Pig manure biochar for contaminated soil management: nutrient release, toxic metal immobilization, and Chinese cabbage cultivation

Abstract

Pig manure could be an effective fertilizer source for soil, but with high concentrations of xic elements. It has been shown that the pyrolysis method could largely reduce the environmental risk of pig manure. However, the comprehensive analysis of both toxic metals immobilization effect and environmental risk of pig manure biochar applied as a soil amendment is rarely addressed. To address the knowledge gap, this study was carried out with pig manure (PM) and pig manure biochar (PMB). The PM was pyrolyzed at 450 ℃ and 700 ℃, the corresponding biochar was abbreviated as PMB450 and PMB700, respectively. The PM and PMB were applied in a pot experiment growing Chinese cabbage (Brassica rape L. ssp. Pekinensis) with clay-loam paddy soil. The application rates of PM were set at 0.5% (S), 2% (L), 4% (M) and 6% (H). With the equivalent mass principle, PMB450 and PMB700 were applied at 0.23% (S), 0.92% (L), 1.84% (M), 2.76% (H), and 0.192% (S), 0.7% (L), 1.4% (M), 2.1% (H), respectively. Parameters of Chinese cabbage biomass and quality, total and available concentrations of toxic metals in soil, and soil chemical properties were systemically measured. The main results of this study showed that compared with PM, PMB700 was more effective than PMB450, which induced the highest reductions of Cu, Zn, Pb, and Cd contents in cabbage by 62.6%, 73.0%, 43.9%, and 74.3%, respectively. Both PM and PMB increased the total contents of metals (Cu, Zn, Pb, and Cd) in soil, and PMB decreased the mobility of Cu, Zn, Pb, and Cd at high application rates (≥2%). Treatment with H-PMB700 reduced CaCl₂ extractable Cu, Zn, Pb, and Cd by 70.0%, 71.6%, 23.3%, and 15.9%, respectively. For Cu, Zn, Pb, and Cd fractions with BCR extraction, PMB treatments, especially PMB700, were more effective than PM in decreasing the available fractions (F1 +F2 +F3) at high application rates (≥2%). Overall, pyrolysis with high temperature (e.g., 700 ℃) could significantly stabilize the toxic elements in PM and enhance PM's effect on toxic metals immobilization. The marked effects of PMB700 on toxic metal immobilization and cabbage quality improvement might be attributed to high ash contents and liming effect.

Keywords: Chinese cabbage; Pig manure; Pig manure biochar; Toxic metals.