Measured and modelled effect of land-use change from temperate grassland to Miscanthus on soil carbon stocks after 12 years

Soil organic carbon (SOC) is an important carbon pool susceptible to land-use change (LUC). There are concerns that converting grasslands into the C-4 bioenergy crop Miscanthus (to meet demands for renewable energy) could negatively impact SOC, resulting in reductions of greenhouse gas mitigation benefits gained from using Miscanthus as a fuel. This work addresses these concerns by sampling soils (0-30 cm) from a site 12 years (T-12) after conversion from marginal agricultural grassland into Miscanthus x giganteus and four other novel Miscanthus hybrids. Soil samples were analysed for changes in below-ground biomass, SOC and Miscanthus contribution to SOC (using a C-13 natural abundance approach). Findings are compared to ECOSSE soil carbon model results (run for a LUC from grassland to Miscanthus scenario and continued grassland counterfactual), and wider implications are considered in the context of life cycle assessments based on the heating value of the dry matter (DM) feedstock. The mean T-12 SOC stock at the site was 8 (+/- 1 standard error) Mg C/ha lower than baseline time zero stocks (T-0), with assessment of the five individual hybrids showing that while all had lower SOC stock than at T-0 the difference was only significant for a single hybrid. Over the longer term, new Miscanthus C-4 carbon replaces pre-existing C-3 carbon, though not at a high enough rate to completely offset losses by the end of year 12. At the end of simulated crop lifetime (15 years), the difference in SOC stocks between the two scenarios was 4 Mg C/ha (5 g CO2-eq/MJ). Including modelled LUC-induced SOC loss, along with carbon costs relating to soil nitrous oxide emissions, doubled the greenhouse gas intensity of Miscanthus to give a total global warming potential of 10 g CO2-eq/MJ (180 kg CO2-eq/Mg DM).