Changes in belowground biomass after coppice in two Populus genotypes

Despite the potential of high-density, short-rotation woody biomass plantations to sequester carbon (C) in the soil, few studies have examined their belowground components. We were particularly interested in the biomass allocation patterns after the change from a single-stem (pre-coppice) to a multi-shoot (post-coppice) system, as well as in the fine root (Fr; ∅ < 2 mm) mortality after coppice for their implications in the belowground C cycle. The root system of selected trees from two poplar (Populus spp.) genotypes - Skado and Koster - were excavated for the determination of coarse (Cr, ∅ > 5 mm) and medium-sized (Mr, ∅2-5 mm) roots. After two 2-year rotations, the Cr biomass (dry mass, DM) was higher in the P. trichocarpa × P. maximowiczii genotype Skado (187.0 g DM m−2) than in the P. deltoides × P. nigra genotype Koster (155.4 g DM m−2). Both genotypes showed a relatively shallow, but extensive root system. Allometric equations were fitted between DM of Cr and Mr, and basal area. The root:shoot ratio decreased exponentially with basal area, showing the same trend for single-stem and multi-shoot trees. The soil coring technique was used to determine Fr mass at different dates and at different soil depths. The highest Fr biomass was detected in the upper 15 cm of the soil; no genotypic differences in Fr mass were detected at any soil depth. After coppice Fr mortality was significantly increased and weed root biomass significantly reduced. The coppice of the aboveground stems and shoots resulted in a high input of C into the soil and a large amount of C was stored in belowground tree biomass.