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Mobilization and mobility of colloidal phosphorus in sandy soils
Subsurface losses of phosphorus (P) contribute to the translocation of P from terrestrial to aquatic ecosystems and may cause or enhance the eutrophication of surface waters. In addition to the dissolved fraction, P in seepage water may be mobile as colloidal P (Pcoll). Since the sorption of P to dispersible solids such as iron (Fe) and aluminum (Al) oxides and hydroxides changes their surface charge, P itself may contribute to the mobilization and mobility of colloids.
In this study I tested the hypotheses that 1) the sorption of P to goethite and to sandy soils disperses goethite and soil particles, 2) inositol hexaphosphate (IHP) is a more efficient dispersing agent than ortho-phosphate (ortho-P), 3) an increasing P saturation of sandy soils increases the release of Pcoll under batch conditions and 4) an accumulation of P in soils increases the leaching of Pcoll under field conditions.
To test 1) and 2) I conducted two batch experiments, in which I added increasing concentrations of IHP and ortho-P to model systems consisting of quartz sand synthetically coated with goethite and to natural subsoils. In another batch experiment I investigated the colloid mobilization of a set of soil samples with a wide range of P saturation, but without any addition of P (3). Further, I tested hypothesis 4 with a column experiment using undisturbed soils of different P saturations. In all experiments I measured dissolved P (Pdiss) and Pcoll concentrations as well as colloid-characterizing parameters such as zeta potential and optical density. To evaluate the best-suited colloid sampling system for the column experiment of hypothesis 4, I compared different sampling systems in a column experiment using colloidal 59Fe-goethite.
The addition of P caused the mobilization of colloids in the first two batch experiments. Larger equilibrium concentrations of Pdiss were necessary to induce colloid dispersion in the batch experiment with natural subsoils (0.07-2.22 mg Pdiss l-1) than in the experiment with coated quartz sand (0.01-0.03 mg Pdiss l-1). In both experiments the critical P saturation, above which colloids were mobilized, corresponded to a zeta potential of colloids of about -20 mV. The sorption of IHP reduced the zeta potential of colloids more effectively and caused the release of larger colloid concentrations than ortho-P. In the batch experiment without any addition of P, Pcoll concentrations in supernatants increased with increasing P saturation because additional colloids were released and because the P content of the colloids increased.
The test of different lysimeter systems showed that after an application of 10 mg colloids per liter to different lysimeter systems, zero-tension and 10 mm membrane lysimeters collected the largest amount of applied 59Fe (9.1% and 6.8%) and wick lysimeters the smallest amount of applied 59Fe in the outflow (0.7%), which was related to a trapping of colloids in the wick. In contrast to the results of batch experiments, the leaching of Pcoll in the column experiment was not significantly affected by the P saturation of soils. Colloidal P concentrations ranged from 0.01 to 0.31 mg P l-1 and contributed between 1 and 37% to the leaching of total P < 1.2 µm.
The lack of an enhancing effect of P accumulation on Pcoll mobility in the column experiment I ascribe to i) a missing application of P in the column experiment compared to the first two batch experiments. Furthermore ii) physical disturbance, which probably enhanced colloid mobilization in all batch experiments, was lacking in the column experiment and iii) factors such as water content or pH additionally affected colloid transport and deposition thereby superimposing the colloid mobilizing effect of P accumulation in soils. An increasing complexity and similarity of experimental approaches to reality generally tended to obliterate the originally strong effect of P sorption on the mobilization of colloids in simple systems. In addition to soil physical or hydraulical constraints, pH and soil organic matter affect the surface charge and aggregation of oxides and hydroxides thereby masking P effects. Furthermore the increasing diversity of P sorbents from model systems to real soils blurs colloid mobilization by P sorption.
My results of batch experiments with model systems and various soils point to a colloid-mobilizing effect of P accumulation in sandy soils. Organically-bound P, contained in manure, likely has a stronger dispersing effect than inorganic P. However, due to the superimposing effects of other factors controlling the mobility of colloids, a P-induced mobilization of colloids does not necessarily result in an increased leaching of Pcoll.
Based on the improved process understanding and the presented findings concerning colloid sampling systems, future research should clarify and quantify the environmental relevance of P-induced colloid mobilization on the field scale using the best-suited colloid sampling system.
Keywords: phosphate , colloids , phosphorus saturation , inositol hexaphosphate , metal oxide