Stored carbon decouples soil CO2 efflux in chronically disturbed ecosystems

Abstract

The tight coupling of belowground autotrophic respiration with the availability of recently assimilated carbon, 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 canopy photosynthesis and soil CO2 efflux. Using longleaf pine (Pinus palustris L.), a species with an evolutionary history of frequent disturbance and large reserves of belowground stored carbon, we experimentally manipulated the transfer of recently assimilated carbon belowground. The first experiment temporarily disrupted the transfer of recently assimilated carbon belowground via canopy scorching, whereas the second experiment permanently terminated the transfer of recently assimilated carbon belowground via stem girdling and root trenching. Canopy photosynthesis was reduced to 20% of that in the control treatment immediately after the scorch treatment was applied; however, recovery occurred surprisingly rapidly, reaching 90% of that in the control treatment within one month. Despite the immediate reduction in canopy photosynthesis, soil CO2 efflux was not impacted by the scorch treatment. Similarly, the scorch treatment did not impact fine root production, mortality, or standing crop. Fine root N concentrations were not diminished by scorching, but we observed some evidence that TNC concentrations in 3rd and 5th order roots decreased as TNC was presumably reallocated to maintain more metabolically active 1st order roots. Soil CO2 efflux was not influenced by girdling or trenching treatments. Sugar and starch concentrations were impacted by girdling and trenching treatments, but 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 results indicate that large belowground carbon reserves decouple the linkage between canopy photosynthesis and soil CO2 efflux.