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 patterns 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, adjacent old-growth eastern hemlock [Tsuga canadensis(L.) Carr.] or sugar maple (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, respectively) 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, were 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, respectively). Litterfall N concentration was not significantly different (P > 0.10) between the two forest types. In situ gross rates of N mineralization were 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 mineralization varied independently of forest type and stand-level litterfall N concentration. Cover type–dependent differences in detritus production and detritus C quality appear to result in different N turnover rates, but the balance between gross mineralization and immobilization of N is very sensitive to within stand variability and varies at a scale smaller than cover type alone can predict. Received 3 Feburary 1999; accepted 27 August 1999.