Biopore Mediated Subsurface Transport of Dissolved Orthophosphate

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 32 P‐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 32 P 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 32 P‐radiation were exposed horizontally at four depths within the upper 15 cm; below this depth a Geiger‐Müller detector showed no increased radioactivity. The 32 P‐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 32 P 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).