Ameliorating soil acidity and physical properties of two contrasting texture Ultisols with wastewater sludge biochar

The production of biochar is a safe and beneficial disposal way for wastewater sludge. The biochar produced from wastewater sludge can be used as soil amendments for improving soil properties and for increasing crop yield. This work investigated the influences of wastewater sludge biochar (WSB) on the pH, exchangeable acidity, and physical properties of strongly acidic Ultisols with contrasting texture (clayey soil and sandy loam). Two soils were mixed with WSB at the rate of 0, 10, 20, and 40 g biochar kg−1 soil and incubated for 240 days at 75% field water capacity. Incubation experimental results indicated that WSB significantly increased soil pH and exchangeable Ca2+ and Mg2+ contents, and decreased soil exchangeable H+ and Al3+, compared with the control. The application of WSB enhanced the formation of 5–2-mm macroaggregate, and decreased the content of <0.25-mm microaggregate. WSB application significantly increased aggregate stability of soils, determined by mean weight diameter (MWD) of aggregate. WSB increased the field water capacity and available water content (AWC) of sandy loam while WSB was not found to increase significantly water-holding capacity and AWC of clayey soil. WSB significantly reduced plastic index and tensile strength (TS) of clayey soil and did not alter the TS of sandy loam. Overall results suggest that WSB is a suitable amendment for strongly acidic Ultisols with poor physical properties. However, the soil texture affected greatly the improvement effect of WSB on poor physical properties in soils.