Overestimate of C₄ Plant Abundance Caused by Soil Degradation-Induced Carbon Isotope Fractionation

Abstract

The carbon isotopic composition (δ¹³C) of soil organic matter (SOM) is a widely used proxy for terrestrial vegetation. However, SOM decomposition can cause C isotope fractionation, which remains poorly constrained in ancient soils thus clouds the interpretations of paleosol-δ¹³C_(SOM) records. Here, we report new δ¹³C_(SOM) records of the Holocene paleosols from the Chinese Loess Plateau and investigate how the decomposition of SOM affects the preserved δ¹³C signal and the inferred vegetation changes. Our results reveal significant C isotope fractionation, as SOM in bulk paleosol samples are systematically enriched in ¹³C (up to 3‰) compared with contemporary SOM occluded in calcite nodules, the latter of which are thought to resist degradation. Such fractionation is likely due to the selective preservation of ¹³C-enriched microbial biomass by fine-grained minerals. Previous studies based on paleosol-δ¹³C_(SOM) records that underestimate or neglect the ¹³C-fractionation related to SOM decomposition probably overestimated the abundance of regional C₄ biomass.

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