PARENT SESSION
Oral Session # 66: Ecological Studies at Biosphere II.
Presiding: ML Martinez Vazquez
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 104.

Responses of ecosystem gas exchanges to simulated drought disturbance in a rainforest mesocosm.

Lin, Guanghui^{*,1, 2}, van Haren, Joost¹, Pierce, Danielle¹, Morris, Tiffany¹, Wright, Allen¹, Berry, Joseph³, ¹ Biosphere 2 Center, Oracle, Arizona, USA² Lamont Doherty Earth Observatory, Palisades, New York, USA³ Department of Global Ecology, Stanford, California, USA

ABSTRACT- Model and preliminary field studies indicate that drought disturbance associated with El-Nino events plays a significant role in controlling ecosystem exchanges of CO₂, water and other trace gases. In this study, responses of ecosystem gas exchanges to a series of simulated drought episodes in a rainforest mesocosm were investigated using the facility of Biosphere 2. Net ecosystem exchange of CO₂, (NEE), soil respiration, evapotranspiration (ET) and emission of N₂O were measured before, during and after four repeated month-long drought episodes in 2000-2002. Night NEE showed a step-wise decrease to the drought, indicating differential sensitivities of litter decomposition, plant respiration and oxidation of soil organic matter to reduced soil moisture. Ecosystem respiration recovered quickly to pre-drought level after rewet treatment. Soil respiration showed significant reduction at the end of each drought episode. Canopy CO₂ uptake also showed a significant (40-60 %) reduction in response to the month-long drought. In most cases, drought-induced reduction in ecosystem respiration was balanced by the reduction in canopy CO₂ uptake, resulting in little change in carbon sink-source relationship at the whole system level. A large decrease in carbon isotope ratio of ecosystem respired CO₂ (about 5 per mil) right after rewet events suggested that (1) ¹³C-depleted carbons from root exudates accumulated during the drought were quickly released by soil microbes and (2) litter decomposition responded very quickly to the rewet treatment. ET showed a significant decrease in response to drought and ecosystem water use efficiency decreased as soil moisture decreased. The drought resulted in a 60-70 % decrease in N₂O production but a large pulse of N₂O was observed within the first 36 hours immediate after rewetting the mesocosm. Changes in the isotope ratios of N₂O indicated a rapid shift between nitrification- and denitrification-dominated processes in response to drought and rewet episodes.

Key words: ecosystem gas exchange, tropical rainforest, drought disturbance, carbon cycling