Younger carbon dominates global soil carbon efflux

Abstract

Soil carbon (C) is comprised of a continuum of organic compounds with distinct ages (i.e., the time a C atom has experienced in soil since the C atom entered soil). The contribution of different age groups to soil C efflux is critical for understanding soil C stability and persistence, but is poorly understood due to the complexity of soil C pool age structure and potential distinct turnover behaviors of age groups. Here, we build upon the quantification of soil C transit times to infer the age of C atoms in soil C efflux (aefflux) from seven sequential soil layer depths down to 2 m at a global scale, and compare this age with radiocarbon‐inferred ages of C retained in corresponding soil layers (asoil). In the whole 0–2 m soil profile, the mean aefflux is 194211021 (mean with 5%–95% quantiles) year and is just about one‐eighth of asoil (14767172547 year), demonstrating that younger C dominates soil C efflux. With increasing soil depth, both aefflux and asoil are increased, but their disparities are markedly narrowed. That is, the proportional contribution of relatively younger soil C to efflux is decreased in deeper layers, demonstrating that C inputs (new and young) stay longer in deeper layers. Across the globe, we find large spatial variability of the contribution of soil C age groups to C efflux. Especially, in deep soil layers of cold regions (e.g., boreal forests and tundra), aefflux may be older than asoil, suggesting that older C dominates C efflux only under a limited range of conditions. These results imply that most C inputs may not contribute to long‐term soil C storage, particularly in upper layers that hold the majority of new C inputs.

Emergent Climatic Controls on Soil Carbon Turnover and Its Variability in Warm Climates

“…The weak temperature sensitivity of τ TO in relatively warm regions was evident from previous studies (Davidson & Janssens, 2006;Koven et al, 2017;Varney et al, 2020). Here it was perhaps surprising to find the stronger sensitivity of τ* to the AI, although this was recently hinted at by Xiao et al (2022), whoanalyzing also τ*-found important differences across biomes (which usually have different levels of ecosystem aridity/wetness).…”

“…The theory has also been leveraged in the context of the C cycle, especially as an important approach to constrain ecosystem models (Manzoni & Porporato, 2009; Metzler et al., 2018; Sierra et al., 2017) and to understand mechanisms of C mineralization (Xiao et al., 2022). In fact, the relative magnitude of τ TO and τ age ( τ * = τ TO / τ age ) indicates whether and to what extent soil microbial communities prefer to assimilate younger or older C in the soil (Sierra et al., 2017).…”

A field incubation approach to evaluate the depth dependence of soil biogeochemical responses to climate change

“…SOM in deeper layers have slower turnover rates (or older ages) than that in upper layers (Shi et al, 2020; Xiao et al, 2022). The mechanism underpinning such phenomenon has been widely debated.…”

“…than that in upper layers (Shi et al, 2020;Xiao et al, 2022). The mechanism underpinning such phenomenon has been widely debated.…”

Increased atmospheric CO2 and the transit time of carbon in terrestrial ecosystems

“…Keenan et al, 2021; Luo et al, 2003; Norby et al, 2005; Ryan, 2013). On the contrary, some studies contend that this gain in biomass does not occur at the ecosystem scale, particularly where nutrients are limited (Ellsworth et al, 2017; Fleischer et al, 2019; Fleischer & Terrer, 2022; Jiang, Caldararu, et al, 2020; Jiang, Medlyn, et al, 2020; Körner et al, 2007; Maschler et al, 2022; Xiao et al, 2022).…”