Interactions between Biochar, Soil, and Water

Biochar has been proposed as an approach for carbon sequestration and soil amendment. Biochar impacts soil carbon cycling, and in addition, the interaction between biochar and soil water can affect the soil hydrologic cycle as well as plant growth. For instance, after being applied into soil, biochar releases dissolved organic carbon (DOC) through leachate into ground water. This may influence the global carbon cycle. In addition, Biochar’s grain size and shape evolve due to natural processes which will then have feedback on the hydrologic properties of soil. Therefore, it is important to understand the effect of biochar on soil hydrologic properties as well as how the properties of biochar change in soil. Through laboratory experiments and numerical modeling, I investigated the effect of biochar on soil hydraulic conductivity and soil water retention. Meanwhile, I studied change of biochar in soil. For example, I measured DOC as a way of biochar carbon transport in the leachate of biochar-sand mixtures. Also, I tested how biochar’s grain size was altered by freeze and thaw cycling. Coupled with the mechanisms driving these changes, we can better understand the effect of biochar amendment to soil on our living environment. My results show that biochar’s effect on soil hydraulic conductivity (K), soil water retention, and DOC release vary with biochar grain size. Fine mesquite biochar changed K the most (72 ± 2% decrease), medium mesquite biochar did not cause significant change of K, and coarse mesquite biochar decreased K decreased by 15 ± 2%. Hydraulic conductivity also decreased with biochar concentration increase, by up to 72 ± 3% from 0-10 wt% mesquite biochar addition. Fine mesquite biochar did not affect plant available water significantly. The addition of medium and coarse mesquite biochar, however, increased plant available water by 75% and 125%, respectively. DOC in the leachate decreased as mesquite biochar particle size increased. The fraction of carbon lost as DOC ranged from 0.06 to 0.18 wt% of mesquite biochar. I propose that the decrease of K through the addition of fine biochar was because finer biochar particles filled spaces between sand particles which increased tortuosity and reduced pore throat size of the mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle size distribution of biochar-sand mixture, resulting in more compact packing and increased tortuosity. The volume of pores inside biochar (intraporosity) and the shape of biochar particles control the observed changes in water retention. Intraporosity drives the increase in water retention of biochar-amended soils at more negative soil water potential values. At less negative soil water potential values, biochar particles’ elongated shape increases water retention by reducing the efficiency of particle packing, creating large gaps where water can be stored. My results show that biochar grain size plays an important role in controlling soil hydrologic properties and DOC leaching. However, the effect may not stay the same over a long term because biochar grain size can be changed by freeze and thaw cycling. The effect of freeze and thaw cycling on biochar grain size varies with biochar feedstock type. For instance, median grain diameter of pine biochar decreased by up to 28.8%, median grain diameter of miscanthus biochar decreased by up to 45.8%, and median grain diameter of mesquite biochar decreased by up to 32% from 0 to 20 freeze and thaw cycles. However, there was no significant change in grain size observed for sewage waste biochar after five freeze and thaw cycles. These results suggest that mixing sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface longer, increase soil water storage with little loss of biochar-derived carbon to groundwater and streams. However, the reduction of biochar grain size by freeze and thaw cycling will drive changes in soil properties such as hydraulic conductivity and soil water retention