Belowground carbon allocation of Rocky Mountain Douglas-fir

Carbon allocation to fine roots and mycorrhizae constitute one of the large st carbon fluxes in forest ecosystems, but these fluxes are also among the most difficult to measure. We measured belowground carbon fluxes in two Pse udotsuga menziesii (Mirb.) Franco var. glauca stands. We used a carbon bala nce approach to estimate total belowground carbon allocation (TBCA) and car bon 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 st ands. Annual root respiration equaled 269 and 333 g C .m(-2). year(-1) in t he 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. Calculat ed 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 ca rbon allocation appeared to be distributed evenly between respiration and p roduction despite differences in biomass and turnover. Sensitivity analysis indicated the NPPfr estimate is dependent foremost on the annual predictio n of soil CO2 flux. The carbon balance approach provided a simple nonintrus ive method for separating the belowground autotrophic and heterotrophic car bon budget.