Surface soil δ13C reveals C4 plants decompose and digest more easily than C3 plants

The positive or negative δ13C shifts between total organic carbon (TOC) in surface soils and the corresponding overlying plants/vegetation have been widely reported and are thought to result from various factors, such as atmospheric, climatic, and soil changes. Conducting studies on a larger spatial scale can better reveal the dominant factors. In this study, 107 surface soil and herbivore fecal samples were collected across several thousand kilometers of inland China. δ13C analyses were performed on surface soil TOC, bulk plant litter separated from surface soil (PL), bulk herbivore feces (HF), and α-cellulose extracted from both surface soil plant litter and herbivore feces. Data analysis showed that the δ13C shift between surface soil TOC and bulk plant litter (Δ13CTOC-PL) and the δ13C shift between bulk herbivore feces and bulk plant litter in surface soils (Δ13CHF-PL) both exhibited a negative correlation with δ13CPL. Similarly, the δ13C shift between herbivore feces α-cellulose and surface soil plant α-cellulose (Δ13CHF-PL-cell) also showed a negative correlation with δ13CPL-cell. The most reasonable explanation for these negative correlations is that during decomposition in surface soils and digestion within herbivores, C4 plants decompose/digest faster than C3 plants. The findings suggest that the relative abundance of C4 plants, estimated based on modern surface soils, sediments, and animal remains δ13C data, may have been commonly underestimated, and this should be appropriately considered and evaluated in future studies. © 2024 Science Press. All rights reserved.