Converting Si-rich biomass into a multifunctional soil amendment and its effect on plant growth

Silicon (Si)-rich biomass feedstocks are generated annually from agricultural crop production. In this study, we explored the potential of converting Si-rich biomass to biochar as a soil amendment of both nutrient supply and disease resistance for crop production. Silicon-rich rice straw/husk, sugarcane harvest residue, and miscanthus were collected and converted to biochar.  Pretreatment of biomass with KOH and other alkali reagents was carried out to increase Si bioavailability and enrich nutrients of biochar. Converted biochar products were characterized for their Si-releasing behavior and plant uptake. Results showed that depending on total Si contents of feedstocks utilized, varying amounts of plant-available Si were released. Alkali pretreatment caused the bleaching of phytolith-Si to increase Si release from biochar. Alkali-enhanced biochars from rice straw or husk yielded the higher extractable Si as pyrolysis temperature increased from 350^oC to 550^oC.  More soluble Si was also released in unbuffered weak acid and neutral salt solutions for alkali-enhanced biochars produced. Potting studies with perennial ryegrass showed that alkali-enhanced biochars increased tissue Si content and suppressed gray leaf spot development. Rice greenhouse trials also illustrated that the application of alkali-enhanced biochars, especially those made from rice husk at 550^oC, increased Si uptake and rice grain yields compared to lower temperature and non-enhanced biochars. Overall, alkali-enhanced-biochar could be used as a multifunctional soil amendment for improving plant growth.