HR: 0800h
AN: B31A-0069    [Abstracts]
TI: Modeling Evaluation of Retention and Release of Atmospheric Deposition N in Mixed Coniferous Forests, San Bernardino National Forest, CA
AU: * Yuan, F
EM: fmyuan@hwr.arizona.edu
AF: University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721,
AU: Meixner, T
EM: tmeixner@hwr.arizona.edu
AF: University of Arizona, Department of Hydrology and Water Resources, Tucson, AZ 85721,
AU: Fenn, M E
EM: mfenn@fs.fed.us
AF: USDA Forest Service, Forest Fire Laboratory, Pacific Southwest Research Station, Riverside, CA 92521,
AU: Allen, E B
EM: edith.allen@ucr.edu
AF: University of California-Riverside, Department of Botany and Plant Sciences, Riverside, CA 92507,
AB: Atmospheric N deposition usually benefits N limited forests; however, sustained high deposition may cause N saturation in ecosystems. This study presents a modeling analysis on soil-plant ecosystem retention of N deposition and its releases using DAYCENT at two mixed coniferous forest sites, Camp Paivika (CP) and Barton Flats (BF). These sites represent high and low levels of N depositions (currently about 70 and 10 kgN ha-1 yr-1) in the San Bernardino Mountains, California. In addition to differences in N deposition CP has annual precipitation of 105.7 cm while BF average 48.7 cm. Using available information, DAYCENT was calibrated to simulate biomass and SOM accumulation reasonably well, but underestimated litter C likely due to lack of ozone injury induced litter-fall in the model. The model also competently predicted biomass C/N ratios. Model showed that the ecosystem retained 80-90% of deposition N with SOM the largest retention pool (40-50%) at both sites. At the high-N and wet CP, the litter stored higher percentage (20-25%)of deposition N than plants (10-15%), while in reverse at the low-N and dry BF site. This difference could be related to plant N uptake per biomass production (1.8 gN per kgC at CP and 1.2 gN per kgC at BF) and immobilization/mineralization ratios (0.83 at CP and 0.74 at BF) due to the contrasted litter/SOM accumulation and soil wetness. It showed that about 40% of added N was captured by plants for about 10 years since N deposition, and up to 50% was incorporated into litter in about 20 years. While it took 40~50 years for SOM pool to immobilize 40% of total N deposited. Meanwhile soil inorganic N was gradually accumulating up to 9 mg kg-1 at CP while only 1.5 mg kg-1 at BF. The model demonstrated that a large amount of N was released to the atmosphere (5-15 kgN ha-1 at CP and <1-4 kgN ha-1 at BF), because of seasonal inconsistency of N addition and plant production, and high soil saturation during winter period. However, N losses into aquatic systems occurred in the late winters of the wet years, and were therefore highly variable from negligible to 5-12 kgN ha-1 when high precipitation occurred at CP, but only 0.1-0.3 kgN ha-1 at BF. This modeling analysis demonstrated that forest atmospheric deposition induced N saturation and possible environmental impacts are a comprehensive expression of fast plant physiological responses with lagged soil biogeochemical processes. These responses occur in a context of environmental driving forces, specifically the precipitation regime in this study.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0469 Nitrogen cycling
SC: Biogeosciences [B]
MN: 2007 Fall Meeting