Response of soil microorganisms to an elevated nitrate input in an open Pinus banksiana - Cladina forest

The two communities of the open Pinus banksiana - Cladina forest have contr asting nitrogen (N) cycles. While the 'lichen patches' occurring on bedrock surfaces have high rates of net N mineralization and net nitrification, ne arby 'forest islands' have low rates of net N mineralization and no net nit rification. Using buried soil bag incubations, we compared the changes in n et N mineralization and net nitrification in forest islands and lichen patc hes during a short-term experimental addition of N (40 kg N ha(-1) per year as NaNO3) to a small catchment. While in N-amended lichen patches net nitr ification increased or remained similar to references, N-amended forest isl ands had a strong tendency to consume NO3- and produce NH4+. We hypothesize that assimilatory NO3- reduction coupled with a decreased gross NH4+ uptak e rare was the combination of mechanisms generating the net conversion of N O3- into NH4+ in forest islands. Observations from the open P. banksiana - Cladina forest suggest that boreal ecosystems may respond in a heterogeneou s fashion to anthropogenic N inputs. (C) 2000 Elsevier Science B.V. All rig hts reserved.