Stored carbohydrates decouple current photosynthate from soil CO2 efflux in frequently disturbed ecosystems

Background/Question/Methods

Soil CO₂ efflux, the combination of belowground autotrophic and heterotrophic respiration, is the largest terrestrial source of carbon dioxide (CO₂) to the atmosphere, yet the mechanistic controls of this important flux are currently under debate. Until recently, the dynamics and controls of the heterotrophic component of soil CO₂ efflux have received the most research attention as it was believed to dominate the flux; however, this view has been challenged as the overall contribution of the autotrophic component may be similar or even exceed that of the heterotrophic component. The tight coupling of root respiration with canopy photosynthesis, and its contribution towards soil CO₂ efflux, 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 foliar leaf area and thus, photosynthate assimilation, via foliar scorching over two consecutive growing seasons. We monitored the impact of scorching on photosynthate assimilation, soil CO₂ efflux, and fine root production, mortality, standing crop, nitrogen (N) concentration, and total nonstructural carbohydrate (TNC) concentration.

Results/Conclusions

Canopy photosynthesis was reduced to 20% of that in controls immediately after the scorch treatment was applied; however, recovery occurred surprisingly rapidly reaching 90% of that in control plots within one month. Soil CO₂ efflux was not immediately impacted by the scorch treatment and seasonal dynamics remained independent of the scorch treatment. The scorch treatment did not impact fine root production, mortality, or standing crop. Fine root N concentrations were not diminished by scorching, but TNC concentrations in 3^rd and 5^th order roots decreased as TNC was presumably allocated to maintain more metabolically active 1^st order roots. We estimated that belowground carbon storage represents at least one-half the annual belowground autotrophic respiratory demand in this ecosystem. The scorching treatment temporarily terminated the belowground supply of photosynthate, the process of leaf area reconstruction represented an additional drain on stored carbon pools and some of this carbon may have been mobilized from belowground. However, the lack of belowground response suggests that stored carbon of the dominant plant species in chronically disturbed ecosystems may decouple the direct linkage between current photosynthate assimilation and soil CO₂ efflux.