NITROGEN MINERALIZATION IN 2 UNPOLLUTED OLD-GROWTH FORESTS OF CONTRASTING BIODIVERSITY AND DYNAMICS

Studies in unpolluted, old-growth forests in the coastal range of sout hern Chile (42 degrees 30'S) can provide a baseline for understanding how forest ecosystems are changing due to the acceleration of nitrogen (N) inputs that has taken place over the last century. Chilean temper ate forests, in contrast to their northern hemisphere counterparts, ex hibit extremely low losses of inorganic N to stream waters. The object ives of this study were (a) to determine whether low inorganic N outpu ts in these forests were due to low rates of N mineralization or nitri fication, and (b) to examine how biodiversity (defined as number of do minant tree species) and forest structure influence N mineralization a nd overall patterns of N cycling. Studies were conducted in a species- poor, conifer-dominated (Fitzroya cupressoides) forest with an even-ag ed canopy, and in a mixed-angiosperm (Nothofagus nitida) forest with a floristically more diverse and unstable canopy. Nitrogen mineralizati on rates measured in laboratory assays varied seasonally, reaching 6.0 mu g N/g DW/day in both forests during late summer. Higher values wer e related to higher microbial activity, larger pools of labile inorgan ic N, and increased fine litter inputs. Field assays, conducted monthl y, indicated positive net flux from N mineralization mainly from Decem ber to January in both forests. Annual net flux of N from mineralizati on varied from 20 to 23 kg/ha/year for the Fitzroya forest and from 31 to 37 kg/ha/year for the Nothofagus forest. Despite low losses of ino rganic N to streams, N mineralization and nitrification are not inhibi ted in these forests, implying the existence of strong sinks for NO3- in the ecosystem. Field N mineralization rates were two times higher i n the Nothofagus forest than in the Fitzroya forest, and correlated wi th greater N input via litterfall, slightly higher soil pH, and narrow er carbon (C)-nitrogen ratios of soils and litter in the former. Diffe rences in N mineralization between the two forest types are attributed to differences in biotic structure, stand dynamics, and site factors. Median values of net N mineralization rates in these southern hemisph ere forests were lower than median rates for forests in industrialized regions of North America, such as the eastern and central USA. We sug gest that these high N mineralization rates may be a consequence of en hanced atmospheric N deposition.