Ecosystem perspectives on carbon and nutrient cycling below-ground

Primary production in most terrestrial ecosystems is limited by below-ground resource availability. In this dissertation, I describe studies that focus on biological, chemical and physical influences regulating carbon and nutrient transformations below-ground in three different ecosystems.; In a Californian annual grassland community, the influence of arbuscular mycorrhizal associations on plant nutrient acquisition was investigated using factorial combinations of nitrogen (N), phosphorus (P) and benomyl fungicide. Plant production in control plots was N-limited. However, production in plots treated with fungicide was limited by both N and P supply. These results suggest that effective P acquisition by arbuscular mycorrhizae contributes to the strong N-limitation of production observed in many terrestrial ecosystems.; In a recently burned Californian Bishop Pine (P. muricata) forest, the influence of surface ash on soil, microbial and plant N pools was examined. Primary production and ecosystem N retention were significantly enhanced by the presence of ash in the first growing season after fire. Analyses of ash samples indicated that production was stimulated by direct ash N inputs to soils and indirect physico-chemical effects on soil N availability to plants. Comparisons of isotopic {dollar}rmsp{lcub}15{rcub}N{dollar} natural abundance values in burned and mature P. muricata forest stands indicate that fractionation of plant N sources increases as surface organic layers develop after fire. This study suggests that post-fire dispersal of ash by wind and rain can result in marked heterogeneity in soil N availability and may be an important mechanism contributing to vegetation patchiness in fire-prone ecosystems.; In arctic Alaskan tundra, the impact of season on climate and vegetation influences over CO{dollar}sb2{dollar} release from below-ground was assessed. Climate had strong effects on CO{dollar}sb2{dollar} efflux from below-ground in both seasons, whereas differences in vegetation-type had greatest effect in summer. A major finding from this study is that below-ground CO{dollar}sb2{dollar} release in winter will be sensitive to the increases in temperature and precipitation predicted by General Circulation Models. These results demonstrate that seasonality is a critical factor controlling regional patterns of below-ground CO{dollar}sb2{dollar} release and modulating feedbacks from arctic ecosystems to global warming.