DOES NITROGEN AVAILABILITY CONTROL RATES OF LITTER DECOMPOSITION IN FORESTS

The effects of increased exogenous N availability on rates of litter d ecomposition were assessed in several field fertilization trials. In a jack pine (Pinus banksiana Lamb.) forest, needle litter decomposed at the same rate in control plots and in plots fertilized with urea and ammonium nitrate (1350 kg N ha(-1)) with or without P and K. Mixed nee dle litter of western hemlock (Tsuga heterophylla (Raf.) Sarg.), weste rn red cedar (Thuja plicata Donn) and Douglas-fir (Pseudotsuga menzies ii (Mirb.) France) incubated in plots recently amended with sewage slu dge (500 kg N ha(-1)) lost less weight during 3 years than did litter in control plots. Forest floor material also decomposed more slowly in plots amended with sewage sludge. Paper birch (Betula papyrifera Mars h.) leaf litter placed on sewage sludge (1000 kg N ha(-1)), pulp sludg e, or sewage-pulp sludge mixtures decomposed at the same rate as leaf litter in control plots. These experiments demonstrate little effect o f exogenous N availability on rates of litter decomposition. The influ ence of endogenous N availability on rates of litter decomposition was examined in a microcosm experiment. Lodgepole pine (Pinus contorta va r. latifolia Engelm.) needle litter collected from N-fertilized trees (525 kg N ha(-1) in ammonium nitrate) were 5 times richer in N than ne edles from control trees (1.56% N versus 0.33% N in control trees), bu t decomposed at the same rate. Green needles from fertilized trees con tained twice as much N as needles from control trees (1.91% N versus 0 .88% N), but decomposed at the same rate. These experiments suggest th at N availability alone, either exogenous or endogenous, does not cont rol rates of litter decomposition. Increased N availability, through f ertilization or deposition, in the absence of changes in vegetation co mposition, will not alter rates of litter decomposition in forests.