Uffe Nielsen, Dylan Bristol, Michaela Blyton, Brendan Delroy, Jeff R Powell 

Litter decomposition is a key process governing carbon and nutrient cycles in forest ecosystems that is expected to be impacted by increasing atmospheric carbon dioxide (CO2) concentrations. We conducted two complementary field studies to assess the effects of elevated CO2 on Eucalyptus tereticornis litter decomposition processes. First, we used bags of two different mesh sizes to assess the effect of macrofauna and elevated CO2 over 24 months on mass loss of litter grown under ambient CO2. Both macrofauna and elevated CO2 enhanced mass loss at late decay stages, with no interactive effect. We then assessed the effect of elevated CO2 during decomposition of litter grown under each combination of (i) ambient CO2 or elevated CO2 and (ii) during a psyllid outbreak that triggered significant canopy loss or later in canopy developing when psyllid densities were low. Again, mass loss was greater at elevated CO2 at late decay stages, particularly for non-psyllid impacted litter grown at elevated CO2. In both studies, CO2 concentration during decomposition influenced fungal assemblages and these effects were observed before any effects on decomposition were observed, with some fungi linked to saprotrophic guilds being found with higher frequency under elevated CO2. CO2 concentrations under which leaves developed and whether leaves were psyllid-impacted was also important in shaping fungal assemblages. The positive effect on mass loss at late decay stages are contrary to previous findings where elevated CO2 generally reduce decomposition rates. Our results show that elevated CO2 effects on decay rates are context specific. Further research is required to establish the mechanisms through which this occurs to better model elevated CO2 effects on global carbon dynamics.