Soil carbon sequestration to mitigate climate change: a critical re‐examination to identify the true and the false

TLDR

Carbon sequestration is defined as any increase in soil organic carbon from land‑management changes, yet it only mitigates climate change if it represents an additional net transfer of atmospheric carbon; however, its finite, reversible nature, potential to increase other greenhouse gases, indirect land‑use effects, and the fact that adding residues may not add atmospheric carbon all limit its effectiveness, and over‑emphasis on it can divert attention from more impactful measures such as deforestation reduction and nitrogen‑use efficiency. Converting cropland to forest, grassland, or perennial crops and re‑vegetating degraded land genuinely removes atmospheric CO₂, whereas increases in soil organic carbon from reduced tillage are smaller than previously claimed and may be offset by higher nitrous oxide emissions, and the benefits of fertilizer‑driven SOC gains must be weighed against associated greenhouse gas emissions.

Abstract

The term ‘carbon sequestration’ is commonly used to describe any increase in soil organic carbon (SOC) content caused by a change in land management, with the implication that increased soil carbon (C) storage mitigates climate change. However, this is only true if the management practice causes an additional net transfer of C from the atmosphere to land. Limitations of C sequestration for climate change mitigation include the following constraints: (i) the quantity of C stored in soil is finite, (ii) the process is reversible and (iii) even if SOC is increased there may be changes in the fluxes of other greenhouse gases, especially nitrous oxide (N 2 O) and methane. Removing land from annual cropping and converting to forest, grassland or perennial crops will remove C from atmospheric CO 2 and genuinely contribute to climate change mitigation. However, indirect effects such as conversion of land elsewhere under native vegetation to agriculture could negate the benefit through increased CO 2 emission. Re‐vegetating degraded land, of limited value for food production, avoids this problem. Adding organic materials such as crop residues or animal manure to soil, whilst increasing SOC, generally does not constitute an additional transfer of C from the atmosphere to land, depending on the alternative fate of the residue. Increases in SOC from reduced tillage now appear to be much smaller than previously claimed, at least in temperate regions, and in some situations increased N 2 O emission may negate any increase in stored C. The climate change benefit of increased SOC from enhanced crop growth (for example from the use of fertilizers) must be balanced against greenhouse gas emissions associated with manufacture and use of fertilizer. An over‐emphasis on the benefits of soil C sequestration may detract from other measures that are at least as effective in combating climate change, including slowing deforestation and increasing efficiency of N use in order to decrease N 2 O emissions.