Detritus production and soil N transformations in old-growth eastern hemlock and sugar maple stands

To examine the linkage between forest cover type, litter inputs, and patter ns of net N mineralization versus the turnover of N among soil microbes, we measured both the net and gross rates of N mineralization in replicated, a djacent old-growth eastern hemlock [Tsuga canadensis (L.) Carr.] or sugar m aple (Acer saccharum Marsh.) stands in upper Michigan. Mean aboveground net primary production and annual litterfall mass were significantly higher (P < 0.01) in the maple forests (870 g.m(-2).y(-1) and 439 g.m(-2).y(-1), res pectively) than in the hemlock forests (480 g.m(-2).y(-1) and 344 g.m(-2).y (-1), respectively). Forest floor and coarse woody debris mass, however, we re significantly lower (P < 0.05) in the maple forests (2.2 and 0.1 kg.m(-2 ), respectively) than in the hemlock forests (2.9 and 0.2 kg.m(-2), respect ively). Litterfall N concentration was not significantly different (P > 0.1 0) between the two forest types. In situ gross rates of N mineralization we re higher (P < 0.06) in the maple forests than in the hemlock forests (7.5 and 6.1 mg N.kg soil(-1).d(-1) respectively), but in situ net N mineralizat ion varied independently of forest type and stand-level litterfall N concen tration. Cover type-dependent differences in detritus production and detrit us C quality appear to result in different N turnover rates, bur the balanc e between gross mineralization and immobilization of N is very sensitive to within stand variability and varies at a scale smaller than cover type alo ne can predict.