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Biopore Mediated Subsurface Transport of Dissolved Orthophosphate

Dep. of Agricultural Sciences, Soil Fertility and Plant Nutrition Lab., Royal Veterinary and Agricultural Univ., Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark.
Geological Inst., Univ. of Copenhagen, Østervoldgade 10, DK-1350 København K., Denmark.
Chemistry Dep., Royal Veterinary and Agricultural Univ., Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark.

^* Corresponding author (mbj{at}fsl.dk).

ABSTRACT

Leaching of phosphorous (P) from structured agricultural soil may cause eutrophication of receiving fresh waters. Especially subsurface storm flows seem capable of subsurface transfer of surface-located P. We used ³²P-autoradiography to trace preferential pathways supporting dissolved orthophosphate (P_i) transport through an undisturbed column of water-saturated clayey subsoil (Aerie Giossaqualf, 0.48 m diam., height 0.73 m, water content 50.2 L). At a flow rate near the saturated hydraulic conductivity (K_sat) of the column, a pulse of ³²P was applied to the column surface. To stain the water flow paths a pulse of Brilliant Blue dye was applied afterwards. Upon drainage, hyperfilms sensitive to ³²P-radiation were exposed horizontally at four depths within the upper 15 cm; below this depth a Geiger-Müller detector showed no increased radioactivity. The ³²P-sorption pattern appearing on the hyperfilms consisted of discrete, circular spots, which coincided with the position of larger biopores (diam. >3 mm). In contrast, the water infiltration pattern depicted by the dye tracer was not restricted to biopores, but covered much larger areas of the column cross sections. Solute transport was further characterized by analyzing breakthrough curves of pulse-applied ³²P and tritium. The results of the study suggest that only macropores with wide apertures can provide the necessary conditions for vertical long distance transport of P_i through structured soil. Estimates suggest the rate-limiting step of the P_i-sorption process to be the diffusion from central stream tubes to sorption sites at macropore wall. Nonequilibrium caused by this sorption barrier is termed macropore nonequilibrium (MNE).

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