Plant ecophysiology and soil microorganisms control the time-lagged response of ecosystem respiration to environmental change

Background/Question/Methods

Studies of carbon exchange between atmosphere and biosphere often used stable isotopes as a natural tracer to investigate environmental controls on carbon turnover times in terrestrial ecosystems. This tracer-function results from changes in assimilated carbon isotopic signatures (δ¹³C) due to plant physiological responses to environmental changes through photosynthetic discrimination. Previous studies have shown responses to environmental change of δ¹³C in ecosystem respiration (δ¹³C_R) with a time lag of 1 to 10 days. The observed time lag was attributed to the transfer of newly assimilated carbon from leaves to respiration of heterotrophic soil microorganisms. However, our understanding of the mechanisms underlying δ¹³C_R time-lagged responses to environmental change is still rather limited. This study aimed to address plant ecophysiological and soil microbial controls over these time-lagged responses.

Two experiments were performed. In the first, wheat plants with different ecophysiological status, obtained by different nutrient and water availability conditions during growth, were submitted to a ¹³C pulse-labeling and δ¹³C_R response measured. In the second, beech saplings suffering from drought at different temperatures were exposed to a water pulse mimicking a rain event. To test the importance of carbon transfer from leaves to belowground components for δ¹³C_R response, we used girdled and ungirdled trees.

Results/Conclusions

In the first experiment, δ¹³C in leaf and soil-respired CO₂ response the ¹³C pulse-labeling showed that the release of the ¹³C-label by respiration, hence the carbon turnover time, was strongly related to plant ecophysiology, in particular to leaf conductance (g_s): stressed plants allocated less carbon to belowground respiration and at slower rate. In the second experiment, we measured a response of δ¹³C of belowground-respired CO₂ to the water pulse, even for girdled trees. This response was temperature dependent and appeared to be driven by changes of microbial biomass δ¹³C. Therefore, we found that ecosystem belowground compartments, and especially soil microorganisms, played an important role in the δ¹³C_R response to the water pulse.

Our results clearly show that the δ¹³C_R and carbon turnover time responses to environmental change are not only depending on plant physiological responses to environmental impacts, but are also depending on plant ecophysiological status preceding that event. These results also emphasize the importance of ecosystem belowground components and their response to the environmental event. Furthermore, these results might help to interpret the observed discrepancies among different field studies on the δ¹³C_R responses to environmental change, and improve our understanding of the controls of carbon residence time in ecosystem.