Increased soil carbon (C) gains projected to occur with elevated CO₂ (eCO₂) might be negated by higher C losses via soil organic matter (SOM) decomposition. The impact of eCO₂ on soil C is usually measured as a change in total soil C which inevitably confounds C gains and losses occurring simultaneously. Furthermore, the impacts of eCO₂ on C gains and losses may differ for contrasting soil organic substrates due to their distinctive physicochemical and microbiological characteristics. Projecting the impacts of eCO₂ on soil C greatly depends on quantifying its individual effects on C gains and losses, which is rarely done in field settings. We used a novel combination of litter bag and isotopic approaches to study temporal C dynamics in two organic substrates: root litter and mineral soil C (SOM-C) to assess C dynamics in a mature Eucalyptus woodland under CO₂ enrichment (EucFACE). We aimed to isotopically separate different C pools for both substrates: a new C pool (C_new, plant-derived C from the study site) and a pre-existent, old C pool (C_old, present in the original substrates). We also performed comprehensive microbial community analyses to investigate relationships between microbial community structure and individual C pools. We found higher root litter mass losses and slightly greater SOM-C C_old losses with eCO₂ during summer months. Gains of C_new were not affected by eCO₂, except for SOM-C during summer, which is linked with the observed C_old losses from SOM-C. Moreover, fungal abundance in root litter increased with eCO₂ but decreased in SOM-C. Temporal changes rather than eCO₂-induced changes in microbial communities were related to C dynamics. Taken together our results suggest that eCO₂ did not lead to significant C_new gains but instead lead to moderate enhancement of C_new and C_old losses, suggesting limited capacity for increased C sequestration of this mature woodland.

通过土壤有机质（SOM）分解产生的更高的碳损失可能会抵消因CO ₂（eCO ₂）升高而导致的土壤碳（C）的增加。eCO ₂对土壤C的影响通常以土壤总C的变化来衡量，这不可避免地会混淆同时发生的C得失。此外，由于土壤有机底物具有独特的理化和微生物学特性，对于对比土壤有机底物，eCO ₂对碳得失的影响可能有所不同。预测eCO ₂的影响土壤碳的含量很大程度上取决于量化其对碳得失的影响，这在田间环境中很少见。我们使用垃圾袋和同位素方法的新颖组合研究了两种有机基质（根凋落物和矿质土壤C（SOM-C））中的时间C动态，以评估在CO ₂富集（EucFACE）下成熟的桉树林地中的C动态。我们旨在通过同位素分离两种底物的不同C池：一个新的C池（来自研究地点的C _new，来自植物的C）和一个已存在的旧C池（C _old，存在于原始底物中）。我们还进行了全面的微生物群落分析，以调查微生物群落结构与单个碳库之间的关系。我们发现在夏季月份，使用eCO _2会导致更高的根凋落物质量损失和略微更大的SOM-C C_旧损失。除了夏季的SOM-C以外，e new ₂的C _new增益不受eCO _2的影响，这与从SOM-C观察到的C _old损失有关。此外，根凋落物中的真菌丰度随着eCO _2的增加而增加，而在SOM-C中则降低。微生物群落的时间变化而不是eCO ₂诱导的变化与碳动力学有关。综合我们的结果表明，eCO₂并未导致大量的C_新增加，而是导致C_新损失和C_老损失的适度增加，表明该成熟林地增加C固存的能力有限。