Soil organic carbon assessment in perennial agriculture — a base study of Kernza in Alnarp, Sweden

Abstract Increased soil organic carbon (SOC) sequestration reduces atmospheric CO 2 and builds soil health. A conversion from annual crops to perennial crops may decrease negative environmental impacts and promote desired ecosystem services. Soil carbon credit systems and environmental monitoring require reliable verification of SOC change. Here we report a baseline survey of SOC for an agricultural study site in southern Sweden comprised of a perennial grain (Kernza), and a traditional rotation of annual crops as reference. We performed a systematic baseline soil sampling and calculated the minimal detectable difference to determine sample requirements for significant verification of SOC sequestration over time. The potential benefits of stratified sampling, based on a satellite-based vegetation index, flow accumulation and soil type, were investigated. Results indicate that 48% of the SOC was located in the upper 30 cm and > 70% in the upper 60 cm. SOC below 1 m soil depth was low. The minimal detectable difference, based on 10 soil samples, was 3.4 mg C g soil −1 , equivalent to 1530 g C m −2 in the upper 30 cm. Stratified sampling showed no difference among strata for vegetation index and flow accumulation. We conclude that the baseline survey of SOC will benefit forthcoming studies of carbon cycling within the study site. The pay back from applying stratified sampling seems low, as this is a rather homogenous study area. The minimal detectable difference approach requires an unreasonable number of soil samples to allow significant and valid verification of differences in SOC sequestration over time.