HR: 0800h
AN: B11A-0992    [Abstracts]
TI: Linking Rainfall Variability With the Flux and (δ¹³C) Isotopic Signature of Ecosystem Respiration
AU: * Brenner, R E
EM: rbrenner@berkeley.edu
AF: University of California Berkeley, Department of Integrative Biology, Berkeley, CA 94720-3140 United States
AU: Tu, K P
EM: kevintu@berkeley.edu
AF: University of California Berkeley, Department of Integrative Biology, Berkeley, CA 94720-3140 United States
AU: Dawson, T E
EM: tdawson@berkeley.edu
AF: University of California Berkeley, Department of Integrative Biology, Berkeley, CA 94720-3140 United States
AB: The accurate determination of the seasonal variability in the isotopic signature (δ¹³C) of biosphere-atmosphere CO₂ exchange is critical for quantifying sources and sinks of atmospheric CO₂ based on its carbon isotope composition. Our measurements of the carbon isotope signature of CO₂ respired from a temperate grassland revealed that seasonal variations in this signature were linked to the amount and timing of precipitation events. During wet winter months rainfall was sufficient to support active plant growth and the signature of ecosystem respiration (heterotrophic + autotrophic) reflected the predominance of recently fixed plant carbon substrates (δ¹³CO₂ of -29 to -28). In contrast, during the summer months, when there was no rainfall, the isotopic signature of the ecosystem solely reflected subsurface microbial decomposition of older soil organic matter (δ¹³CO₂ of -26 to -23). The overall rate of respiration was also much reduced in the dry season ( 0.1-0.5 micromol CO₂ m^-2 s^-1 ) relative to the rainy season (3-5 micromol CO₂ m^-2 s^-1 ). The timing of the shift back to the plant-dominated signal was determined by the occurrence of the first rain event in the late fall when sufficient moisture was present to allow microbial decomposition of dead plant material at the soil surface. A laboratory soil incubation study allowed us to trace the ecosystem respiration signature during the dry season back to particular depth intervals within the soil profile. We were further able to use a clipping manipulation in combination with isotopic measurements to partition soil respiration into rhizosphere and non-rhizosphere components. Our results indicate that the isotopic signature of this ecosystem depends upon on the timing of precipitation events which control the seasonal dynamics of plant growth, litter and SOM decomposition, and water availability within the upper 50 cm of the soil profile. These dynamics should be represented in models of the global carbon cycle which constrain estimates of carbon sources and sinks based on the isotopic composition of atmospheric CO₂.
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0429 Climate dynamics (1620)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: Fall Meeting 2005