Does water repellency of pyrochars and hydrochars counter their positive effects on soil hydraulic properties?

Adding carbon-rich solids from pyrolysis (pyrochars) and hydrothermal carbonization (hydrochars) to soils can improve their hydraulic properties and increase the plant available soil water capacity (AWC). However, the effects of the specific char characteristics on soil hydraulic properties are still little understood and thus, difficult to optimize for soil amendment. It is often assumed that the water repellency of chars is an important factor influencing the water retention of soils. In using the evaporation method and saturated conductivity measurements, we investigated how changes of hydraulic properties of a loamy sand with 2% (w/w) char addition depend on (i) production processes, (ii) feedstocks, (iii) particle size, and (iv) wetting properties of added chars. Applied chars comprised pyrochars carbonized at 750 °C and hydrochars carbonized at 200 °C and 250 °C from two different feedstocks (digestate and woodchips) and two particle sizes (b0.5 mm and 0.5–1.0 mm). The wetting properties of the pure chars and soil–char composites were determined by the sessile drop method. To investigate the impact of the water repellency of chars, pure chars and soil–char composites were saturated by capillary water uptake under atmospheric conditions and by saturation from the bottom under vacuum conditions. The chars showed highly variable wetting properties with the contact angles being strongly correlated to the specific surface area of the chars. The saturated hydraulic conductivity was significantly increased after char addition for both vacuum and capillary saturated samples, while being independent of wettability characteristics. The effects for the unsaturated conductivity were negligible. In contrast to expectation, soil amendments with more water repellent chars caused higher increases of AWC(up to 15%), both after vacuum and capillary saturated conditions. Highest increase of AWC occurred for chars from digestate material, particle size fractions b0.5mm, and chars hydrothermally carbonized at 200 °C. The increase of AWC only occurred in the lowpressure range (pF 1.8 to 2.5), whereas it was decreased in the high pressure range (pF 2.5 to 4.2). Our study indicates that water repellency of pyrochars and hydrochars have little counteracting effect on the overall improvement of soil water retention and saturated conductivity, and other factors that also depend on production process and feedstock, such as particle size, intra-particle porosity, shape, and plasticity are more important