Water repellency and distribution of hydrophilic and hydrophobic compounds in soil aggregates from different tillage systems

Water repellency properties of individual soil aggregates offer increased reduction of carbon (C) mineralization within soils. These studies explore the spatial distributions of C and groupings of C–H and CO compounds at multiple soil depths, on surfaces and within multiple sized soil aggregates. Measurements were conducted on soil aggregates, sampled from three horizons in a silty loam Anthrosol under long term grassland pastures and short term conservation tillage wheat and in a silty loam Luvisol subjected to long term conventional tillage of maize. Soil aggregates were fractionated into seven size classes by dry sieving and single aggregates were mechanically separated into external, transitional and interior regions by soil aggregate erosion (SAE) chambers. Soil materials from different aggregate-size classes and aggregate layers were evaluated for total C and nitrogen (N) contents. Functional hydrophobic and hydrophilic groups in each of these soils were identified by DRIFT spectrometry. Water repellency was identified on the surfaces of different size-class aggregates and each of their interior regions by the ethanol/water sorptivity method. Total organic C contents within individual aggregates and size classes varied considerably and were influenced by soil depths and management practices. Whole microaggregates had greater C contents than macroaggregates. Total organic C content was highly correlated to hydrophobic (C–H) groups (R2 = 0.78), but not hydrophilic groups. External surfaces of aggregates from no tilled soils of wheat and grass exhibited the largest gradients of hydrophobic C–H groups when compared to their interiors. More hydrophilic CO groups were observed in maize and grassland biomes where plant biomass was greater. Although highly variable, the largest gradients of CO groups were measured in the Ap and Ah horizons of maize and grasslands. Water repellency indices were greater in aggregate sizes ranging from 8 to 5 mm for agricultural soil management systems and were similar for most aggregate sizes measured for grasslands. Soil aggregate water repellency appears to be controlled by the uniform distribution of these compounds at surfaces of all soil aggregate-size fractions and possibly by the concentrations of hydrophobic compounds on surfaces of aggregates at multiple soil depths. Further inquires into the mechanisms controlling soil wetting require more than a quantification of total organic C. These studies demonstrated spatial interactions between the locations and quantities of hydrophobic and hydrophobic compounds distributed among soil aggregate-size fractions located within the soil profile.