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

Abstract

Soils represent the largest terrestrial carbon pool, a majority of which come from atmospheric CO 2 that is fixed through photosynthesis by the terrestrial biosphere (Stockmann et al., 2013). The exchange of carbon between the atmosphere and the terrestrial biosphere makes soils an archive of past environmental changes (Pachauri et al., 2014). For example, the stable carbon isotopic composition (δ 13 C) of soil organic matter (SOM) has been used to approximate the δ 13 C of CO 2 produced by soil respiration-a key parameter in using paleosols to reconstruct paleoatmospheric CO 2 levels (Breecker, 2010;Cerling, 1992). Moreover, the δ 13 C SOM of ancient soils, or paleosols, have been widely used to narrate the evolution history of aboveground vegetation habitat as well as hominin environments (e.g., Cerling, 1991;Ehleringer et al., 1997).The decomposition of SOM represents a significant obstacle when using δ 13 C SOM records to reconstruct paleoenvironments (Bowen & Beerling, 2004). SOM is a mixture of organic compounds (e.g., lipids, lignins) of both plant and microbial origins, featuring distinct δ 13 C signals, physicochemical properties, and reaction kinetics (Ehleringer et al., 2000). Changes in the relative abundances of those compounds during SOM degradation could cause 13 C fractionation, altering the δ 13 C signal of fresh, undegraded soils. In well-drained, modern soil profiles, the decrease of total organic carbon content (TOC) along depth usually occurs concomitantly with a progressive increase of δ 13

Particle‐Size‐Specific Radiocarbon Constraints Imply an Active Subsoil Organic Carbon Pool

“…We observe consistent trends in [TOC], [TN], and δ 13 C versus depth for fSOM and cSOM, which are similar to the published records across the CLP (Da et al., 2021). The [TOC] and [TN] generally decrease with depth, albeit showing an increasing segment at the top of the Holocene S0 unit (Figures 1b and 1c).…”

“…This discrepancy is likely due to a higher abundance of microbial products within fSOM than bulk SOM, resulting in a higher degree of isotopic fractionation and a lower α. The decomposition-related 13 C-enrichment is also in line with our previous work, showing that SOM occluded in the calcite nodules (shielded from microbial decomposition) are depleted in 13 C compared to bulk SOM across the CLP (Da et al, 2021). Notably, the fSOM and cSOM of the S0 and L1 units show opposite trends from the Rayleigh distillation lines in the ln(R/R 0 )-ln(C/C 0 ) space (Figure 5), suggesting the first-order control of aboveground vegetation (i.e., C 3 vs. C 4 plants) in response to regional climate that led to higher [TOC] and δ 13 C SOM in the S0 unit (S. Yang et al, 2015).…”

“…To better understand the SOC dynamics and the associated carbon isotope fractionation at deeper depths, this work targets the top 2 m of an eolian-born loess tableland on the Chinese Loess Plateau (CLP). Our previous work in this region reveals substantial δ 13 C SOM increase after burial, implying possible influences from decomposition and vertical mixing (Da et al, 2021). To elucidate the processes responsible for carbon isotope fractionation of SOM, here we examined the vertical distributions of the radiocarbon ( 14 C) activities of SOM in two size fractions: the clay-sized fraction (<2 μm, hereafter as fSOM) and the remaining coarser material (the majority ranges between 2 and 50 μm, hereafter as cSOM).…”

“…Notably, modern, well‐drained soils show consistent down‐profile 13 C‐enrichment in SOM (e.g., Wynn et al., 2006), which has been attributed to various mechanisms, including decomposition‐related 13 C/ 12 C fractionation (Wu et al., 2019; Wynn, 2007), the incorporation of 13 C‐enriched soil CO 2 into microbial biomass and consequently SOM (also known as “dark CO 2 fixation”) (Ehleringer et al., 2000; Nel & Cramer, 2019), and the selective sorption and associated fractionation of dissolved organic matter (Kaiser et al., 2001). However, such fractionation remains loosely constrained (Philben et al., 2022) and has rarely been accounted for when using δ 13 C SOM for paleoenvironmental reconstructions, introducing potential biases (Da et al., 2021). In addition to the various processes associated with biogenic carbon, the organic carbon from the parent material (petrogenic carbon) likely remained in the soil under arid climate (Liu et al., 2007), bringing additional noise to the SOM‐based proxies.…”

Significant carbon isotopic fractionation during early formation of biological soil crusts with indications for dryland carbon cycling

“… 10 , 11 However, a recent study proposes strong fractionation effects during the decomposition of soil organic matter. 12 Therefore, the fractionation of stable carbon isotopes and their relative composition (δ 13 C) in soil organic matter may help to trace the processes of carbon cycling and underscore the role of microbes in carbon accumulation especially during early stages of soil formation. 13 For example, previous studies have effectively employed 13 C labeling to investigate the allocation of photosynthetic carbon within plant-soil system.…”