Clarifying the role of microbial communities in carbon loss from rhizosphere priming of contrasting crop species under elevated atmospheric CO2

<p dir="ltr">Background: The increasing risk of soil organic carbon (SOC) loss in farming soils under elevated atmospheric CO2 (eCO2) environments calls for mechanistic studies on how the increased plant-C input in response to eCO2 alters microbial phylogenetic diversity and metabolic activity, and how that relates to SOC decomposition via eCO2-induced priming. </p><p dir="ltr">Methods: A natural ¹³C-enriched soil was used to quantify plant- and soil-originated C from belowground respiration. Canola, white lupin and wheat were grown under ambient CO2 (400 ppm) and eCO2 (800 ppm) until the flowering stage. </p><p dir="ltr">Results: Elevated CO2 increased rhizosphere priming by 2.0- and 2.3-fold (p < 0.05) in lupin-grown soil at 48 and 63 days, respectively, but had no significant effect in canola- or wheat-grown soils. The greater ratio of dissolved organic C to mineral N (101 compared to 22 and 36 for canola and wheat, respectively) might contribute to the enhanced priming effect in the lupin rhizosphere. Phylogenetic differential abundance analysis indicated that Streptomyces, Geodermatophilus and Mycobacterium, affiliated to Actinobacteriota, in the lupin rhizosphere were enriched under eCO2, and hence may play an important role in SOC decomposition. In the fungal community, Udeniozyma, a saprotrophic yeast genus, in the lupin rhizosphere was more abundant under eCO2 than ambient CO2, and accounted for 2.3% of the fungal community, further contributing to the priming effect. </p><p dir="ltr">Conclusion: The increase of dissolved organic C relative to mineral N may drive Actinobacteriota genera to facilitate SOC mineralization in the rhizosphere of lupin grown in the eCO2 environment.</p>