Seed Germination and Early Seedling Growth of Rhododendron Species in Biochar-Amended Peat Substrates

Peat is a natural resource that regenerates very slowly, and the research for seeking low-cost and environmentally friendly alternative products is encouraged. This study evaluated the feasibility of corncob biochar (CB) and rice-husk biochar (RB) as peat substitutes in growth media for rhododendron species cultivation. We used aqueous extract (1/10 w/v) from biochar to assess its potential toxicity on seed germination of four rhododendron species by petri dish bioassay. ForR. calophytum Franch., R. delavayi Franch. and R. molle G. Don, RB and CB had positive or neutral effect on seed germination, respectively. In contrast, both biochar additions inhibitedR. decorum Franch. seed germination. A greenhouse pot experiment was conducted to assess the effects of CB and RB at varying rates (0%, 10%, 20%, and 40% by volume) on early seedling growth ofR. decorumandR. molle. In the RB-containing substrates, the shoot biomass and chlorophyll content ofR. mollewere increased at the rates of 10-40%, andR. decorumperformed well with a maximum does of 20%. However, the growth of the two rhododendron species seedlings was significantly inhibited in peat: CB (60:40 by volume) mixed substrates, which is attributed to the high alkaline and salinity, unsatisfactory hydrophysical properties, together with decreased N and P availability. The results indicated RB could be safely used as a peat substitute at the rates of 20-40% by volume for the cultivation of rhododendron species, due to ideal physico-chemical properties and balanced nutrient supply of the peat:RB mixed substrates.