Seasonal and interannual variations of carbon and oxygen isotopes of respired CO₂ in a tallgrass prairie: Measurements and modeling results from 3 years with contrasting water availability

We made weekly measurements of carbon (δ 13 C) and oxygen (δ 18 O) isotopes of atmospheric CO 2 in a C 3 /C 4 tallgrass prairie during the growing season for 3 years with contrasting soil moisture conditions. Air samples above and within canopies were collected using 100‐ml flasks at night to characterize isotopic composition of ecosystem respiration. We used a two‐source mixing line (Keeling plot) approach to estimate isotope ratios of ecosystem respired CO 2 for both carbon (δ 13 C R ) and oxygen (δ 18 O R ). Measured net ecosystem CO 2 exchange (NEE) showed the largest net carbon uptake in 2004, followed by 2003 and 2002. This interannual difference in NEE strongly depends on the amount and distribution of precipitation received by this tallgrass prairie. Precipitation also affects the timing of the seasonal transition from C 3 dominance in spring to C 4 dominance in summer. Variations of δ 13 C R showed that C 4 plants dominated ecosystem respiration in 2003 and 2004, except in early spring when C 3 plants were more active. In contrast, contributions of C 3 plants were relatively higher for an extended period in the summer of 2002, when a severe drought occurred. Typically, C 3 forbs extract water and nutrients from soil layers below that of the C 4 grasses and remain photosynthetically active in periods when C 4 grasses have water stress that limits photosynthesis. Drought‐reduced C 4 grass photosynthesis was lower than temperature‐limited C 3 forb growth during this period. We used an integrated isotope land surface model (ISOLSM) to simulate (and compare to measurements) net CO 2 fluxes, δ 18 O values of leaf and soil water, and δ 18 O values of aboveground and soil respiration. The Keeling plot analysis becomes less reliable for estimating δ 18 O R values when the surface soil is dry. We suspect this is due to low CO 2 production in the soil when water is limiting, in which case the invasion (abiotic) effect is more significant. ISOLSM reasonably captured seasonal variations of measured δ 18 O R in all 3 years, indicating the model's consistency of predicting δ 18 O R in different soil water conditions. Model simulations also showed that nighttime δ 18 O values of aboveground respiration were variable, often becoming very positive in water‐stressed conditions primarily because of the low relative humidity and resultant elevated δ 18 O values of leaf water.