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TECHNICAL REPORTS

Ground Water Quality

Is Colloid-Facilitated Phosphorus Leaching Triggered by Phosphorus Accumulation in Sandy Soils?

Dep. of Soil Science, Institute of Ecology, Berlin Univ. of Technology, Salzufer 11-12, D-10587 Berlin, Germany

^* Corresponding author (jan.siemens{at}tu-berlin.de).

Received for publication October 12, 2007. The leaching of colloidal phosphorus (P_coll) contributes to P losses from agricultural soils. In an irrigation experiment with undisturbed soil columns, we investigated whether the accumulation of P in soils due to excess P additions enhances the leaching of colloids and P_coll from sandy soils. Furthermore, we hypothesized that large concentrations of P_coll occur at the onset of leaching events and that P_coll mobilized from topsoils is retained in subsoils. Soil columns of different P saturation and depth (0–25 and 0–40 cm) were collected at a former disposal site for liquid manure and at the Thyrow fertilization experiment in northeastern Germany. Concentrations of total dissolved P, P_coll, Fe_coll, Al_coll, optical density, zeta potential, pH, and electrical conductivity of the leachates were determined. Colloidal P concentrations ranged from 0.46 to 10 µmol L^–1 and contributed between 1 and 37% to total P leaching. Large P_coll concentrations leached from the P-rich soil of the manure disposal site were rather related to a large P-content of colloids than to the mobilization of additional colloids. Concentrations of colloids and P_coll in leachates from P-poor and P-rich columns from Thyrow did not differ significantly. In contrast, accumulation of P in the Werbellin and the Thyrow soil consistently increased dissolved P concentrations to maximum values as high as 300 µmol L^–1. We observed no first-flush of colloids and P_coll at the beginning of the leaching event. Concentrations of P_coll leached from 40-cm soil columns were not smaller than those leached from 25-cm columns. Our results illustrate that an accumulation of P in sandy soils does not necessarily lead to an enhanced leaching of colloids and P_coll, because a multitude of factors independent from the P status of soils control the mobility of colloids. In contrast, P accumulation generally increases dissolved P concentrations in noncalcareous soils due to the saturation of the P sorption capacity. This indicates that leaching of dissolved P might be a more widespread environmental problem in areas with P-saturated sandy soils than leaching of P_coll.

Abbreviations: DPS, degree of phosphorus saturation • P_coll, colloidal phosphorus • Al_coll, colloidal aluminum • Fe_coll, colloidal iron • C_coll, colloidal carbon • P_diss, dissolved phosphorus