Increasing tree productivity does not translate into greater soil organic carbon storage

Increasing soil organic carbon (SOC) storage is one of the promising solutions to mitigate climate change. Fast-growing trees are a potential tool in this context as they rapidly accumulate C in their biomass and could transfer more organic matter (OM) into the soil. However, the relationship between aboveground productivity and SOC storage remains poorly understood. Five clones with different growth rates were selected from a 14-year-old hybrid poplar plantation located in New Liskeard, ON, Canada. We collected soil cores at 87.5 and 175.0 cm distance from the stem and at 0-20, 20-40 and 40-60 cm soil depth for soil C concentration analysis. The most productive clone DN2 (Populus deltoides x P. nigra) stored less SOC (83 Mg ha -1) between 0 and 60 cm depth than the mid-productive clones 1079 (Populus x jackii (P. balsamifera x P. deltoides)) and 915005 (P. maximowiczii x P. balsamifera) (95 and 96 Mg ha- 1 respectively), while the least productive clone 747210 (P. balsamifera x P. trichocarpa) also had a lower SOC stock (85 Mg ha -1) compared to the other clones, but not significantly. There was no relationship between aboveground productivity and SOC stocks and total SOC stocks increased by 6 % when the sampling distance was closer to the tree stems. The difference in SOC stocks between clones was mostly observed at the 20-40 cm depth suggesting the significant effect of roots on SOC storage. Soil C/N ratios were significantly different between clones at 0-20 and 20-40 cm depths suggesting differences in OM decomposition rates between clones. There could be a trade-off between aboveground productivity and litter decomposition rate to increase SOC storage.