Temperature, nitrogen availability and mixture effects on the decomposition of heather [Calluna vulgaris (L.) Hull] and bracken [Pteridium aquilinum (L.) Kuhn] litters

1. Weather moorlands in the upland regions of the United Kingdom are under threat from changes in land use and management, the potential effects of atmospheric nitrogen deposition and climate warming. Displacement of heather (Calluna vulgaris) stands by bracken (Pteridium aquilinum) is also widespread. 2. Field (Upper Teesdale National Nature Reserve) and laboratory studies were carried out to investigate decomposition rates of heather and bracken litters. The effects of fertilization with mineral N (50 kg ha(-1) year(-1) added in four increments) and temperature responses of microbial respiration in decomposing litters were determined. 3. The qualities of heather and bracken litters were similar with 1.5% N, 22.3% cellulose and 40.6% lignin in heather, and 1.32% N, 28.9% cellulose and 49.0% lignin in bracken. Total concentrations of phenylpropanoid derivatives (PPDs) from lignin (by alkaline CuO oxidation) and total hydrolysable carbohydrates (using TFA) were similar for the two litter types, but there were qualitative differences in the constituents of the lignins and hemicelluloses. 4. Bracken and heather litters in 1 mm mesh bags both lost about 34% of initial mass over 18 months but 1:1 mixtures of the two Litters showed 14% higher losses over the same period. These mixture effects could not be explained in terms of litter quality. Laboratory studies, on litter mixtures incubated on sterile sand and soil organic matter, suggested that the mechanisms involved interactions between components of litter microbial communities; possibly fungal endophytes. 5. The decomposition rates of heather and bracken Litters were about 10% higher in die N-fertilized plots than control plots while the mixtures showed nearly 15% higher mass losses, 6. Heather and bracken litters incubated in the laboratory with temperature cycles varying from 2 degrees C to 24 degrees C for heather and 2 degrees C to 27 degrees C, far bracken, showed rapid depletion of available carbohydrates over 2 months. Despite the changes in litter quality the temperature response coefficient (Q(10) congruent to 2.5) showed little change over this period. 7. It is concluded chat synergistic interactions between litter types, which may occur when bracken invades heather stands, can significantly increase surface litter decomposition rates and that N fertilization can further enhance mass losses. The extent to which these interactions affect litter standing crops, plant species interactions and organic matter dynamics in peaty upland soils, remains to be determined.