Effect of crop residue biochar on soil acidity amelioration in strongly acidic tea garden soils

Strongly acidic soil (e.g. pH<5.0) is detrimental to tea productivity and quality. Wheat, rice and peanut biochar produced at low temperature (max 300 degrees C) and differing in alkalinity content were incorporated into Xuan-cheng (Ultisol; initial pH(soil/water = 1/2.5) 4.12) and Ying-tan soil (Ultisol; initial pH (soil/water = 1/2.5) 4.75) at 10 and 20g/kg (w/w) to quantify their liming effect and evaluate their effectiveness for acidity amelioration of tea garden soils. After a 65-day incubation at 25 degrees C, biochar application significantly (P<0.05) increased soil pH and exchangeable cations and reduced Al saturation of both tea soils. Association of H+ ions with biochar and decarboxylation processes was likely to be the main factor neutralizing soil acidity. Further, biochar application reduced acidity production from the N cycle. Significant (P<0.05) increases in exchangeable cations and reductions in exchangeable acidity and Al saturation were observed as the rate of biochar increased, but there were no further effects on soil pH. The lack of change in soil pH at the higher biochar rate may be due to the displacement of exchangeable acidity and the high buffering capacity of biochar, thereby retarding a further liming effect. Hence, a significant linear correlation between reduced exchangeable acidity and alkalinity balance was found in biochar-amended soils (P<0.05). Low-temperature biochar of crop residues is suggested as a potential amendment to ameliorate acidic tea garden soils.