Some effects of changing soil chemistry on decomposition of plant litters and cellulose on a Scottish moor.

Nitrogen (N), phosphorus (P), calcium (Ca) and soluble carbohydrates (CHO) were each added at three levels to a moorland podzol, and the decomposition of three constrasting untreated substrates (Calluna vulgaris stems,Molinia caerulea leaves, and cotton strips) compared between treated and untreated plots. All soil treatments increased decay rates of all three substrates, except for the highest levels of P and CHO, which appeared to inhibit decomposition of cotton andMolinia. The results generally indicated use by the decomposers of nutrients or energy sources from the soil to aid decomposition of untreated substrates. With all additives (N, P, Ca, CHO) maximum degree of change was inversely related to substrate quality. All responses were nonlinear. Optimal levels of N and Ca were in the same order as substrate quality, i.e. optimum forCalluna<cotton<Molinia, but this was not so with P and CHO. The patterns of change in decomposition rates with soil treatments could not be explained entirely by edaphic and substrate quality effects; it was also necessary to consider selection of decomposer organisms, both by substrate and by treatment. More generally, there were no simple 'limiting factors'. Rather, decay rates were controlled by the combined 'availability' of a number of resources (including availability of suitable decomposer organisms). The consequences of this, especially the importance of indirect and interactive effects, are discussed.