An Investigation of the Biotic Factors Determining the Rates of Plant Decomposition on Blanket Bog

(1) The microbial and animal contributions to plant decomposition in peat and mineral soils in the northern Pennines were assessed by exposing litter of seven different plant species in nylon bags of two mesh sizes, one of which excluded all soil animals. (2) The average rate of microbial decomposition of the same substrates was similar at the peat and at the mineral sites, but the rates of decomposition of litter from different plant species showed large differences. (3) The role of animals in the decomposition of the plant materials differed markedly according both to the soil type and to the plant species of the substrate. Little of the decomposition of Eriophorum vaginatum and Sphagnum recurvum on peat soils was attributable to animal activity, whereas Calluna vulgaris and Rubus chamaemorus owed an appreciable part of their decomposition to the activity of soil animals. (4) The decomposition losses attributed to micro-organisms and to animals were not significantly correlated. (5) Significantly greater densities of soil animals were found on R. chamaemorus leaves that had been exposed on peat as compared with mineral soils, despite the much lower overall density of soil animals in peat soils. (6) The rates of microbial decomposition of the plant substrates were highly correlated with their P or N concentrations on both peat and mineral soils. When the animal contribution to decomposition was included, the high correlation with N or P was maintained on the mineral soils, but the correlation was lower and not significant on the peat. (7) The concentrations of N and P in the plants used were highly correlated. Their effects were separated by comparison between decomposition rates of litter derived from plants growing on areas fertilized with either N or P, and of litter of control material from the same plant species. The material enriched with N, but not that enriched with P, showed increased decomposition rates. (8) The addition of nitrogen fertilizer to blanket bog produced an increase in the density of soil invertebrates. It also caused increased larval growth in Tipula subnodicornis and Lasiocampa quercus, and increased fecundity in the former. Addition of a phosphorus fertilizer had less marked effects, but where these were significant they usually took the form of a decrease in density or in growth rate. (9) It is concluded that the chemical composition of a plant species is of paramount importance in determining the rate of its decomposition on blanket bog. Peat accumulation on blanket bog is primarily the result of the intrinsic slow decay rate of some of the species in the plant communities on such areas.