Soil invertebrates control peatland C fluxes in response to warming

1. Peatland soils are estimated to store a third of all terrestrial carbon stocks and are very sensitive to climate change. In these systems, one group of soil mesofauna, enchytraeid worms (Annelida, Oligochaeta), represent up to 70% of total soil fauna biomass and previous studies have highlighted their potential use as 'biological indicators' for functionally important changes in the C cycle. 2. To examine the link between temperature, enchytraeids and carbon fluxes we performed a microcosm experiment in which we assessed the influence of temperature on enchytraeid populations and soil CO₂ and DOC release from a Galician peatland soil over 90 days. Additionally, to unravel the potential underlying mechanisms responsible for DOC production, we also tested the effects of increasing temperatures and enchytraeid activities on the presence of organic chelating metals (iron and aluminium) and hydrogen ions (i.e. acidity) in the soil solution. 3. Enchytraeid population numbers and biomass increased over time at both temperature treatments (14 and 19 °C), with the greatest increase produced at the highest temperature (to over five and seven times higher initial values, respectively, by day 88). Results also showed that, under warmer conditions, enchytraeid activities increased both CO₂ fluxes and DOC release by twofold (Q₁₀ values of 3·9 and 3·6, respectively). 4. The combined effect of temperature and enchytraeids promoted the breakdown of organic substances and consequently, more DOC and iron were leached. An important decline in H⁺ release was observed when enchytraeids were present and possibly eliminating one of the critical mechanisms restricting DOC release. These leachate pH values were also responsible for aluminium immobilisation and hence for its insignificant role in the export of DOC from the peat soil. 5. We conclude that temperature change alone does not explain all the observed increases in soil respiration and DOC production but rather soil invertebrate responses to warming are crucial in controlling C fluxes in peatland soils. This is the result of temperature induced changes of enchytraeid populations and activities which have the potential of speeding up the decomposition of organic matter and altering soil aeration, metal mobilization and acidity of the soil solution, with important implications for the global carbon cycle. 6. There is an urgent need for incorporating the response of soil biology in climate change modelling to make better predictions of future changes in terrestrial carbon pools.