Belowground carbon allocation response to soil acidification stress in a meadow grassland

Soil acidification caused by atmospheric sulfur (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 for how plants allocate photosynthetic C among belowground functional sinks and whether they can leverage these limited C resources to adapt abiotic stresses. We conducted a ¹³CO₂ pulse-labelling experiment in a grassland field to investigate the effects of simulated soil acidification by S addition on photosynthetic C allocation and analyzed the trade-offs among plant belowground functional sinks. We also elucidated the contribution of belowground C allocation to SOC formation. We found that soil acidification decreased the absolute amount of excess ¹³C allocated to both shoots and soils, possibly due to less photosynthetic C assimilation and aboveground biomass production. In contrast, S addition partially increased the excess ¹³C allocated to roots, indicating that a greater proportion of C was allocated to root biomass construction to combat acidification stress. The excess ¹³C in roots related negatively to soil ¹³C but positively to both root biomass and non-structural carbohydrates (NSC), suggesting a possible trade-off relationship in belowground ¹³C allocation between rhizodeposition and root growth. Our research confirms that under soil acidification stress, less photosynthetic C in roots was converted into rhizodeposition C entering the soil, while more was invested in root growth, respiration, and storage to improve their survival and ability to resist environmental stress. Although with lower excess ¹³C allocated to both shoots and soils, soil acidification had no effect on SOC stocks, possibly due to less SOC decomposition accompanied with suppressed microbial activity. These results provide an invaluable insight into plant C allocation strategy and its impact on belowground C dynamics under soil acidification stress.