Amendment by biochar made by thermal degradation of biomass is expected to enhance carbon sequestration through stimulating carbon assimilation by plants. We clarified the effect of biochar amendment on the photosynthesis of trees in forest ecosystems. Biochar was applied to young oak trees (Quercus serrata) in temperate deciduous forest at rates of 0, 5, 10 and 20 Mg ha−1 in four plots (C0, C5, C10, and C20). The variation in photosynthetic parameters (the maximum photosynthetic rate: Pmax, maximum carboxylation rate: Vcmax and the potential rate of electron transport: Jmax) and leaf traits (the stomatal conductance: gc, leaf mass per area (LMA) and leaf nutrient concentrations) were examined every month during the growing seasons for 3 years. Pmax generally increased in C5 and C10 and did not increase in C20. Similarly, Vcmax and Jmax increased in C5 and C10 and correlated significantly positively with Pmax, suggesting that biochar amendment basically increased the photosynthetic rate through improvements in physiological activities but that there was a maximum useful dosage. We also found that gc, LMA and leaf nutrient (N, Mg, and S) showed significant positive correlations with Pmax, indicating that an increase in photosynthetic rates would be supported by these leaf traits. However, stimulation of photosynthesis became smaller year by year, indicating that the effects of biochar amendment faded gradually. We concluded that biochar amendment basically improved the photosynthesis of oak trees in the forest through the change of all gc, LMA and leaf nutrient concentrations but declined yearly.
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