Background The fifth IPCC assessment report pointed out that the atmospheric CO₂ concentration would exceed 700 μmol/mol at the end of the 21st century, and the global mean surface temperature would rise by 0.3 ℃-4.8 ℃ on the current basis. Climate change induces multiple stresses on plant growth and soil health quality. It is well known that soil microelements are crucial indicators for plant growth and soil health quality. Previous studies generally focused on the isolated effect of elevated CO₂ concentration and temperature on soil microelement contents. Therefore, clarifying the coupling effects of elevated CO₂ and temperature on soil microelements is crucial for the evaluation of soil health quality under the climate change background.

Methods A pot experiment was conducted in artificial climate chambers to investigate the coupling effects of elevated CO₂ and temperature on 4 kinds of soil microelement availability (Fe, Mn, Cu and Zn). The experimental treatments included 3 climate scenarios, i.e., the controlled experiment (400 μmol/mol CO₂ and 22 ℃, CK), elevated CO₂ (700 μmol/mol CO₂ and 22 ℃, EC), both elevated CO₂ and temperature (700 μmol/mol CO₂ and 26 ℃, EC+ET), each climate scenario covered two water conditions, i.e., sufficient water supply (70% of field capacity) and light water stress (50% of field capacity). The total and available contents of soil microelements were measured at maturity stage of millet.

Results 1) Compared with CK, only elevated CO₂ treatment increased soil available microelement contents under the 2 water conditions. Under the light water stress condition, elevated CO₂ significantly increased the contents of soil available Cu and Zn by 20.4% and 107.4%, respectively (P < 0.05); and under the sufficient water supply condition, elevated CO₂ concentration significantly increased 18.6% and 133.4% of soil available Cu and Zn contents, respectively (P < 0.05). 2) Compared with CK, both elevated CO₂ and temperature increased soil available Mn, Cu and Zn contents under the two water conditions, while it reduced the soil available Fe content. Under the sufficient water supply condition, soil available Cu and Zn contents significantly increased by 19.4% and 109.3%, respectively (P < 0.05). Under the light water stress condition, soil available Mn, Cu and Zn contents significantly increased by 14.4%, 14.5% and 118.3%, respectively (P < 0.05). 3) Under the sufficient water supply condition, soil available Zn content had a significantly negative correlation with millet belowground biomass, while it had a significantly positive correlation with soil organic matter content; under the light water stress condition, soil available Mn content showed a significant decreasing trend with the increase of millet aboveground biomass.

Conclusions Elevated CO₂ promotes the accumulation of soil available microelement contents, but this promotion is weakened when the temperature increases simultaneously. Compared with soil available Fe and Mn contents, soil available Cu and Zn contents are more sensitive to elevated CO₂ and temperature. These results may enhance the understandings on the impacts of climate change on soil microelement availability.