KINETICS OF MICROBIAL RESPIRATION AND NITROGEN MINERALIZATION IN GREAT-LAKES FORESTS

Recent attention has focused on organic matter storage in forested ecosystems because climate change could potentially alter this process. Labile organic matter pools are especially important because they may be most strongly influenced by changes in soil temperature and water availability. We measured rates at which C and N were released from labile organic matter within the forest floor and mineral soil of jack pine (Pinus banksiana Lambert), red pine (P. resinosa Aiton), balsam fir [Abies balsamea (L.) Miller], sugar maple (Acer saccharum Marshall), and quaking aspen (Populus tremuloides Michaux) forests. Forest floor and mineral soil samples were assayed for microbial respiration and N mineralization using a long-term (32 wk at 35-degrees-C) laboratory incubation. Cumulative amounts of respired C and mineralized N were fit to first-order rate equations; pools and rate constants were compared among forests. Labile (respired) C pools in forest floor ranged from 67 (jack pine) to 92 g C m-2 (sugar maple), four to six times less than that measured in mineral soil. Rate constants for microbial respiration were statistically different among forest types, but means ranged narrowly in forest floor (0.269-0.299 wk-1) and mineral soil (0.303-0.350 wk-1). Labile (mineralized) N pools ranged from 2.2 (red pine) to 4.1 g N m-2 (sugar maple) in forest floor, an order of magnitude less than those in mineral soil. Rate constants for N mineralization varied from 0.326 to 0.556 wk-1 in forest floor and from 0.043 to 0.069 wk-1 in mineral soil. Regional climatic variables were weakly correlated with labile C and N pools and with rate constants. Annual in situ estimates of microbial respiration and N mineralization were far less than respired C and mineralized N pools, suggesting that only a fraction of labile soil organic matter is annually metabolized within these forests. Local climate, rather than the chemistry of labile organic matter, appears to be an important factor constraining the annual in situ flux of C and N from this pool.