Belowground carbon allocation of Rocky Mountain Douglas-fir

Carbon allocation to fine roots and mycorrhizae constitute one of the largest carbon fluxes in forest ecosystems, but these fluxes are also among the most difficult to measure. We measured belowground carbon fluxes in two Pseudotsuga menziesii (Mirb.) Franco var. glauca stands. We used a carbon balance approach to estimate total belowground carbon allocation (TBCA) and carbon allocation to fine-root and mycorrhizal production (NPPfr). The stands differed in belowground biomass because of fertilization treatment 8 years prior. Annual soil flux was 856 and 849 g C·m-2·year-1 for the two stands. Annual root respiration equaled 269 and 333 g C·m-2·year-1 in the low- and high-biomass stand, respectively. TBCA equaled 733 and 710 g C·m-2·year-1 in the low- and high-biomass stand, respectively. Calculated NPPfr equaled 431 g C· m-2·year-1 in the low-biomass stand and 334 g C·m-2·year-1 in the high-biomass stand; equivalent to 59 and 47% of TBCA, respectively. Fine-root and mycorrhizal turnover equaled 1.8 and 0.8 year-1 in the low- and high-biomass stands, respectively. Belowground carbon allocation appeared to be distributed evenly between respiration and production despite differences in biomass and turnover. Sensitivity analysis indicated the NPPfr estimate is dependent foremost on the annual prediction of soil CO2 flux. The carbon balance approach provided a simple nonintrusive method for separating the belowground autotrophic and heterotrophic carbon budget.