far, we do not fully understand the mechanism(s) involved. Here, we combine methods for assessing isotopically exchangeable P and P sequential fractionation to gain an insight into the processes which cause this sudden increase in P solubilization. We suggest that Change‐Points simply define the asymtote of rapid desorption isotherms relating to that P which is most readily isotopically exchangeable (i.e at 24 h – 33 P 24 ) with the soil solution. This involves ligand exchange at hydroxyl sites associated with Fe and Al cations, which is kinetically governed by the concentration of surface complexes on soil minerals. Individual Change‐Points reflect the mineralogy and surface chemistry of different soil types. Laboratory and field measurements of the Olsen‐P Change‐Point reflect these surface phenomena and are similar. Olsen‐P extracts the portion of the exchangeable pool that most readily controls solution P, and the Olsen‐P/ 33 P 24 ratio is linearly related to Olsen‐P Change‐Points. This may provide a method for estimating P Change‐Points where gradients of soil P are not available.