Models for Predicting Water Retention in Pyrogenic Carbon (Biochar) and Biochar-Amended Soil at Low Water Contents

The dry end of the soil water retention curve (WRC) plays an important role in various hydrologic, solute transport, plant, and microbial processes. Despite increasing application of biochar as a soil amendment, knowledge about water retention in biochars and biochar-amended soils under dry conditions is lacking. Mechanistic models are presented to predict the WRC for biochars and biochar-amended soils at matric potential (psi) similar to-1 MPa. For biochars, the amount of water retained is linked to biochar surficial oxygen content and pore volume and surface area distributions. The WRC for soils at dry conditions is predicted using specific surface area. The WRC model for biochar-amended soils is the sum of the contributions of models for biochar and soil. The model's utility was examined for three natural soils and a uniform sand, a wood-based biochar, and 10 different combinations of these soils and biochar. The accuracy of the model for biochars was further tested for six other pyrogenic carbonaceous materials (PCMs). The models agreed well with experimental data: for the biochar and PCMs, soils, and biochar-amended soils, the root mean square error normalized to the range of water content was almost always <10%. The line of best fit for predicted versus measured gravimetric water content at permanent wilting point had slope of 0.935 +/- 0.013 and a coefficient of determination of 0.997. The applicability of these models for different biochars, soils, and their mixtures is discussed.