Realistic increases in soil organic carbon % through land management: 0.5% over 20 years and worth every point


This figure is taken from the PNAS study (Sanderman et al, 2017).  It is yet another hockey stick graph that shows the exponential impact of human activity over the last 300 years.  According to this study, since the period before Land USe Change (NoLU), the world's soils have lost 133 Petagrams (133 billion tons) of soil organic carbon (SOC) from the top 2 m and 37 Petagrams from the top 30 cm.  The total SOC stocks in the top 2m and 30cm before land-use change were 3144 and 899 Petagrams respectively so that losses are of the order of 4% of total carbon for both strata.  It seems reasonable that these losses place upper limits on the sequestration potential of soils.

An average silt loam has a density of 1.33 g/cm3, 1.33 tons/m3.  One ha of soil has 3000 m3 in the top 30 cm, so 3990 tons of soil.  A soil with 5% SOC would have had 199.5 tons of SOC/ha.  After land use change it would have had 199.5 tons - (199.5 X 37/899) = 191.3 tons of SOC = 4.7%.

At COP21 in 2015 a global project called 4 in 1000 was launched with a commitment to increase SOC stocks by 0.4% every year, or 4 parts in 1000 (4 per mille) - see Soussana et al  2019 and Minasny et al  2017.  For this same soil now at 4.7% SOC, annual increases of 0.4% would see 0.765 tons added per ha per year.  All papers below agree that soil sequestration will cease after 20 years due to equilibration.  Over 20 years 15 tons of SOC would be added to the 191.3 tons resulting in 206 tons of SOC, more than it started with!  Sanderman et al  point out that it is unlikely that more than 2/3 of lost SOC can be recovered which makes the 4 per mille study seem even more ambitious.  The PNAS study is based on modelled global averages and regional reviews of actual measured studies by Minasny et al give them confidence that the 4 per mille goal is achievable.  An aspiration goal that feels achievable to land managers and other stakeholders is perhaps more important than absolute numbers as there is no question that any steps take to sequester carbon are worth encouraging.  All that said it is worth noting that 206 tons of SOC are only 5.2 %, an increase of 0.5% from 4.7% over 20 years.  Although that sounds modest compared to claimed increases of many percentage points by proponents of mob grazing or no-till cultivation, it is enough to make a huge contribution to climate change mitigation.

References

Sanderman, Jonathan, Tomislav Hengl, and Gregory J. Fiske. ‘Soil Carbon Debt of 12,000 Years of Human Land Use’. Proceedings of the National Academy of Sciences 114, no. 36 (5 September 2017): 9575–80. https://doi.org/10.1073/pnas.1706103114.

Soussana, Jean-François, Suzanne Lutfalla, Fiona Ehrhardt, Todd Rosenstock, Christine Lamanna, PetrHavlík, Meryl Richards, et al. ‘Matching Policy and Science: Rationale for the “4 per 1000 - Soils for Food Security and Climate” Initiative’. Soil and Tillage Research, Soil Carbon and Climate Change: the 4 per Mille Initiative, 188 (1 May 2019): 3–15. https://doi.org/10.1016/j.still.2017.12.002.

Minasny, Budiman, Brendan P. Malone, Alex B. McBratney, Denis A. Angers, Dominique Arrouays, Adam Chambers, Vincent Chaplot, et al. ‘Soil Carbon 4 per Mille’. Geoderma 292 (15 April 2017): 59–86. https://doi.org/10.1016/j.geoderma.2017.01.002.




 


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