Trade‐off in carbon allocation between roots and rhizodeposition underpins plant adaptation to acidification in a meadow steppe

Soil acidification caused by atmospheric sulphur (S) deposition may have a significant impact on plant carbon (C) assimilation and allocation, thereby altering soil organic C (SOC) dynamics. However, it remains largely unknown how plants allocate photosynthetic C among below‐ground C sinks and whether they can leverage these limited C resources to adapt to abiotic stresses. We conducted a 13 CO 2 pulse labelling experiment in a meadow to investigate the effects of soil acidification, induced by S addition, on photosynthetic C allocation and to explore the trade‐offs among plant below‐ground sinks as well as the contribution of below‐ground C allocation to SOC formation. We found that soil acidification decreased rhizodeposit C (13%–60%) and the absolute amount of 13 C allocated to soil (0.8%–51%), due to a 30%–36% reduction in shoot photosynthetic 13 C assimilation. Conversely, S addition increased the proportion of 13 C allocated to roots by 3%–52%. The root biomass, non‐structural carbohydrates (NSC) and respiration increased with S addition rates, and a threshold point appeared around 20 g S m −2 year −1 , after which they began to decline. Root 13 C was negatively correlated with soil 13 C but positively associated with both root biomass and NSC. Soil acidification led to an increase in below‐ground biomass, while SOC stocks remained unchanged, possibly due to reduced rhizodeposit C input accompanied by suppressed SOC decomposition. Synthesis . These results confirm that under soil acidification, less photosynthetic C in roots was converted into rhizodeposit C entering the soil, while proportionally more was invested in root growth, respiration and storage to cope with acidification stress. Overall, the trade‐off in C allocation between root biomass and rhizodeposition may underlie plant adaptation to soil acidification and is a key indicator for predicting SOC dynamics.