Fine-scale patterns of leaf litterfall and nitrogen cycling in an old-growth forest

Spatial patterns of litterfall and soil nitrogen (N) mineralization were investigated in an old growth hemlock-hardwood forest to test the hypothesis that the lignin/N ratio of litterfall is negatively correlated with soil N availability. Litterfall was measured at 50 small subplots on a 7.2-ha mapped plot in Sylvania Wilderness Area, Michigan, where there is pronounced patchiness of species in the canopy. Nitrogen mineralization and nitrification were measured concurrently at the subplots using buried-bag incubations. Total aboveground litterfall was 3723 kg·ha-1 per year with leaf litterfall contributing 69% of the total, and N mineralization was 68.0 kg·ha-1 per year. The lignin/N ratios of leaf litter and total litterfall were negatively correlated with soil N mineralization and nitrification at the 50 subplots. Nitrogen mineralization rates, especially percent nitrification, were higher where basswood and sugar maple litterfall dominated and lower where hemlock litterfall was high, demonstrating the importance of litterfall and leaf litter chemistry to soil N pattern on a fine spatial scale. The effect of leaf litter chemistry on soil N may be a positive feedback mechanism that maintains the landscape-scale canopy mosaic at Sylvania.