Effects of bamboo biochar on soybean root nodulation in multi-elements contaminated soils

Improvements in plant physiological performance by means of biochar application in soils contaminated by multi-elements are determinants of agroecosystem functioning. This study analyzed the effects of bamboo-derived biochar on root nodulation and plant growth in a moderately acidic Andosol (pH = 5.56) contaminated with multi-elements during a 70-day investigation of soybean growth. Bamboo biochar that had been pyrolyzed at a temperature below 500°C was applied to soils at three different and moderately high rates (5%, 10%, and 15%, w/w). Biochar amendment beyond 5% stimulated root nodulation as well as soybean growth. The nodule weight per root system was significantly enhanced by 186% and 243% over the control at the 10% and 15% addition rates, respectively. The primary explanation for these stimulatory effects was attributed to an increase in the K and Mo supplies for plant uptake that was induced by the biochar application, whereas the increased availability of P contributed to a lesser extent. Leaf CO₂ assimilation rate was slightly enhanced at the highest application rate, but this enhancement was not associated with an increase in biomass. The incorporation of biochar into the soil reduced extractable-NH₄NO₃ Cd, Cu, Mn, Ni, and Zn, but not Pb, regardless of the application dose. This change was accompanied by a significant (P < 0.05) suppression of the uptake od trace elements in soybean shoots at the optimum application rate (10%); the degree of reduction followed this order: Pb>Mn>Cd>Zn>Cu>Ni. The increase in soil pH and the diffusion/adsorption of trace elements onto the biochar may have contributed to the lowering of the concentration of trace elements in the soil as well as in soybean shoots.