Radiocarbon partitioning of soil CO2 efflux into root and microbial respiration indicates black spruce allocate less C belowground with warming

Background/Question/Methods
Soil temperature strongly influences ecosystem function in boreal forests.  Cold soil temperatures inhibit decomposition rates, which in turn restrict plant nutrient availability and growth. One aspect of forest growth that appears sensitive to soil temperature is boreal forest C allocation.  Previous research along climate gradients found that for black spruce (Picea mariana); warmer soils corresponded to less C allocated belowground.  Similarly, allocation to fine root growth decreased with an experimental warming of both soil and air in a 17 year-old black spruce forest near Thompson, Manitoba.  In the warming experiment, the decrease in fine root growth likely contributed to the rapid temperature acclimation of soil CO2 efflux.  The objective of this study was to further examine how the warming experiment has affected black spruce C allocation and soil C turnover. In August 2007, we used “bomb” radiocarbon to partition soil CO₂ efflux into microbial and root respiration for the warming experiment.
Results/Conclusions

The D14C of soil CO2 efflux was significantly different (paired t-test, n=4, p=0.02) between the warmed (75.4±5.0 per mil) and control (63.0±7.0 per mil) plots.  The D14C of fine root respiration was 57.5±3.2 per mil averaged across treatment and control, and heterotrophic respiration 78.5±9.0 per mil.  Statistical partitioning of the soil CO2 efflux indicated that 59% of respiration in the control plots was from roots, while 39% of soil CO2 efflux was from roots in the warmed plots.  These results support previous studies that have indicated black spruce allocate less C to roots in warmer soils, and that this is the primary reason for the acclimation of soil CO2 efflux in the warming treatments. Microbial respiration was unlikely responsible for the acclimation of soil CO2 efflux, because in a laboratory incubation, the temperature sensitivity of microbial respiration was similar between the warming treatments and control plots (Q10=3.2±0.4 warmed vs. 3.3±0.2 control).  With soil warming, black spruce may allocate less C to roots because warming increases rates of soil C turnover and nutrient availability.