Field Scale Variation In Water Dispersible Colloids From Intact Soil Samples and Aggregates: Method Comparison and Relevance for Leaching Risk Mapping.

Colloid-facilitated transport can play an important role in the transport of chemicals through the soil profile. The negative surface charge and large surface area makes colloids perfect carriers for strongly sorbing chemicals, like phosphorus and certain pesticides, in highly structured soils. It is, however, difficult to quantify the amount of colloids ready available to participate in colloid-facilitated transport. In literature, the part of the colloidal fraction that readily disperses into suspension is referred to as water-dispersible clay (WDC). In this study we used two methods for measuring the amount of WDC (< 2 µm equivalent particle diameter) on soil sampled from an agricultural 1.69 ha field in a 15 x 15 m grid (65 points). In method no. 1 laser diffraction was applied to continuously measure the particle size distribution of 1-2 mm intact soil aggregates at two initial moisture contents, air dry and equilibrated at -100 H2O of soil matric potential (pF2). Method no. 2 is simple end-over-end shaking of 100 cm3 intact soil samples equilibrated to an initial matric potential of -5 cm H2O. Using method 1, the wet aggregates in average had six times higher WDC than the dry aggregates. For the dry aggregates we observed a trend across the field, where one-third of the field seemed to have markedly higher WDC contents. This also coincides with the part of the field where the largest leaching of pesticides has been observed during a 12-year monitoring program. Leaching experiments on 20 x 20 cm intact soil columns sampled from the same field grid also showed that the largest mass of particles and phosphorus leached from this part of the field. Thus, the presented WDC method comparison and results seem highly relevant in regard to field-scale mapping of leaching risk in regard to colloid-facilitated chemical transport. WDC measurements from the two methods will furthermore be correlated to particles leaching from the intact 20 x 20 cm soil columns.