Overestimate of C 4 Plant Abundance Caused by Soil Degradation‐Induced Carbon Isotope Fractionation

• Published in: Geophysical research letters 2021-12, Vol.48 (24)
• Main Authors: Da, Jiawei, Li, Gen K., Ji, Junfeng
• Format: Article
• Language: English
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093407

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. Soil organic matter inherits the carbon isotope signal ( 13 C/ 12 C) of the aboveground biomass and is widely used to infer past changes in vegetation biomes. However, the isotope fractionation during soil organic matter decomposition remains loosely constrained, biasing its interpretation. Based on the Holocene ancient soil (paleosol) profiles from the Chinese Loess Plateau, we show a systematic 13 C‐enrichment of organic matter in bulk paleosols, compared with organic matter occluded in soil carbonates that resist decomposition. The difference between the two sets of data indicates significant C isotope fractionation during the degradation of organic matter after soil formation and aggradation. We propose that fine‐grained soil minerals are likely responsible for this fractionation, which are inclined to absorb and protect 13 C‐enriched organic compounds produced by soil microbes. Previous works that neglect or underestimate this C isotope fractionation possibly overestimated the relative abundances of C 4 biomass on the Chinese Loess Plateau over the past few million years. Overall, our work highlights the importance of constraining the degradation‐induced fractionation, which is crucial to interpreting carbon isotopic records in the paleosol archive. We report δ 13 C of soil organic matter in both bulk paleosols and calcite nodules from the Holocene profiles on the Chinese Loess Plateau Org