Benchmarking soil organic carbon to support agricultural carbon management: A German case study

Abstract

Background Soil organic carbon (SOC) storage is highly variable across sites and primarily depends on site properties and land use. It is therefore difficult for farmers to evaluate the actual SOC status of a site. To aid the interpretation of measured SOC contents, easy-to-use frameworks for the assessment of SOC contents are needed. Aims The aim of this study was to derive site-specific SOC benchmarks for German mineral soils under agricultural use based on the dataset of the first German Agricultural Soil Inventory. Methods The dataset was stratified into 33 strata by land use, soil texture, C/N ratio and mean annual precipitation. Lower and upper SOC benchmarks were calculated for all strata (0.125 and the 0.875 quantile). Results The SOC benchmark value ranges were lower for cropland (6.8-48.9 g kg(-1)) than for grassland (14.1-76.6 g kg(-1)), and increased with rising clay content and precipitation. Sandy soils with a wide C/N ratio and high SOC content due to their heathland or peatland history were divided into separate strata. The number of strata only decreased the SOC benchmark ranges slightly. Around 15-20 sites were required as a minimum to quantify SOC benchmarks for one stratum. Conclusions The presented framework is easy to use, requiring only four readily available stratification factors to perform a comparative classification of SOC contents. It allows farmers and extension services to compare where their measured SOC contents fall within the expected SOC value range for their site, and can thus help develop an initial evaluation of the SOC status of a site with regard to soil-specific differences.