Dynamics of diffusive soil phosphorus in two grassland experiments determined both in field and laboratory conditions

This work aimed to test the ability of laboratory experiments to predict long term changes in plant-available soil P in two grassland experiments carried out on a Luvisol (Erce, French Pyrenees) and a Brunisol (Gramond, Massif Central, central France) since 1999 and 1998, respectively. Each experiment consisted of two annual rates of N and P fertilization arranged in four randomised blocks. Dry matter yields and P contents were measured for each cut, and P off-take and P balance were calculated every year and accumulated over years of experimentation. The ionic P concentration in solution (Cp) was also determined almost every year for both sites. Results showed that a close linear correlation existed between Cp values and the cumulative P balance. Plant-available P was assessed in laboratory experiments (soil suspension (1 g: 10 mL) at steady-state) as the sum of the amount of P ions in soil solution (Pw) plus the associated gross amount (Pr) of P ions bound to the soil solid phase that buffers the soil solution P by diffusion over time (t). The Pw values were calculated by multiplying the P ion concentration in solution (Cp) by the volume/mass ratio. The isotopic labelling of P ions in solution and the dilution principle were used to determine Pr values. The dynamics of Pr versus (Cp, t) was accurately described by the Freundlich kinetic equation (Pr = vCp(w)t(p), with Pr < total mineral P). The Pr values were greater for the Luvisol than for the Brunisol. This behaviour was mainly ascribed to differences in Al- and Fe-(hydr) oxide contents, which were 545 and 117 mmol kg(-1) for the Luvisol and Brunisol, respectively. The (Pw + Pr) values were used with the mass conservation law to calculate the change in Cp per unit of P balance (Delta Cp) assuming different time scenarios. For the Luvisol, the field and laboratory Delta Cp values did not vary significantly, but a mismatch was observed for the Brunisol and this is discussed. Finally, the approach seems promising to elaborate a mechanism- and process-oriented modelling of the changes in plant-available P with management practices in grassland soils. (c) 2006 Elsevier B.V. All rights reserved.