Isotopic insights into plant and soil carbon metabolism and implications for climate change
Climate warming has the potential to alter the balance between photosynthetic carbon assimilation and respiratory losses in terrestrial ecosystems, leading to uncertainty in predicting their future physiological functioning. The effects of warming on plant and microbial respiration have been well studied, but effects on carbon use efficiency (CUE), allocation and mean residence time (MRT) have received less attention. In this talk I will discuss recent experiments using stable isotope tracers in native Australian systems. In the first study, we evaluated effects of warming on Eucalyptus parramattensis assimilation, sugar metabolism, transport, storage, respiration and root exudation. We applied a 13CO2 pulse to the trees in whole-tree chambers operated at ambient and ambient +3 °C temperature treatments and followed the label in plant sugars, bulk tissues and soil microbes, and in CO2 respired from leaves, canopy, roots and soil over a 3-week period in conjunction with measurements of tree growth. Although warming appeared to reduce the MRT of carbon respired from roots and soil, CUE was not significantly altered. In the second study, we evaluated soil microbial CUE and MRT across a hydrologic gradient from forest to wetland, at different soil depths, using H218O labelling of DNA during a short-term incubation experiment at two temperatures. We found that microbial CUE decreased with increasing temperature and varied with substrate C/N ratios, while the MRT and temperature sensitivity (Q10) were strongly related to soil clay content. These isotopic labelling experiments provide insights into how rising temperatures may affect the fate of assimilated carbon from the leaf to the ecosystem scale and demonstrate potential for improvements in biogeochemical models.