Effects of biochar addition on anaerobic digestion of tobacco stalks and the fertility of biochar on crop growth

This study evaluates methane production from anaerobic digestion of tobacco stalk under different TS (total solid) concentrations and supplemented with or without tobacco stalk biochar (at 5% and 10% TS) at room temperature (18-33 degree Celsius), and investigates fertility of the biochar on crops with pot experiments. 50-day lab-scale reactor experiments showed that 249.3-276.3 mL/g VS (volatile solid) methane was produced in the reactors with 3%, 6% and 9% TS. The productivities are comparable with those reported from similar studies on anaerobic digestion of tobacco stalks carried out under mesophilic (35-37 degree Celsius) condition. Biochar addition removed the inhibition of high TS concentration (6% and 9%) on methane production. Supplementation with 10% biochar raised the methane production level to 319.3-336.3 mL/g VS. No significant difference in methane production was found among the reactors with different TS concentrations. The optimal TS and biochar concentrations for the maximum methane production are 9% and 10% respectively. Quantitative fluorescence in situ hybridization showed that the biochar resulted in enrichment of methanogenic members of Methanosaeta spp., Methanomicrobiales and Methanobacteriales in the reactors with 3%, 6% and 9% TS concentrations respectively. 80-day pot experiments using soil mixed with the biochar-containing biogas residue showed that faba bean (Vicia faba L.) grown in soil with a low concentration (0.63%) of biochar had significantly (P < 0.05) higher above ground height (49.07 vs 44.44 cm), bigger stem width (5.57 vs 4.91 mm) and above ground dry weight (2.50 vs 2.13 g/plant) than the controls grown in the soil mixed with biogas residue without biochar. Illumina high throughput sequencing analyses revealed that the biochar changed the composition and diversity of the rhizosphere prokaryotic and fungal communities, and significantly (P < 0.05) enhanced the activities of the ureolysis, sulfate and sulfite respiration, and nitrate reduction and denitrification of the rhizosphere soil. Especially, biochar significantly enriched biological control bacteria Pseudomonas (by 102.3%), Lysobacteria (by 27.4%) and Stenotrophomonas (by 35.1%), and resulted in a decrease in the abundance of the plant pathogen fungi in the rhizosphere soil of the faba bean. Enrichment of biological control bacteria may help to explain the positive impact of biochar on the crop growth.