Successional Nitrogen Gradients Regulate Ectomycorrhizal Effects on Wood Decomposition

Contrary to a universal Gadgil effect, trenching accelerated mass loss by up to 41% within 365 days only in the youngest, N-poorest stand (80Y), coinciding with a dramatic 781% increase in the saprotrophic ascomycete Cudoniella and > 96% reduction in ECM fungi. This rapid response was accompanied by markedly enhanced wood N enrichment in trenched treatments during early decomposition in N-limited conditions. In older, N-richer stands, trenching effects on decomposition were delayed (significant only at 730 days) or strongly attenuated, with fungal communities shifting toward saprotrophic ascomycete dominance (e.g., Zopfiella in 250Y) and more modest changes in wood N and P enrichment patterns.

Overall, trenching increased saprotrophic ascomycetes by 54% and total saprotrophs by 35%. These findings demonstrate that ECM suppression of decomposition is most pronounced under acute N limitation in early succession and progressively diminishes as soil N accumulates, reconciling literature inconsistencies and emphasizing the need for dynamic, nutrient-sensitive models of fungal controls on temperate forest carbon cycling.