Sequestration of C in soils under Miscanthus can be marginal and is affected by genotype-specific root distribution.

Abstract Miscanthus is a low input energy crop suitable for low fertility marginal arable land and thought to provide carbon sequestration in soil. We analysed a long-term field experiment (14-year) to determine whether differences in genotype, growth habit, and root distribution affected soil carbon spatially under different Miscanthus genotypes. Soil cores were taken centrally and radially to a depth of 1 m, and divided into six vertical segments. Total root length (TRL), root dry matter (RDM) and δ 13 C signature of soil organic carbon (SOC) were measured directly, and root length density (RLD), fractions of Miscanthus -derived soil organic C (SOC M ), and residual soil carbon (SOC orig ) were calculated. Genotype was found to exhibit a statistically significant influence on spatial allocation of SOC. Grouping varieties into ‘tuft-forming’ (T) and ‘non-tuft-forming’ (NT) phenotypes revealed that respective groups accumulated similar amounts of RDM over 14 years (11.4 ± 3.3 vs. 11.9 ± 4.8 Mg ha −1 , respectively). However, phenotype T allocated more carbon to roots in the subsoil than NT (33% vs. 25%). Miscanthus genotypes sequestered between 4.2 and 7.1 g C 4 -SOC kg −1 soil over the same period, which was more than the average loss of C 3 -derived SOC (3.25 g kg −1 ). Carbon stocks in the ‘A horizon’ under Miscanthus increased by about 5 Mg ha −1 above the baseline, while the net increase in the subsoil was marginal. Amounts of Miscanthus root C in the subsoil were small (1.2–1.8 Mg C ha −1 ) but could be important for sustainable sequestration as root density (RLD) explained a high percentage of SOC M ( R 2  = 0.66).