Soil organic carbon (SOC) concentration is the fundamental indicator of soil health,
underpinning food production and climate change mitigation. SOC storage is highly
sensitive to several dynamic environmental drivers, with approximately one third of
soils degraded and losing carbon worldwide. Digital soil mapping illuminates where
hotspots of SOC storage occur and where losses to the atmosphere are most likely.
Yet, attempts to map SOC often produce widely differing results for the same region,
owing to differences in methodology and the representativeness of input data for
predictive mapping. Here we compare national scale SOC concentration map
products for Great Britain – a country where several digital SOC maps are available
and consists of soils spanning the full range of SOC concentrations. Our results
reveal generally strong agreement of data in mineral soils, with progressively poorer
agreement in organo-mineral and organic soils. Divergences in map predictions from
each other and survey data widen in the high SOC content land types we stratified.
Given the disparities are highest in carbon rich soils, efforts are required to reduce
these uncertainties to increase confidence in mapping SOC storage and predicting
where change may be important at national to global scales. This is particularly
important because the decline in SOC stocks from rising temperatures scales
proportionally with the size of the standing SOC stocks; thus, current uncertainties in
total SOC stocks presents a barrier to fully understanding the land carbon-climate
feedback. Our map comparison results could be used to identify SOC risk where
concentrations are high and should be conserved, and where uncertainty is high and
further monitoring should be targeted. Reducing inter-map uncertainty will rely on
addressing limitations with how representative observational data are for a region of
interest, as well as including covariates that capture the convergence of physical
factors that produce high SOC contents.