Elevated CO₂ enhances decomposition and modifies litter‐associated fungal assemblages in a natural Eucalyptus woodland

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 (CO 2 ) concentrations. We conducted two complementary field studies to assess the effects of elevated CO 2 on Eucalyptus tereticornis litter decomposition processes. First, we used bags of two different mesh sizes to assess the effect of macrofauna and elevated CO 2 over 24 months on mass loss of litter grown under ambient CO 2 . We then assessed the effect of elevated CO 2 during the decomposition of litter grown under each combination of (i) ambient CO 2 or elevated CO 2 and (ii) during a psyllid outbreak that triggered significant canopy loss or later in canopy developing when psyllid densities were low. Both macrofauna and elevated CO 2 enhanced mass loss at late decay stages in the first study, with no interactive effect. Again, mass loss was greater at elevated CO 2 at late decay stages in the second study, particularly for non‐psyllid‐impacted litter grown at elevated CO 2 . In both studies, CO 2 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 CO 2 . CO 2 concentrations under which leaves developed and whether leaves were psyllid‐impacted was also important in shaping fungal assemblages. Synthesis . The positive effect on mass loss at late decay stages is contrary to previous findings where elevated CO 2 generally reduced decomposition rates. Our results show that elevated CO 2 effects on decay rates are context‐specific. Further research is required to establish the mechanisms through which this occurs to better model elevated CO 2 effects on global carbon dynamics. Read the free Plain Language Summary for this article on the Journal blog.