Stable isotope ratios of soil organic carbon (SOC) are a potentially powerful, integrative tool for analyzing the soil C cycle. However, limited understanding of the mechanisms for C isotope fractionation in soil prevents their widespread application. Soil organic carbon (SOC) is progressively enriched in ¹³C with age and depth in the soil profile even though CO₂ produced during soil respiration is typically enriched in ¹³C compared to SOC. This results in an apparent mass balance paradox. To resolve this paradox, we hypothesized that the loss of ¹³C-depleted DOC during soil diagenesis, the combination of microbial decomposition and abiotic processes such as leaching, can account for the increase in SOC δ¹³C with depth. We combined three independent approaches (field measurements, a laboratory incubation experiment, and seasonal sampling of DOC across a climate transect) to systematically evaluate the relationship between the δ¹³C of soil respiration, DOC, and the SOC it is derived from. However, DOC was not significantly depleted in ¹³C compared to SOC in any of the three approaches, and mass balance calculations indicated that the DOC flux cannot account for the full extent of ¹³C enrichment of SOC with depth in the soil profile. We suggest that vertical C transport by plant roots or fungi, rather than diagenesis, may be largely responsible for the observed C isotope profile. Future studies aimed at understanding these vertical transport processes should enable increased application of soil δ¹³C, enhancing soil biogeochemical studies.

土壤有机碳 (SOC) 的稳定同位素比率是分析土壤碳循环的潜在强大的综合工具。然而，对土壤中 C 同位素分馏机制的有限了解阻碍了其广泛应用。尽管土壤呼吸过程中产生的 CO ₂与 SOC 相比通常富含¹³ C，但随着土壤剖面中的年龄和深度，土壤有机碳 (SOC) 逐渐富含^{13 C。}这导致了明显的质量平衡悖论。为了解决这个悖论，我们假设土壤成岩过程中¹³ C 耗尽的 DOC 的损失、微生物分解和非生物过程（如浸出）的结合可以解释 SOC δ ^13的增加C 有深度。我们结合了三种独立的方法（现场测量、实验室孵化实验和跨气候样带的 DOC 季节性采样）来系统地评估土壤呼吸的 δ ¹³ C、DOC 和它所衍生的 SOC 之间的关系。然而，与三种方法中的任何一种方法中的 SOC 相比，DOC 在¹³ C 中没有显着耗尽，并且质量平衡计算表明 DOC 通量不能解释^{13的全部范围}土壤剖面中 SOC 随深度的 C 富集。我们认为植物根系或真菌的垂直 C 运输，而不是成岩作用，可能是观察到的 C 同位素分布的主要原因。旨在了解这些垂直迁移过程的未来研究应该能够增加土壤 δ ¹³ C 的应用，从而加强土壤生物地球化学研究。