The soil organic carbon: Clay ratio in North Devon, UK: Implications for marketing soil carbon as an asset class

Building up stocks of agricultural soil organic carbon (SOC) can improve soil conditions as well as contribute to climate change mitigation. As a metric, the ratio of SOC to clay offers a better predictor of soil condition than SOC alone, potentially providing a benchmark for ecosystem service payments. We determined SOC:clay ratios for 50 fields in the North Devon UNESCO World Biosphere Reserve using 30 cm soil cores (divided into 0-10 cm and 10-30 cm depth samples), with soil bulk density, soil moisture and land-use history recorded for each field. All the arable soils exceeded the minimum desirable SOC:clay ratio threshold, and the ley grassland soils generally exceeded it but were inconsistent at 10-30 cm. Land use was the primary factor driving SOC:clay ratios at 0-10 cm, with permanent pasture fields having the highest ratios followed by ley grass and then arable fields. Approximately half of the fields sampled had potential for building up SOC stock at 10-30 cm. However, at this depth, the effect of land use is significantly reduced. Within-field variability in SOC and clay was low (coefficient of variation was similar to 10%) at both 0-10 cm and 10-30 cm, suggesting that SOC:clay ratios precisely characterized the fields. Due to the high SOC:clay ratios found, we conclude that there is limited opportunity to market additional carbon sequestration as an asset class in the North Devon Biosphere or similar areas. Instead, preserving existing SOC stocks would be a more suitable ecosystem service payment basis.