Belowground biomass C outweighs soil organic C of perennial energy crops: insights from a long-term multispecies trial

The cultivation of Perennial Energy Crops (PECs) couples the production of ligno-cellulosic biomass to the provision of multiple ecosystem services, such as the reduction of greenhouse gas emissions and the mitigation of climate change through carbon (C) sequestration in soil. Though C sequestration in soil by PECs has been widely studied, the contribution of their belowground biomass (BGB) to soil C sequestration and their influence on soil nitrogen (N) storage potential has received very little attention. In this study, C and N stocks in soil and BGB fractions (plant belowground organs and fine roots) was measured for six PECs (Populus spp. “Poplar”, Robinia pseudoacacia “Black locust”, Sa-lix spp. “Willow”, Arundo donax “Giant reed”, Miscanthus x giganteus “Miscanthus”, and Panicum virgatum “Switchgrass”) grown on marginal soil, eleven years after establishment. All PECs had a higher soil organic carbon (SOC) stock and soil total nitrogen (STN) stock than arable land in the top (0-10 cm) soil layer. In this same top layer, woody crops had the highest SOC stock. The increase of SOC under PECs led to increased soil porosity in the top-soil layer. On average, 43% of the below-ground C stock of PECs was allocated in the plant belowground organs (PBO) (i.e in the rhizomes of herbaceous PECs and the stump for woody PECs). Giant reed had the highest C stock in PBO, while switchgrass the lowest (22.7 vs 5.9 Mg C ha-1). On the contrary, switchgrass had the highest C stock in fine roots. Giant reed had the highest belowground C stock (sum of soil and BGB contribution) and black locust the highest belowground N stock. After eleven years of PEC cultivation, 68% of the belowground C stock was allocated in the BGB, and 32% was as SOC.