Ozone and Nitrogen Deposition as Modifiers of Biogeochemical Fluxes and Processes in California Forests
Abstract
The combined effects of ozone and N deposition results in major perturbations of C and N cycling in forests of southern and central California. Increased shoot:root ratios of the major trees species, N-stimulation of aboveground growth, and premature foliar abscission result in greater aboveground C and N pools. Fire suppression exacerbates these perturbations and provides the opportunity for chronic N deposition to further increase the stand densification problem. Long-term litter decomposition rates are retarded by N enrichment which contributes further to litter accumulation in the forest floor. Stage 3 of N saturation in California mixed conifer forests occurs as chronic N deposition, in conjunction with co-occurring ozone effects, decreases fine root biomass, interferes with stomatal control, and increases the susceptibility of ponderosa pine trees to drought stress and bark beetle attack, leading to increased stand mortality. Hot moments of N transfers from canopy to the forest floor occur during precipitation events that follow long dry periods, but particularly during fog events. During initial soil wet up, pulses of NO and N2O emissions from the forest floor occur. Streamwater losses of nitrate are highest following storms preceded by dry periods, but also during peak runoff, typically in February and March. However, major losses of accumulated N occur during and after fire events. However, ecosystem N budgets, biogeochemical modeling studies and experimental burns in N-saturated chaparral catchments in southern California demonstrate that symptoms of N excess are not easily reversed by N release in and following fire. Even with decreased N deposition, momentum for elevated N losses from California forests would likely continue, driven by actively nitrifying soils and increased N content of litter and soil organic matter. Initial studies show that during peak runoff, as much as 20-40% of runoff nitrate in some catchments is throughput of unassimilated atmospheric nitrate. Paradoxically, the critical load for nitrate leaching in Mediterranean California forests, is approximately twice that of temperate forests in Europe and the eastern U.S. This may be related to the relatively high C:N ratio of litter in Mediterranean ecosystems and denitrification losses during subsurface nitrate transport.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B52C..03F
- Keywords:
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- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 0470 BIOGEOSCIENCES / Nutrients and nutrient cycling;
- 0478 BIOGEOSCIENCES / Pollution: urban;
- regional and global