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

Abstract

AbstractThe 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‐δ13CSOM records. Here, we report new δ13CSOM 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‐δ13CSOM records that underestimate or neglect the 13C‐fractionation related to SOM decomposition probably overestimated the abundance of regional C4 biomass.