Predicting the impact of biochar additions on soil hydraulic properties

Authors: LIM, TAE-JUN - Rural Development Administration - Korea; Spokas, Kurt; Feyereisen, Gary; Novak, Jeffrey

Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2015
Publication Date: 7/2/2015
Citation: Lim, T., Spokas, K.A., Feyereisen, G.W., Novak, J.M. 2015. Predicting the impact of biochar additions on soil hydraulic properties. Chemosphere. 142:136-144.

Interpretive Summary: Biochar additions can change the hydraulic properties of the soil system. There have been previous studies that have examined the impact of biochar additions and have observed a range of effects from increases to decreases, as well as no changes. However, there has not been an attempt to unify all the biochar studies under one theoretical concept. In this study, we added 4 different biochars to a range of different soil textures to examine the possibility of developing a model to predict the overall impact on water flow following biochar additions. We observed that the impacts of biochar could be predicted by the existing soil models of saturated conductivity. The primary alteration in the soil system was due to the physical particle size of the biochar. Despite its internal pore structures, the biochar behaved like a solid particle when evaluating the saturated hydraulic conductivity. Overall, the impact of biochar is due to the alteration in the packing arrangement of soil particles, with the largest impact being observed in sandy textured soils. This work represents the first step to a unified theory behind the impact of biochar on soil saturated conductivity. These results are significant to farmers and policy makers and will assist scientists and engineers in developing improved models for predicting the impacts of biochar on other hydraulic properties. These models are vital in determining the impact of biochar on soil moisture relationships for water holding relationships.

Technical Abstract: Different physical and chemical properties of biochar, which is made out of a variety of biomass materials, can impact water movement through amended soil. The objective of this research was to develop a decision support tool predicting the impact of biochar additions on soil saturated hydraulic conductivity (Ksat). Four different kinds of biochar were added to four different textured soils (coarse sand, fine sand, loam, and clay textured soils) to assess these effects. Particle size distribution of each biochar was measured prior to soil mixing at the rates of 1, 2, and 5 % (w/w). The results showed that the Ksat of the biochar amended soils were significantly influenced by the rate and type of biochar, as well as the original particle size of soil. The Ksat decreased when biochar was added to coarse and fine sands, as a function of the biochar rate. Biochar with larger particles sizes (60%; >1 mm) decreased Ksat to a larger degree than the smaller particle size biochar (60%; <1 mm) in these two sandy textured soils. Biochar additions reduced the bulk density (P<0.05) and similarly reduced Ksat for sandy soils. The increasing tortuosity in the amended sandy soil could explain this behavior. On the other hand, for the clay loam 1% and 2% biochar additions universally increased the Ksat with higher biochar amounts providing no further increases and decreases dependent on particle size of the biochar. The developed model utilizes soil texture pedotransfer functions that have been developed for predicting agricultural soil Ksat as a function of soil texture. The model accurately predicted the direction of the Ksat influence, even though the exact magnitude still requires future refinement. This represents the first step to a unified theory behind the impact of biochar on soil saturated conductivity following application.