Ectomycorrhizas and retarded decomposition in a Pinus resinosa plantation

• Ectomycorrhizas may interact with saprotrophic microorganisms to influence decomposition. These interactions are poorly understood. • We buried mesh envelopes filled with either litter or F-layer material in the forest floor of a Pinus resinosa plantation. After 16 months we characterized decomposition, C : N ratio, water content, and the density of four ectomycorrhiza morphotypes. • For both substrates there were negative correlations between ectomycorrhiza density and decomposition, and between ectomycorrhiza density and water content. The relative proportion of morphotypes was significantly different in litter and F-layer material buried at the same depth in the forest floor. • We conclude that the negative effect of ectomycorrhizas on decomposition could be mediated by extraction of water, particularly in relatively dry years. The 'Gadgil effect' may be explained by this phenomenon. In wetter years, water extraction may be less consequential. Also, variation in substrate quality may influence the structure of ectomycorrhizal fungal communities.