Assessing the impact of biological exudates associated with soil water repellency

Most soils have some level of water repellency as a result of the deposition of hydrophobic compounds from biological sources. The presence, concentration and the orientation of these compounds will contribute, with varying degrees, to the resulting repellency. A practical way of quantifying possible hydrophobic organic compounds within soils could utilise diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Peaks in the area 3000-2800 cm(3) of the DRIFT signal are indicative of aliphatic C-H bond concentration, which are thought to induce repellency. Peak areas within this region have been shown to give a more significant correlation with soil water repellency than total organic carbon.

Here we present an investigation using a selection of commercially available biological exudates, both plant (Lecithin and Polygalacturonic acid) and fungal (scleroglucan) derived. Using the exudates, we attempted to induce increasing repellency in experimental soils and kaolinite, alongside comparative measures of the DRIFT signal area. Significant increases in DRIFT peak area were detected as a result of increased chemical concentration; peak area was positively correlated with the number of C-H bonds found within exudates. These results indicate that DRIFT is an accurate means of estimating the concentration of potentially hydrophobic compounds within a soil; however, corresponding increases in water repellency were detected in only some of the treatments. This disparity, alongside variations in peak area between experimental soils containing similar concentrations of exudates, suggest that differing particle properties between soils may have a significant effect upon the orientation of molecules and as a consequence the resulting repellency. Although DRIFT quantifies the presence of hydrophobic compounds well, we found that it cannot be used as a surrogate measure of repellency