Biochar addition induced the same plant responses as elevated CO₂ in mine spoil

Nitrogen (N) limitation is one of the major constrain factors for biochar in improving plant growth, the same for elevated atmospheric carbon dioxide (CO₂). Hence, we hypothesized that (1) biochar would induce the same plant responses as elevated CO₂ under N-poor conditions; (2) elevated CO₂ would decrease the potential of biochar application in improving plant growth. To test these hypotheses, we assessed the effects of pinewood biochar, produced at three pyrolytic temperatures (650, 750 and 850 °C), on C and N allocation at the whole-plant level of three plant species (Austrostipa ramossissima, Dichelachne micrantha and Isolepis nodosa) grown in the N poor mine spoil under both ambient (400 μL L^-1) and elevated (700 μL L^-1) CO₂ concentrations. Our data showed that biochar addition (1) significantly decreased leaf total N and δ¹⁵N (P < 0.05); (2) decreased leaf total N and δ¹⁵N more pronouncedly than those of root; and (3) showed more pronounced effects on improving plant biomass under ambient CO₂ than under elevated CO₂ concentration. Hence, it remained a strong possibility that biochar addition induced the same plant physiological responses as elevated CO₂ in the N-deficient mine spoil. As expected, elevated CO₂ decreased the ability of biochar addition in improving plant growth.