Coarser root residues are a major subsoil carbon sink after re-conversion of poplar short rotation coppice plantation to cropland

On agricultural soils, short rotation coppices (SRC) can be an efficient alternative for long-term carbon fixation in roots compared to conventional land-uses. Previous studies on root biomass focused mainly on fine roots or topsoil layers, but a comprehensive assessment requires accounting for total root biomass across both topsoil and subsoil. Three plots of a poplar SRC with rotation length of 2, 4 and 16 years (SRC2, SRC4 and SRC16) were studied to assess total root biomass (finer <7 cm and coarser >7 cm), its distribution down to 1 m depth, and the root decomposition rates 12 and 24 months after re-conversion. The objective of this study was to quantify the relevance and persistence of carbon stocks from SRC root biomass after re-conversion. Total root biomass was 66.53 Mg ha −1 (SRC16), 37.90 Mg ha −1 (SRC4) and 30.24 Mg ha −1 (SRC2), which equals 23.13 Mg C ha −1 (SRC16), 15.49 Mg C ha −1 (SRC4) and 12.39 Mg C ha −1 (SRC2). Most biomass was stored in roots <7 cm, but coarser root residues were still relevant with 12.90 Mg ha −1 (SRC16), 2.11 Mg ha −1 (SRC4), and 3.31 Mg ha −1 (SRC2), especially for the longest rotation. Fifty percent of the subsoil-C stayed stable within 24 months. The observed decrease in decay rate, shows the mid-to long-term effect of carbon storage even after re-cultivation. Coarser roots were more resilient to mineralization than finer roots due to a smaller relative reactive surface and a higher C:N ratio. In conclusion, not only aboveground SRC yield contributes the SRC carbon sink effect, but also belowground biomass, which can be seen by comparison with root studies from forests and grasslands.