Nitrogen cycling in unpolluted old-growth forests, southern Chile.

摘要

The nitrogen cycle is an important driver of the biogeochemical dynamics of temperate forest ecosystems. Due to atmospheric pollution and other human impacts, however, only little is known about the nitrogen cycle of truly undisturbed, old-growth temperate forests. This dissertation examines both patterns and mechanisms of nitrogen cycling in remote forests of southern Chile in order to provide better understanding of how nitrogen cycles in minimally impacted temperate forests.; Climate, biota, topography, parent material, and time are the principal state factors which influence how biogeochemical cycles develop in terrestrial ecosystems. In order to evaluate the influence of these state factors on the nitrogen cycle of unpolluted, old-growth temperate forests, I examined patterns of hydrologic nitrogen loss from 64 forested watersheds across 9 sites throughout temperate southern Chile which encompassed broad natural variations in state factors. Patterns of hydrologic nitrogen loss displayed remarkable constancy across sites, characterized by: (1) exceedingly low levels of NO 3-; (2) ratios of NO3- -N:NH4+-N less than unity; and (3) quantitative dominance by dissolved organic over inorganic forms of nitrogen. Most broadly, these results suggest that characteristic patterns of nitrogen loss can develop in temperate forests, despite considerable natural variation in climate, biota, geology, and other factors.; Since supplies of nitrogen are naturally only very low in unpolluted forests, it is thought that NH4- and NO 3- are tightly recycled and retained by plant and microbial communities, resulting in only low hydrologic losses. I applied 15N pool dilution and pulse-chase tracer techniques as complementary approaches within small plots to understand fluxes and fates of inorganic nitrogen over two years in the surface soil of an unpolluted forest in southern Chile. Strong uptake of 15N by microbial communities, followed by rapid transfer into nitrogen-conserving plant and soil organic matter pools, resulted in efficient long-term retention of nitrogen. Equal retention of 15NH4+ and 15NO3 - tracers, coupled greater production of NH4 + in soils, results in the characteristic dominance of NH4 + over NO3- in hydrologic losses from this forest. My results differ from information gathered in more disturbed regions, and indicate that the nitrogen cycle of minimally impacted forests is very finely-tuned.