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Overestimation of Phosphorus Adsorption Capacity in Reduced Soils: An Artifact of Typical Batch Adsorption Experiments

^a Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew Univ. of Jerusalem, Rehovot 76100, Israel
^b Dep. of Environmental Sciences, Tel-Hai Academic College, Upper Galilee 12210, Israel

View larger version (53K): [in this window] [in a new window]   Fig. 1. The study site in the Hula Valley, showing Lake Agmon, the drainage canals, and the sampling locations of cultivated degraded peat (CDP), fallow degraded peat (FDP), nondegraded, noncalcareous peat (NDNCP), and marl soils. Coordinates according to the Israel grid are given in kilometers.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Microcosm setup: (a) darkened 19-cm high, 15-cm-diameter plastic vessel; (b) redox electrode; (c) pH electrode; (d) porous cup; (e) tube and tap, connected to syringe; (f) CO2 effluent tube; (g) electrical-resistance adaptors, connected to a data logger; (h) data logger; (i) sealing cover.

View larger version (29K): [in this window] [in a new window]   Fig. 3. Timeline of –log of the e– activity in molar units (pe) + pH (solid line) and Fe and P concentrations during soil incubation under reduced and reoxidized conditions in the cultivated degraded peat (CDP), fallow degraded peat (FDP), nondegraded, noncalcareous peat (NDNCP), and marl soils. Time of draining and initiation of the reoxidative incubation is marked by an arrow on each x axis. To fit into the graph, the Fe concentration values were reduced by a factor of 2 for the CDP, FDP, and NDNCP soils, and by a factor of 20 for the marl soil, as shown on the y axes.

View larger version (37K): [in this window] [in a new window]   Fig. 4. Saturation indices (SI) of P minerals in the equilibrated adsorption solutions of preincubated, reduced, and reoxidized(A) cultivated degraded peat (CDP), (B) fallow degraded peat (FDP), (C) nondegraded noncalcareous peat (NDNCP), and (D) marl soils. The SI values for the P minerals were calculated according to the thermodynamic stability constants of Lindsay (1979): brushite, CaHPO4·2H2O; octacalcium phosphate, Ca4H(PO4)3·2.5H2O; beta-calcium phosphate, ß-Ca3(PO4)2; hydroxyapatite, Ca5(PO4)3OH; MnHPO4; strengite, FePO4·2H2O; and vivianite, Fe3(PO4)2·8H2O. Data for the oxidized soils were taken from adsorption experiments conducted under aerobic conditions and data for the reduced soils were taken from experiments conducted under N2 atmosphere.