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

Recent attention has focused on organic matter storage in forested eco systems because climate change could potentially alter this process. L abile organic matter pools are especially important because they may b e most strongly influenced by changes in soil temperature and water av ailability. We measured rates at which C and N were released from labi le organic matter within the forest floor and mineral soil of jack pin e (Pinus banksiana Lambert), red pine (P. resinosa Aiton), balsam fir [Abies balsamea (L.) Miller], sugar maple (Acer saccharum Marshall), a nd quaking aspen (Populus tremuloides Michaux) forests. Forest floor a nd mineral soil samples were assayed for microbial respiration and N m ineralization using a long-term (32 wk at 35-degrees-C) laboratory inc ubation. Cumulative amounts of respired C and mineralized N were fit t o first-order rate equations; pools and rate constants were compared a mong 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 respiratio n were statistically different among forest types, but means ranged na rrowly in forest floor (0.269-0.299 wk-1) and mineral soil (0.303-0.35 0 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 th an those in mineral soil. Rate constants for N mineralization varied f rom 0.326 to 0.556 wk-1 in forest floor and from 0.043 to 0.069 wk-1 i n mineral soil. Regional climatic variables were weakly correlated wit h labile C and N pools and with rate constants. Annual in situ estimat es of microbial respiration and N mineralization were far less than re spired C and mineralized N pools, suggesting that only a fraction of l abile soil organic matter is annually metabolized within these forests . Local climate, rather than the chemistry of labile organic matter, a ppears to be an important factor constraining the annual in situ flux of C and N from this pool.