finding 12.4 : key-message-12-4

Large uncertainties remain regarding soil carbon budgets, particularly the impact of lateral movement and transport of carbon (via erosion and management) across the landscape and into waterways. By 2015, cumulative regeneration of soil carbon at eroded agricultural sites and the preservation of buried, eroded soil carbon may have represented an offset of 37 ± 10% of carbon returned to the atmosphere by human-caused land-use change (medium confidence).

This finding is from chapter 12 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: Best estimates of the effects of erosion are summarized in Billings et al. (2010), Van Oost et al. (2007), and Wang et al. (2017). Erosion can significantly affect productivity in agricultural regions, and some authors have argued that loss of eroded carbon represents a true loss to the atmosphere (Lal and Pimentel 2008). However, work based on multiple eroding profiles indicates that approximately 26% of eroded SOC can be replaced at the eroding site, representing a small but significant carbon sink (Van Oost et al., 2007). Harden et al. (1999) suggest that U.S. cropping patterns before 1950 likely resulted in about a 20% to 30% reduction of original SOC but that on-site recovery of soil organic matter (SOM) levels occurred after the 1950s. In Canada, VandenBygaart et al. (2012) also note a net carbon sink for eroded agricultural soils. Van Oost et al. (2007) suggest that replacement of eroded SOC, along with damped SOC mineralization upon burial, may combine to generate a small net carbon sink up to about 0.1 Pg C per year. Wang et al. (2017) calculate that cumulative, agriculturally accelerated erosion prompted SOC replacement and buried SOC preservation, representing an offset of 70 ± 16% of carbon emissions by anthropogenic land-cover change up to AD 1600; after this period, the cumulative value represented a smaller offset (37 ± 10% in 2015).

New information and remaining uncertainties: The fate of eroded agricultural soil can only be modeled, not directly measured, and the production of new soil carbon after exposure of new mineral surfaces also cannot be directly measured.

Assessment of confidence based on evidence: Erosion of soil is known to occur, but the fate of the eroded SOC is less clear. Currently, findings conclude that the eroded SOM appears to represent a small sink of carbon but that not all material is accounted for, and the geographic extent of full carbon budget studies is quite limited. Although subsurface soil carbon appears to be relatively stable, the responses to future changes in management and climate are not well understood.

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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