Carbon sequestration in the subsoil and the time required to stabilize carbon for climate change mitigation

Abstract

Soils store large quantities of carbon in the subsoil (below 0.2 m depth) that is generally old and believed to be stabilized over centuries to millennia, which suggests that subsoil carbon sequestration (CS) can be used as a strategy for climate change mitigation. In this article, we review the main biophysical processes that contribute to carbon storage in subsoil and the main mathematical models used to represent these processes. Our guiding objective is to review whether a process understanding of soil carbon movement in the vertical profile can help us to assess carbon storage and persistence at timescales relevant for climate change mitigation. Bioturbation, liquid phase transport, belowground carbon inputs, mineral association, and microbial activity are the main processes contributing to the formation of soil carbon profiles, and these processes are represented in models using the diffusion–advection–reaction paradigm. Based on simulation examples and measurements from carbon and radiocarbon profiles across biomes, we found that advective and diffusive transport may only play a secondary role in the formation of soil carbon profiles. The difference between vertical root inputs and decomposition seems to play a primary role in determining the shape of carbon change with depth. Using the transit time of carbon to assess the timescales of carbon storage of new inputs, we show that only small quantities of new carbon inputs travel through the profile and can be stabilized for time horizons longer than 50 years, implying that activities that promote CS in the subsoil must take into consideration the very small quantities that can be stabilized in the long term.

Controls and relationships of soil organic carbon abundance and persistence vary across pedo‐climatic regions

“…This is an indication that the deeper soil layers at 1 m are less connected to surface SOC dynamics. Possible explanations include slower decomposition rates, higher SOC adsorption to minerals, slower vertical C transport and/or less C inputs with soil depth as suggested by our compartment models (Figure 6; Ahrens et al, 2020;Sierra et al, 2024). However, the importance of these different controls on SOC abundance and persistence varies widely for the three pedo-climatic groups.…”

“…Our results further highlight differences in the relationship between SOC abundance and persistence across pedo-climatic regions (Figure 3). The identified patterns can be related to differences in C inputs (above-and belowground), decomposition rates, as well as vertical and horizontal transfer of C that are characteristic for the different regions (Figure 6; Sierra et al, 2024). In the following section, we discuss the underlying mechanisms controlling SOC abundance and persistence across regions and their implications for assessing the response of SOC abundance and persistence within their pedo-climatic boundary conditions to changes in soil management and climate.…”

Storage and persistence of organic carbon in the upper three meters of soil under arable and native prairie land use

“…Such comparisons can be used to delineate soil environmental conditions under which the highest SOC gains or losses can be expected in response to land use or management changes (Follett et al, 2012;Amelung et al, 2020;Maharjan et al, 2020). Subsoils store most of the OC for centuries to millennia, making them ideal locations for long-term carbon storage, which has implications for climate change mitigation (Scheibe et al, 2023;Sierra et al, 2024). While subsoils generally have lower concentrations of SOC compared to topsoils, their larger volume results in greater SOC storage (Angst et al, 2018;Moreland et al, 2021).…”