Acidity, water retention, and mechanical physical quality of a strongly acidic Ultisol amended with biochars derived from different feedstocks

Strongly acidic Ultisols in tropical and subtropical regions of China present one of the most important degraded soils. The improvement of soil quality for these soils is a key goal for sustainable agriculture. The purpose of this study is to evaluate the beneficial effects of biochar amendments on the soil acidity, plant available nutrient contents, and physical properties of a strongly acidic Ultisol. A Typic Plinthudult with low soil fertility and poor physical properties was amended by three biochars made from straw (SB), woodchips (WCB), and wastewater sludge (WSB) at the rate of 0, 2, 4, and 6 % biochar, respectively. After 180 days of incubation, the chemical, nutrient contents, water retention, consistency, tensile strength, and shear strength of biochar-amended soils were determined. Experimental results indicate that biochars significantly (p < 0.05) increase the pH of the soil and decrease the contents of exchangeable H+ and Al3+. The WCB treatment results in higher pH values than the SB and WSB treatments. The biochars significantly increase total C, available P, K, and exchangeable K, Ca, and Mg contents. Biochar applications significantly enhance water-holding capacity of soil, while not increasing the available water content (AWC) of the soil. Biochar application significantly (p < 0.05) increases the liquid limit (LL) and plastic index (PI) of the soil. The effectiveness of biochar on LL and PL is more pronounced in the SB-amended soils. With application of biochar, the tensile strength (TS) of Ultisol decreases from original 466 kPa to 233, 164, and 175 kPa for 6 % WCB-, SB-, and WSB-amended soils, respectively. Direct shear tests indicate WCB significantly reduces the cohesion (c) of the soils, while biochars do not alter the internal friction angle (phi) of soil. Analyses of scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) reveal that soil mineral particles are inserted inside the pores of biochar and attached on the surface of biochar, indicating that biochar greatly changes the microstructure and pore system of soil. It is suggested that biochar amendment generally improves the quality of degraded Ultisols with strong acidity, low fertility, and poor physical properties. The physical dilution effect and microstructure change caused by the porous and less dense biochar are identified to be the main mechanism for the biochar to improve the physical properties of strongly acidic Ultisols.