IMPACTS OF BIOCHAR CONCENTRATION AND PARTICLE SIZE ON HYDRAULIC CONDUCTIVITY OF SOIL-BIOCHAR MIXTURES

Amending soil with biochar has been proposed as a method to capture and sequester carbon, and to improve soil hydrologic properties by changing soil physical and chemical characteristics. We present hydraulic conductivity (K) of soil-biochar mixtures determined by the falling-head method. Our soil mixtures were sand with biochar amendment rates from 0-10 wt%. We find that K decreases by an average of 63 % as biochar concentration increases from 0 to 10 wt%. We also determined porosity and saturation of samples using helium pycnometry, mass measurements, and phase relations. As biochar concentration increases from 0-10 wt%, sample porosity increases from 0.36 ± 0.03 to 0.46 ± 0.02 and water saturation decreases from 102 ± 3 % to 85.4 ± 0.4 %. We interpret that this decrease in saturation is controlling the decrease in K. At 2 wt% amendment rate, our results also show that K decreased significantly (by 67 %, p<0.001) when biochar particles (<0.251 mm) are smaller than sand particles (0.251-0.853 mm) and decreased by 15 % (p<0.001) when biochar particles (0.853-2.00 mm) are bigger than sand particles (0.251-0.853 mm). When biochar and sand particle sizes are comparable, no significant changes in hydraulic conductivity were observed (p=0.25). We propose that the decrease of K by adding smaller biochar is driven by the combination of several mechanisms including partial saturation, smaller particle size and grain shape of biochar which will reduce the available space for flow, reduce pore throat size, and increase toturosity. We also propose that the decrease of K by adding larger biochar is caused by non-uniform particle size distribution yielding a net decrease in pore throat size. Together these results confirm that biochar amendment does influence hydraulic conductivity, and the mechanisms that control it are largely physical including the relative size of biochar and soil particles and their proportions.