Substrate quality and not dominant plant community determines the vertical distribution and C assimilation of enchytraeids in peatlands

Peatlands are important carbon (C) stores as a result of acidity, waterlogging conditions and low temperatures slowing decomposition rates. However, climate change is predicted to bring not only changes in abiotic conditions but also the replacement of peat‐forming vegetation, such as Sphagnum mosses, with vascular plants. In these systems, enchytraeid worms represent the dominant mesofaunal group, and previous climate change manipulations have shown that their abundances and vertical distribution are strongly influenced by temperature and moisture gradients, with important implications for C cycling. Therefore, determining their behavioural responses to changes in both abiotic and biotic factors is crucial to quantify their contribution to decomposition processes. We investigated the importance of vertical edaphic gradients (soil moisture and labile C) on enchytraeids total numbers, vertical distribution and feeding activities by inverting intact soil cores and hence, reversing both substrate quality but leaving the microclimatic gradients intact. This manipulative experiment was set up in two different peatland areas, one dominated by heather (Erica mackaiana ) and another by mosses (Sphagnum sp.) to determine the influence of different plant functional growth forms on these responses. Our results showed that most enchytraeid species were ‘stayers’ (i.e. in the ‘Upturned cores’, they remained in the same layer where they are ‘Normally’ located), and hence, their vertical distribution was more dependent on substrate quality than a change in microclimate. This response was more evident under drier conditions (i.e. the highest locations of the height gradient at the heather site) than at the wettest locations (i.e. the bottom of the slope and under mosses) due to more pronounced vertical gradients in substrate quality. Furthermore, radiocarbon measurements on enchytraeid tissues showed that to avoid competition among the growing populations for the limited labile resources present in the top layers, enchytraeids selected older C sources. These findings indicate that future climate change scenarios will not only bring important changes in the abiotic conditions (temperature and moisture) and biotic properties of peatlands (both vegetation composition and below‐ground soil biota community structure and vertical distribution) but also alterations in the feeding preferences of key decomposers that could result in the mobilization of previously unavailable C pools

Disentangling thermal acclimation and substrate limitation effects on C and N cycling in peatlands

Temperature and substrate availability are among the key factors controlling microbial metabolism. The relative importance of these two drivers on soil organic matter turnover is, however, hotly debated. In this study, we investigated the effect of temperature changes on the potential enzyme activities involved in C (phenol-oxidase) and N (protease and amidase) cycling by incubating peat soils collected in winter and summer at the two typical temperatures recorded in the field during these two distinct periods (4 and 19 °C, respectively). In addition, to evaluate the effect of substrate limitations, we also compared the respiration rates of the thermally adapted soils with and without plant litter additions. Results showed that both collection season and incubation temperature had a significant effect on the two enzymes involved in N-cycling, with summer and increasing temperatures having detrimental effects on the potential activities of protease and amidase, whereas none of these factors affected phenol-oxidase activity. Furthermore, while adding readily decomposable substrate accelerated decomposition rates, C