Importance of short-term dynamics in carbon isotope ratios of ecosystem respiration (delta C-13(R)) in a Mediterranean oak woodland and linkage to environmental factors

Temporal dynamics in carbon isotope ratios of ecosystem respiration (delta C-13(R)) were evaluated on hourly, daily and annual timescales in a Mediterranean woodland. Emphasis was given to the periods of transition from wet to dry season and vice versa, when the system turns from a net carbon sink to a source. The constancy of nocturnal delta C-13(R) was tested. The relationship between delta C-13(R) (determined through Keeling plots) and environmental factors was evaluated through time-lag analysis. delta C-13(R) exhibited high annual variation (> 7). During the transition periods, delta C-13(R) correlated significantly with factors influencing photosynthetic discrimination, soil respiration, and whole-canopy conductance. Time-lags differed between below- and above-ground variables, and between seasons. A shift in regression parameters with environmental factors indicated seasonal differences in ecosystem responsiveness (e.g. temperature acclimation). delta C-13(R) exhibited substantial nocturnal enrichment (> 4) from dusk to dawn. These data indicate pronounced short-term dynamics in delta C-13(R) at hourly to daily timescales and a modulated response to environmental drivers. Substantial short-term changes in nocturnal delta C-13(R) may have important implications for the sampling protocols of nocturnal Keeling plots.