Predicting water retention of biochar-amended soil from independent measurements of biochar and soil properties

Biochar is black carbon produced from pyrolysis of biomass and may be added to soil to sequester carbon and improve soil water retention. To date models to predict changes in soil water retention with biochar amendment are still missing and therefore direct measurements are required for every biochar/soil combination, which can be time-consuming. Here, a predictive model for biochar's effect on soil water retention was developed and tested that includes water retained in biochar intrapores and biochar's impact on interpores between particles. The independently measured parameters needed for the model are the particle size distributions (PSDs) and particle densities for biochar and soil, water retention data for biochar-free soil, biochar intrapore volume distribution from mercury porosimetry, amount of biochar added, bulk density of the biochar/soil mixture, and dew point potentiometer measurements of biochar. The model was tested using poultry litter and wood biochars amended to two soils (sand and sandy loam) at 2 and 7% mass fraction. The model predicted changes in the soil water characteristic well for the biochar amendment, with RMSE decreasing by ~50% when the full model was used. Model predictions of the change in available water capacity with biochar amendment for eight biochar/soil combinations had an average absolute error of 0.017 ± 0.006 and an average relative error of 10¯0 ± 40%. The model correctly predicted the increase in available water content when sandy loam was amended with wood biochar, and the decrease if amended with poultry litter biochar. The model provides an improved understanding of the mechanisms by which biochar alters water retention, and a means to estimate the initial change in available water capacity for a particular biochar/soil combination if necessary biochar and soil properties are measured.