BIOPORE MEDIATED SUBSURFACE TRANSPORT OF DISSOLVED ORTHOPHOSPHATE

Leaching of phosphorous (P) from structured agricultural soil may caus e eutrophication of receiving fresh waters. Especially subsurface stor m flows seem capable of subsurface transfer of surface-located P. We u sed P-32-autoradiography to trace preferential pathways supporting dis solved orthophosphate (Pi) transport through an undisturbed column of water-saturated clayey subsoil (Aeric Glossaqualf, 0.48 m dam., height 0.73 m, water content 50.2 L). At a now rate near the saturated hydra ulic conductivity (K-sat) of the column, a pulse of P-32 was applied t o the column surface. To stain the water now paths a pulse of Brillian t Blue dye was applied afterwards. Upon drainage, hyperfilms sensitive to P-32-radiation were exposed horizontally at four depths within the upper 15 cm; below this depth a Geiger-Muller detector showed no incr eased radioactivity. The P-32-sorption pattern appearing on the hyperf ilms consisted of discrete, circular spats, which coincided with the p osition of larger biopores (diam. >3 mm). In contrast, the water infil tration pattern depicted by the dye tracer mas not restricted to biopo res, but covered much larger areas of the column cross sections. Solut e transport was further characterized by analyzing breakthrough curves of pulse-applied P-32 and tritium. The results of the study suggest t hat only macropores with wide apertures can provide the necessary cond itions for vertical long distance transport of Pi through structured s oil. Estimates suggest the rate-limiting step of the Pi-sorption proce ss to be the diffusion from central stream tubes to sorption sites at macropore wall. Nonequilibrium caused by this sorption barrier is term ed macropore nonequilibrium (MNE).