Litter quality and interactive effects in litter mixtures: more negative interactions under elevated CO 2 ?

Summary1 The decomposition rate of litter mixtures can depend on interactions between compounds in the component species, i.e. in litter chemistry. Differences in litter N concentration are likely to lead to positive interaction (i.e. faster than expected decomposition) and differences in phenolic concentrations to a negative interaction. These interactions may become less positive (or more negative) when the litter is produced under elevated atmospheric CO 2 concentrations. 2 We measured respiration rates of litter from two contrasting species pairs, produced under ambient and elevated CO 2 . 3 As expected, mixtures of Calamagrostis epigejos and Vicia lathyroides , which differed strongly in litter N concentration, decomposed faster than expected, but the interaction disappeared under elevated CO 2 . 4 Despite the large difference in litter N concentration between Sphagnum recurvum and Carex rostrata , no interaction was observed in the ambient CO 2 litter mixtures, and under elevated CO 2 , there was an unexpected positive interaction. 5 Interactions in litter mixtures can therefore change under elevated CO 2 , albeit not in a predictable manner, which would require correct prediction of species‐specific litter chemistry and its response to elevated CO 2 .