Linking two wood biochars physicochemical properties with soil water evaporation dynamics from amended sandy soil

Abstract Conflicting reports on biochar's effect on soil evaporation (E) highlight a poor understanding of how its physicochemical properties govern E dynamics in sandy soils. Thus, the aim of this study was to provide mechanistic insights by correlating biochar physicochemical properties with the distinct stages of evaporation. Biochars from black wattle (BW) and red river gum (RG), produced via vacuum pyrolysis at 500°C–800°C, were applied (3% by mass) to a sandy Arenosol. The control and biochar‐amended treatments were then subjected to saturation, followed by drying for 20 days in soil columns. Soil volumetric water content was measured at three soil depths throughout the drying period. Cumulative E was partitioned into Stage I (capillary‐flow‐driven) and Stage II (diffusion‐limited). Results revealed feedstock‐dependent, opposing effects. High‐temperature (700°C–800°C) RG biochars increased Stage I E by 207%–392%, correlated with increased saturated hydraulic conductivity, acting as “hydraulic bridges” that depleted the water reservoir. Conversely, high‐temperature BW biochars (e.g., BW800) increased water loss during Stage II E by 106%, linked to their high specific surface area and micropore volume, functioning as “vapor pumps” that sustained E loss. Consequently, both high‐temperature biochars at 800°C increased total cumulative E by 122%–181% compared to the control, whereas the 500°C BW biochar reduced it by 21%. These findings demonstrate that for enhancing water conservation, a biochar's influence on dynamic water transport is more critical than its static water retention capacity. We conclude that lower temperature, more degradable biochars are better suited for reducing evaporative water loss in sandy soils than their high‐temperature counterparts.