Biochar amendment boosts photosynthesis and biomass in C3 but not C4 plants: a global synthesis

Biochar is a carbon (C)-rich solid produced from the thermochemical pyrolysis of biomass. Its amendment to soils has been proposed as a promising mean to mitigate greenhouse gas emissions and simultaneously benefit agricultural crops. However, how biochar amendment affects plant photosynthesis and growth remains unclear, especially on a global scale. In this study, we conducted a global synthesis of 74 publications with 347 paired comparisons to acquire an overall tendency of plant photosynthesis and growth following biochar amendment. Overall, we found that biochar amendment significantly increased photosynthetic rate by 27.1%, and improved stomatal conductance, transpiration rate, water use efficiency, and chlorophyll concentration by 19.6%, 26.9%, 26.8%, and 16.1%, respectively. Meanwhile, plant total biomass, shoot biomass, and root biomass increased by 25.4%, 22.1%, and 34.4%, respectively. Interestingly, plant types (C3 and C4 plants) showed greater control over plant photosynthesis and biomass than a broad suite of soil and biochar factors. Biochar amendment largely boosted photosynthesis and biomass on C3 plants, but had a limited effect on C4 plants. Our results highlight the importance of the differential response of plant types to biochar amendment with respect to plant growth and photosynthesis, providing a scientific foundation for making reasonable strategies towards an extensive application of biochar for agricultural production management.