Fjölrit RALA - 15.06.2004, Page 119

P adsorption and desorption capacities of andisols from European
volcanic areas
E. Auxtero and M. Madeira
Departamento de Ciéncias do Ambiente, Instituto Superior de Agronomia, Tapada da Ajuda, Lisboa, Portugal
Most andisols contain large amounts of active A1 and Fe, allophane, and organic C, which
greatly influence the capacity of soils to adsorb and desorb P. Phosphorus deficiency has been
identified as one of the limiting factors to crop production in andisols. Additionally, potential
losses of P from agriculturally P enriched areas to nearby bodies of water may enhance
eutrophication thereby degrading water quality. In order to provide insight for developing P
management strategies, the ability of the soil to adsorb P and its relative desorptive
characteristics should be studied. Having this in view, a study on surface (Ah or Ap ) and
subsurface (Bw or BC) horizons of eighteen pedons of andisols selected from representative
volcanic areas of Italy, Portugal, Iceland, Spain (Tenerife), Greece, France and Hungary was
conducted to 1) determine the P adsorption capacity using Langmuir isotherm, 2) determine
the P desorption capacity using eight successive extractions with 0.01 M CaCl2, and 3) assess
the relationships between adsorption-desorption parameters and selected soil properties.
Phosphorus sorption data were determined by the method of Fox and Kamprath (1970).
This was done by adding 20 ml of 0.01 M CaCL solutions containing various concentrations
of phosphate as KH2PO4 to 2 g of soil in 50 ml plastic centrifuge tubes. The suspensions were
shaken reciprocally for 30 min twice daily within 6 days at room temperature. Filtered P in
the supematant solution was determined by the ascorbic acid blue color method (AAB).
Adsorbed P, Ads P (g kg4) was estimated as Ads P = (Ci-Cfj'V/W, where:
Ads P: adsorbed P (g kg1)
Ci: initial P concentration added (|lg mL1)
Cf: P concentration in supematant solution after equilibration period (pg mL1)
V: volume of P added (mL1)
W: the oven-dried weight of the soil (g)
Calculated adsorption data were then fitted to the hnear form of Langmuir equation as
C/Ads P - C/Ads max + 1 /k Ads max, where:
C: equilibrium P concentration (pg mL1)
Ads P: amount of P sorbed (g kg1)
Ads max: Langmuir adsorption maximum (g kg1)
k: constant related to the P binding strength
The values of C/Ads P were plotted against C and a linear curve is fitted to the scattered
points to obtain y = a + bC. The value of b obtained from this Unear fit was calculated as 1/b,
representing the Langmuir adsorption maximum (Ads max). The k was then estimated by
multiplying the Ads max value with the intercept from the linear fiL
Phosphorus desorption isotherms were obtained by equilibrating duphcate 2 g of soil with
20 ml of 0.01 M CaCl2 solutions containing respective Langmuir Ads max concentration of
phosphate as KH2PO4 in 50 ml plastic centrifuge tubes, for 6 days at room temperature. The
suspensions were shaken for 30 min twice daily within the equilibration period using
reciprocal shaker. Filtered P in the supematant solution, determined by the AAB method
represented the initial value for adsorbed P. P saturated soils were then subjected to sequential
desorption. This was done by adding 20 ml of 0.01 M CaCl2 to these saturated soils, followed
by 2 h of shaking using a reciprocal shaker. Samples were then centrifuged for 10 min and
filtered. This procedure was repeated for eight successive extractions. Filtered P in the
95