Increasing soil organic carbon (SOC) confers benefits to soil health, biodiversity, underpins carbon sequestration and ameliorates land degradation. One recommendation is to increase SOC such that the SOC to clay ratio (SOC/clay) exceeds 1/13, yet normalising SOC levels based on clay alone gives misleading indications of soil structure and the potential to store additional carbon. Building on work by Poeplau & Don (2023) to benchmark observed against predicted SOC, we advance an alternative indicator: the ratio between observed and “typical” SOC (O/T SOC) for pan-European application. Here, “typical” SOC is the average concentration in different pedo-climate zones, PCZs (which, unlike existing SOC indicators, incorporate land cover and climate, alongside soil texture) across Europe, determined from mineral (<20 % organic matter) topsoils (0–20 cm) sampled during 2009–2018 in LUCAS, Europe's largest soil monitoring scheme (n = 19,855). Regression tree modelling derived 12 PCZs, with typical SOC values ranging 5.99–39.65 g/kg. New index classes for comparison with SOC/clay grades were established from the quartiles of each PCZ's O/T SOC distribution; these were termed: “Low” (below the 25th percentile), “Intermediate” (between the 25th and 50th percentiles), “High” (between the 50th and 75th percentiles), and “Very high” (above the 75th percentile). Compared with SOC/clay, O/T SOC was less sensitive to clay content, land cover, and climate, less geographically skewed, and better reflected differences in soil porosity and SOC stock, supporting 2 EU Soil Health Mission objectives (consolidating SOC stocks; improving soil structure for crops and biota). These patterns held for 2 independent datasets, and O/T SOC grades were sensitive enough to reflect land management differences across several long-term field experiments. O/T SOC used in conjunction with several other physical, chemical and biological soil health indicators can help support the EU Soil Monitoring Law and achieve several United Nations Sustainable Development Goals.

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.