Original Articles: Nitrogen Mineralization in Two Unpolluted Old-Growth Forests of Contrasting Biodiversity and Dynamics

ABSTRACT

Studies in unpolluted, old-growth forests in the coastal range of southern Chile (42°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 temperate forests, in contrast to their northern hemisphere counterparts, exhibit extremely low losses of inorganic N to stream waters. The objectives of this study were (a) to determine whether low inorganic N outputs in these forests were due to low rates of N mineralization or nitrification, and (b) to examine how biodiversity (defined as number of dominant tree species) and forest structure influence N mineralization and overall patterns of N cycling. Studies were conducted in a species-poor, conifer-dominated (Fitzroya cupressoides) forest with an even-aged canopy, and in a mixed-angiosperm (Nothofagus nitida) forest with a floristically more diverse and unstable canopy. Nitrogen mineralization rates measured in laboratory assays varied seasonally, reaching 6.0 μg N/g DW/day in both forests during late summer. Higher values were related to higher microbial activity, larger pools of labile inorganic N, and increased fine litter inputs. Field assays, conducted monthly, indicated positive net flux from N mineralization mainly from December to January in both forests. Annual net flux of N from mineralization 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 inorganic N to streams, N mineralization and nitrification are not inhibited in these forests, implying the existence of strong sinks for NO₃ ⁻ in the ecosystem. Field N mineralization rates were two times higher in the Nothofagus forest than in the Fitzroya forest, and correlated with greater N input via litterfall, slightly higher soil pH, and narrower carbon (C)–nitrogen ratios of soils and litter in the former. Differences 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 hemisphere forests were lower than median rates for forests in industrialized regions of North America, such as the eastern and central USA. We suggest that these high N mineralization rates may be a consequence of enhanced atmospheric N deposition.