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Phosphorus Desorption Dynamics in Soil and the Link to a Dynamic Concept of Bioavailability

^a Alterra, Wageningen University and Research Centre (WUR), P.O. Box 47, 6700 AA, Wageningen, the Netherlands
^b Department of Soil Quality, Wageningen University, WUR, P.O. Box 8005, 6700 EC, Wageningen, the Netherlands

View larger version (23K): [in a new window]   Fig. 1. Desorption isotherm of the soils of the pot experiment (Koopmans et al., 2004) describing the relationship between the P concentration measured in 1:10 (w/v) 0.01 M CaCl2 extracts (vigorous shaking) and the total pool of sorbed P (open symbols). The Langmuir equation was fitted to the data. Furthermore, the total pool of sorbed P is plotted against the P concentration as a function of time measured in 1:10 (w/v) 0.01 M CaCl2 extracts obtained under mild shaking conditions in the batch experiments (closed symbols).

View larger version (18K): [in a new window]   Fig. 2. Measured (symbols) and predicted P concentrations (lines) in 0.01 M CaCl2 extracts with varying soil to solution ratios as a function of time at a fitted pore volume fraction of 0.035. The CaCl2 extracts were obtained from the initial soil under mild shaking conditions in the batch experiments. The open triangle represents the P concentration measured in the 1:10 (w/v) 0.01 M CaCl2 extract with vigorous shaking. The dotted line represents the predicted P concentration at a soil to solution ratio of 1:10 (w/v) whereby the radius of the spherical aggregate was set at 0.1 mm.

View larger version (15K): [in a new window]   Fig. 3. Measured (symbols) and predicted P concentrations (lines) in 1:10 (w/v) 0.01 M CaCl2 extracts as a function of time. The CaCl2 extracts were obtained from the soils of the 5- and 10-cm treatments sampled after 151 and 978 d of plant uptake of P in the batch experiments.

View larger version (19K): [in a new window]   Fig. 4. Profile of the P concentration along the radius of the spherical aggregate simulated for P desorption from the initial soil to an external solution with a soil to solution ratio of 1:10 (w/v) (x = 0 mm corresponds with the center of the spherical aggregate).

View larger version (16K): [in a new window]   Fig. 5. Profile of the P concentration (A) and the amount of P sorbed to the solid phase (B) along the radius of the spherical aggregate simulated for the measured plant uptake of P in the 5-cm treatment (x = 0 mm corresponds with the center of the spherical aggregate).

View larger version (27K): [in a new window]   Fig. 6. Simulated maximal P desorption rate from the spherical aggregate, the actual plant uptake rate, and the P content measured in the grass harvested in the 10-cm treatment as a function of time. The inserted figure shows the dynamic bioavailability index (DBI) as a function of time.