Overestimate of C<sub>4</sub> Plant Abundance Caused by Soil Degradation‐Induced Carbon Isotope Fractionation
Jiawei Da, Gen Li, Junfeng Ji
Abstract
Abstract The carbon isotopic composition (δ 13 C) 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‐δ 13 C SOM records. Here, we report new δ 13 C SOM records of the Holocene paleosols from the Chinese Loess Plateau and investigate how the decomposition of SOM affects the preserved δ 13 C signal and the inferred vegetation changes. Our results reveal significant C isotope fractionation, as SOM in bulk paleosol samples are systematically enriched in 13 C (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 13 C‐enriched microbial biomass by fine‐grained minerals. Previous studies based on paleosol‐δ 13 C SOM records that underestimate or neglect the 13 C‐fractionation related to SOM decomposition probably overestimated the abundance of regional C 4 biomass.