Litter mixing effects on decomposition in a peatland partially drained 30 years ago

Litter decomposition is a key process controlling carbon (C) sequestration in Sphagnum dominated peatlands. Peatland drainage lowers the water table, increases vascular plant richness, and alters species evenness. We hypothesized that abiotic and biotic changes following peatland drainage would greatly accelerate litter decomposition. This hypothesis was tested by a litter decomposition experiment in an undrained and a drained area of a peatland which was partially drained 30 years ago. Litter bags filled with shoots of the peat moss Sphagnum magellanicum and leaves of the dwarf shrub Potentilla fruticosa as single species and species mixtures with three evenness treatments were placed at the surface of the peatland. After 6 months (from August onwards) of decomposition, the single species litter of Potentilla fruticosa, with low initial C/N ratio, decomposed much faster than S. magellanicum litter, with high initial C/N ratio in both undrained and drained areas. Overall, mixed litter produced antagonistic effects (observed values less than expected) on decomposition and C/N ratio. Litter mixtures dominated by S. magellanicum showed lower mass loss than other mixtures. Contrary to our expectation, the undrained area showed higher decomposition than the drained area. Our study suggests that drainage-induced alteration in plant diversity slows down litter decomposition in a short term. However, in drained peatlands, lower C/N ratio of mixed litter may enhance decomposition and reduce C sequestration over time.