Patterns produced by foraging mycelial systems of decomposer fungi might be the visual evidence of strategies for obtaining differently distributed carbon sources. Mycelial systems might also be responsive to unevenly distributed mineral nutrients in the soil. The nature of the mycelial response might differ between species inhabiting eutrophicated disturbed sites, such as Stropharia caerulea (Kriesel), and inhabitants of more typical woodland soils, such as Phanerochaete velutina DC.: (Pers.) Parmasto. We examined the effect of enrichment of soil by addition of different concentrations of N, P and potassium on inoculum decay rate and on extension rate, biomass production and fractal dimension (determined by image analysis) of extra-resource mycelia of S. caerulea and P. velutina extending from wood blocks across compacted non-sterile soil in trays.

S. caerulea was more responsive than P. velutina to both type and quantity of soil nutrient enrichment. P. velutina grew more rapidly, produced more biomass and decayed inocula more rapidly than S. caerulea under most conditions, rates of growth and resource decay of the former remaining stable over a range of soil nutrient enrichments. In contrast, morphology of S. caerulea altered on enriched soils compared to controls with greater and more pronounced fanning (divergence of separate hyphae) of the foraging mycelial front. These changes were reflected in fractal values. Extension rate and biomass production of S. caerulea was less than controls on N-amended soil, but greater on P-amended soils, with the greatest biomass produced on 0·13–0·27 mg P g−1 air-dried soil. Decay rate of inocula was often positively related with mycelial production rates. These responses indicate extra-resource mycelium of S. caerulea is able to respond rapidly to a range of soil nutrient enrichments, and this is discussed in terms of the different foraging strategies exhibited by the two fungi.