Stocks of organic carbon in German agricultural soils - Key results of the first comprehensive inventory

Background: There is considerable uncertainty about the actual size of the global soil organiccarbon (SOC) pool and its spatial distribution due to insufficient and heterogeneous data cover-age. Aims: We aimed to assess the size of the German agricultural SOC stock and develop a stratification approach that could be used in national greenhouse gas reporting. Methods: Soils from a total of 3104 sites, comprising 2234 croplands, 820 permanent grasslands and 50 sites with permanent crops (vineyards, orchards) were sampled in a grid of 8 x 8 km to a depth of 100 cm in fixed depth increments. In addition, a decade of management data was recorded in a questionnaire completed by farmers. Two different approaches were used to stratify cropland and grassland mineral soils and derive homogeneous groups: stratification via soil type (pedogenesis) and via SOC-relevant soil properties. Results: A total of 146 soils were identified as organic soils, which stored by far the highest average SOC stock of528–201 Mg ha^–1 in 0–100 cm depth. Of the mineral soils, croplands and permanent crops stored on average 61–25 and 62–25 Mg ha^–1 in 0–30 cm (topsoil) and 35–30 and 44–28 Mg ha^–1 in 30–100 cm (subsoil), while permanent grasslands stored significantly more SOC (88–32 and 47–50 Mg ha^–1 in topsoil and subsoil). Overall, topsoils stored 67–14% and subsoils 33–14% of total SOC stocks. Soil C:N ratio, clay content and ground-water level were major factors that explained the spatial variability of SOC stocks in mineral soils. Accordingly, Podzols, Gleysols and Vertisols were found to have the highest SOC stocks. Conclusions: Stratification via soil properties yielded the most comparable cropland and grassland strata and is thus preferable for estimating land-use change effects, e.g., for greenhouse gas inventories. In total, 2.5 Pg C are stored in the upper 100 cm of German agricultural soils, making them the largest organic carbon pool in terrestrial ecosystems of Germany. This bares a large responsibility for the agricultural sector and society as a whole to maintain and, if possible, enhance this pool.