Stored carbon decouples the linkage between recent photosynthate and soil CO2 efflux in frequently disturbed ecosystems

Background/Question/Methods

The combined contributions of belowground autotrophic and heterotrophic respiration (soil CO₂ efflux) account for the largest annual terrestrial CO₂ flux from the biosphere to the atmosphere; however, the mechanistic controls of this flux remain unclear. The tight coupling of root respiration, the primary component of belowground autotrophic respiration, with canopy photosynthesis and recently produced photosynthate is an emerging paradigm in the ecophysiological literature. Here, we test the hypothesis that stored carbohydrates decouple the direct linkage between current photosynthate assimilation and soil CO₂ efflux in frequently disturbed ecosystems. Using longleaf pine (Pinus palustris L.) as a model for species with an evolutionary history of chronic disturbance, we experimentally manipulated the transfer of recently assimilated photosynthate belowground using stem girdling and root trenching. Girdling treatments were applied in both May and August to examine potential seasonal dynamics in belowground dependence on recent photosynthate. Trenching was applied in May and differed from stem girdling in that it not only terminated the supply of photosynthate belowground, but also severed connections between the root branching network and the tap root. We monitored the impact of belowground photosynthate exclusion treatments on soil CO₂ efflux, and root sugar and starch concentrations.

Results/Conclusions

Soil CO₂ efflux was not influenced by photosynthate exclusion treatments and seasonal dynamics were independent of treatment. Efflux rates ranged from 1.1 to 3.9 µmol m^-2 sec^-1 during the observation period. At the end of our observation period (243 days), the cumulative soil CO₂ efflux in the photosynthate exclusion treatments were statistically similar to that of the control treatment. Sugar and starch concentrations ranged from 2.5 to 25% and 8 to 35%, respectively, during our observation period and were impacted by photosynthate exclusion treatments, but treatment effects depended on root type and temporal period. For example, sugar and starch concentrations of root orders 1-3 were not impacted by photosynthate exclusion treatments, but concentrations of root orders 4-6 and lateral roots decreased after treatment application suggesting that sugar and starch were mobilized from larger roots to maintain smaller, more metabolically active, roots. Our observations following girdling of a tree species with an evolutionary history of chronic disturbance via fire are in stark contrast to those obtained in girdling studies of tree species with evolutionary histories of infrequent, but catastrophic disturbance and suggest life history traits may influence the time lag and coupling between canopy photosynthesis and soil CO₂ efflux.