Evaluating Causes of Enhanced Soil Aggregation in Biochar-Amended Roadway Soils

Biochar amendment to roadway soils has been proposed to increase infiltration and retention of stormwater runoff, thus reducing discharge to nearby water bodies. Increased infiltration is believed to result from enhanced soil aggregation, which may play a significant role in increasing hydraulic conductivity. However, the mechanisms by which biochar enhances soil aggregation are not well understood, making it difficult to determine the biochar and soil properties critical to this process. A wood-derived biochar pyrolyzed at 500⁰C was amended to the top 30 cm of a sandy loam roadway soil at a Delaware field site, and to 15 roadway soils collected from four states with a wide range of soil texture, mineralogy and organic carbon content that were repacked into laboratory columns. Biochar amendment to the Delaware roadway soil substantially altered stormwater hydrology, reducing cumulative runoff and peak flow rate by ~ 40% compared to an unamended soil over 84 storm events and 2 years of testing. Disc infiltrometer measurements indicated that the saturated hydraulic conductivity was ~ 50% larger in biochar-amended versus unamended Delaware roadway soil. Cores taken from the Delaware site were wet sieved to determine water-stable macroaggregates, and subsamples tested to evaluate the role of organo-mineral association; glomalin, a soil protein produced by fungi; and polysaccharides generated by bacteria on aggregation. Biochar-amended soil had between 20 and 133% more macroaggregates (> 250 µm) than unamended soil, depending on sampling depth. While increased soil aggregation was correlated with glomalin and polysaccharide concentrations over 7.5 - 30 cm depth, at the surface (0-7.5 cm) aggregation was only correlated with increases in organo-mineral association. Biochar amendment increased the degree of organo-mineral association between 20 and 30% over all depths with the most significant increases at the surface, which is the location most critical for increasing hydraulic conductivity and stormwater infiltration. These data and results from the ongoing incubation study of 15 roadway soils are discussed to help elucidate biochar and soil properties important for enhanced soil aggregation.