Comparison of the ameliorating effects on an acidic ultisol between four crop straws and their biochars

Abstract

The amelioration effects of crop straws and their biochars on an acidic ultisol were compared in incubation experiments to determine suitable organic amendments for acid soils. Four crop straws, including non-legumes (canola straw and rice straw) and legumes (soybean straw and pea straw) were used to prepare biochars using a low temperature (350°C) oxygen-limited pyrolysis method. Two application rates of 1% and 2% were used for both crop straws and their biochars in incubation experiments lasting 90 days. Soil pH (1:2.5 soil to water), soil exchangeable acidity, soil exchangeable base cations, and soil cation exchange capacity (CEC) were determined to evaluate the amelioration effects of these crop straws and their biochars on an acidic ultisol. The incorporation of crop straws increased or decreased the soil pH depending on the relative contribution of alkalinity of the straws, mineralization of organic N and nitrification of NH4 +. The incorporation of biochars produced from crop straws increased the soil pH, and their ameliorating effects increased with the application rates of biochars. The biochars from legume straws induced more increase in soil pH than non-legume biochars. The addition of both crop straws and their biochars decreased soil exchangeable acidity and exchangeable Al3+, and increased soil exchangeable base cations and base saturation degree. The biochars (especially legumes) induced a greater decrease in soil exchangeable acidity and a greater increase in soil exchangeable base cations compared to their feedstock due to their much higher contents of base cations. The CEC of biochars were 10–20 times that of soil CEC and thus biochar incorporation increased the soil CEC significantly, as well as the retention of Ca2+, Mg2+, K+, and NH4 + by acid soils. The biochars produced from legume crop straws were better choices as amendments for acid soils than their feedstock. Organic anions and carbonates were the main forms of alkali in the biochar; both contributed to neutralizing soil acidity and increasing soil pH. The incorporation of biochar cannot only neutralize soil acidity, but can also improve soil fertility.