Wednesday, November 7, 2007
285-3
Diffusion of Multiple Anionic Tracers in Differently Structured Soil.
Mette Laegdsmand1, Per Moldrup2, and Per Schjønning1. (1) University of Aarhus, University of Aarhus, F.Agri.Sci., PO Box 50, Tjele, (Non U.S.) 8830, DENMARK, (2) DENMARK,Aalborg Univ., Dept. of Biotech. Chem. and Environ. Engineering, Aalborg Univ Sohngaardsholmsvej 57 D-building, Aalborg, DK9000, DENMARK
Diffusion of solutes in soils may restrict the kinetics of chemical and microbiological processes in the soil. Sorption and degradation processes depend on diffusion to transport the sorbent/substrate to the site of sorption or degradation. We wanted to investigate how the diffusion of solutes in structured soil was affected by (i) the structure of the soil, (ii) the water content, and (iii) the type of solute molecule. Intact soil samples were collected in a field experiment with two different tillage strategies: (i) moldboard ploughing to 20 cm depth and (ii) harrowing to 8-10 cm depth. The samples were taken in 12-16 cm depth in three replicate blocks for each treatment. Solute diffusivity measurements were conducted with a modification of the classical half-cell method. In the modified method concentrations of three different anionic tracers could be measured at the same time (bromide (Br), 2,6-difluoro benzoic acid (DFBA) and pentafluoro benzoic acid (PFBA)) and individual diffusion coefficients could be estimated for each tracer and for each of the two intact half-cells. For each soil sample the impedance factor (f) was estimated with a model for solute diffusion in two finite cells separated by a sheet, using the measured water contents and concentration profiles of bromide, DFBA and PFBA. The impedance factors for all the soil samples and for all tracers were used to estimate the slope (H) and the limiting water content where solute diffusion ceases (θlim) in a linear model for impedance factor as a function of water content. The data fitted well to the linear model. The values of H and θlim for Br were significantly different from those estimated for DFBA and PFBA. We interpret this as being the result of solute size affecting both the diffusion pathway of the solute molecules as well as the amount of non-accessible water in the soil. The tillage-derived differences in soil structure also induced different values of H and θlim.