Effects of elevated CO₂ on fine root biomass are reduced by aridity but enhanced by soil nitrogen: A global...

Plant roots play a crucial role in regulating key ecosystem processes such as carbon (C) sequestration and nutrient solubilisation. Elevated (e)CO₂ is expected to alter the biomass of fine, coarse and total roots to meet increased demand for other resources such as water and nitrogen (N), however, the magnitude and direction of observed changes vary considerably between ecosystems. Here, we assessed how climate and soil properties mediate root responses to eCO₂ by comparing 24 field-based CO₂ experiments across the globe including a wide range of ecosystem types. We calculated response ratios (i.e. effect size) and used structural equation modelling (SEM) to achieve a system-level understanding of how aridity, mean annual temperature and total soil nitrogen simultaneously drive the response of total, coarse and fine root biomass to eCO₂. Models indicated that increasing aridity limits the positive response of fine and total root biomass to eCO₂, and that fine (but not coarse or total) root responses to eCO₂ are positively related to soil total N. Our results provide evidence that consideration of factors such as aridity and soil N status is crucial for predicting plant and ecosystem-scale responses to future changes in atmospheric CO₂ concentrations, and thus feedbacks to climate change.