Enrichment of 13C with depth in soil organic horizons is not explained by CO2 or DOC losses during decomposition

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 13Cdepleted DOC during soil diagenesis, the combination of microbial decomposition and abiotic processes such as leaching, can account for the increase in SOC 813C 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 813C 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 813C, enhancing soil biogeochemical studies.

Automated summary

Stable isotope ratios of soil organic carbon (SOC) are a powerful tool for understanding the soil carbon cycle, but limited understanding of the mechanisms for C isotope fractionation in soil restricts their application. This study examined the relationship between soil respiration, dissolved organic carbon (DOC), and SOC using three independent approaches. The results suggest that vertical C transport by plant roots or fungi may be the main driver of the observed C isotope profile.