More exceptions than rules: Variable effects of ectomycorrhizal fungi on leaf litter decomposition in temperate pine forests

Ectomycorrhizal (ECM) fungi have long been thought to reduce litter decomposition in nitrogen (N)-limited ecosystems by outcompeting saprotrophs for litter N (a phenomenon known as the ‘Gadgil effect’). However, recent research has called the generality of this effect into question, by demonstrating that ECM fungi can increase or decrease organic matter decomposition in different forests. The ecological factors driving such variation in the size and direction of ECM fungal effects on decomposition remain unclear. Here, we tested the hypothesis that ECM fungi would suppress decomposition of N-poor, recalcitrant litter more in forests with lower N-availability by exacerbating saprotrophic N limitation. We conducted an in situ ECM fungal and root reduction experiment (via soil trenching) in nine pine forests across three US states, which varied in soil and litter N content, climate and pine host (Pinus muricata in California, P. elliottii in Florida and P. resinosa in Minnesota). In each site, we decomposed needle litter from (1) a pine species native to that site and (2) a common litter, P. strobus, for 1 year. Contrary to our expectations, ECM fungi either stimulated (California) or had no effect on (Florida and Minnesota) pine needle decomposition. Across sites, ECM fungal stimulation of decomposition did increase with total soil N content, but was unrelated to inorganic N availability. Furthermore, despite previous work suggesting that competition for N structures ECM fungal–saprotroph interactions, trenching effects on decomposition did not differ between pine litter types, despite large differences in initial litter C:N ratios, recalcitrance and net litter N immobilization. Synthesis. Taken together, our results add to a growing body of evidence that the ‘Gadgil effect’ is not universal, even in the N-poor litter of temperate pine forests where it was first described and is often invoked. Furthermore, the inconsistency of relationships between trenching effects with different metrics of decomposer N supply and demand calls into question the central role of N in structuring fungal interguild interactions.