Litter quality and interactive effects in litter mixtures: more negative interactions under elevated CO2?

Abstract

Summary 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 CO2 concentrations. We measured respiration rates of litter from two contrasting species pairs, produced under ambient and elevated CO2. 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 CO2. Despite the large difference in litter N concentration between Sphagnum recurvum and Carex rostrata, no interaction was observed in the ambient CO2 litter mixtures, and under elevated CO2, there was an unexpected positive interaction. Interactions in litter mixtures can therefore change under elevated CO2, albeit not in a predictable manner, which would require correct prediction of species‐specific litter chemistry and its response to elevated CO2.