Effects of Application of Bagasse- and Sunflower Residue-derived Biochar to Soil on Growth and Yield of Oilseed Sunflower

Oilseed sunflower (Helianthus annuus L.) has a lower seed yield and harvest index than other oil crops, and consequently produces a large amount of residue such as stem, receptacle, and oil cake. Our study attempted to use these residues as biochar for improving soil fertility, sunflower growth, and yield in subtropical island, Okinawa, Japan. To reveal the synergistic effect of biochar and inorganic fertilizer, we established four treatments: biochar (B), biochar + fertilizer (BF), fertilizer (F), and control, using biochar derived from bagasse (Exp. 1) and sunflower residue (Exp. 2). As a result, biochar application improved soil fertility, including soil electrical conductivity and mineral concentration. In spite of the initial positive effect of biochar application to plant growth this effect became negative later. The reason for this response might be the reduction of the net assimilation rate (NAR) during the later growth stage due to nitrogen deficiency after biochar application. Biochar application had a positive effect on sunflower growth during the early growth stage when the leaf area index (LAI) was dominant for the crop growth rate (CGR), while this effect became negative during the later growth stage when the NAR was dominant for the CGR. Further research is required to determine the effects of biochar application depending upon application methods, plant growth stages, and plant species.