In situ and laboratory non-additive litter mixture effect on C dynamics of Sphagnum rubellum and Molinia caerulea litters

The accumulation of carbon in peatlands originates from the slow rate of Sphagnum litter decomposition. Vegetation shifts can alter the Sphagnum decomposition rate through a litter mixture effect. This is rarely studied in peatlands. In a site colonised by vascular plants, we examined the effect of mixing litters of Sphagnum species with those of Molinia caerulea and Betula spp. on litter carbon (C) dynamics. We tested that water content and pH may explain the potential non-additive litter mixing effect. Litter bags with Sphagnum cuspidatum or Sphagnum rubellum and M. caerulea or Betula spp. were placed in situ, in a Sphagnum decomposing environment and retrieved after 1 year of incubation. In the laboratory, the specific interaction between S. rubellum and M. caerulea was investigated. Solid, soluble and gaseous forms of C were studied in addition to the fluorescence of the dissolved organic matter (FDOM). Laboratory and field experiments showed that there is a non-additive effect of mixing S. rubellum and M. caerulea litter on C dynamics. The analyses of FDOM suggested a relatively higher living biomass in the laboratory-measured mixture than in the expected one. The in situ S. rubellum moist environment could stimulate the decomposition of M. caerulea that experiences much drier conditions in its native environment. In the laboratory experiment, M. caerulea were kept moist, and no significant difference in water content between the measured and expected mixture was found. Also, pH decreased in the measured mixture, ruling out any direct effect on microbial activity. The non-additive mixture effect observed in the laboratory may be triggered by an increase of the microbial biomass. This increase was not explained by direct moisture or a pH effect. The suggested hypothesis that a lower pH could affect the availability of labile organic substrate through increased organic matter (OM) hydrolysis and thus stimulate microbial growth has to be further studied.