Neglected carbon pools and fluxes in the soil balance of short-rotation woody biomass crops

The cultivation of dedicated bioenergy crops is being stimulated because of their potential to replace fossil fuels and to maintain or to sequester carbon (C) in the soil, and thus help to mitigate the rising atmospheric CO2 levels. There are, however, still a lot of inaccuracies with regard to the dynamics of C in the soil, and thus with the potential to sequester soil C in these bioenergy crops. Using experimental data observed at the intensively monitored short-rotation woody crops (SRWC) plantation of the POPFULL project, we demonstrate that frequently neglected C pools and fluxes can be of crucial importance for the soil C balance. We highlight three specific cases. First, C inputs into the soil due to weed roots may equal or exceed those due to poplar fine roots, especially during the establishment phase of the plantation. Secondly, harvesting influences the dynamics of above- and belowground C inputs, as well as the soil environment. Large amounts of C are stored in the belowground woody biomass, which represents a long-term C pool. Thirdly, spatial differences related to the planting design are an important source of error in the upscaling of soil variables. We call upon researchers to consider and measure these neglected C pools and fluxes.