Seasonal variation in the isotope ratio of ecosystem respiration and canopy-scale discrimination of a corn-soybean ecosystem

Micrometeorological and stable isotope techniques were combined to study the seasonal variation in the net ecosystem CO₂ exchange (NEE) of a corn (C₄ grass) – soybean (C₃ dicot) rotation ecosystem in the Upper Midwest United States. Results are reported for the corn phase of the 2003 growing season, which was warmer and drier than normal. Isotopic fluxes of ¹²CO₂ and ¹³CO₂ were measured using a tunable diode laser absorption spectroscopy (TDLAS) system and gradient technique. Continuous isotopic fluxes and mixing ratios of ¹²CO₂ and ¹³CO₂ were used to quantify the isotope ratio of ecosystem respiration (δ¹³C_R) and canopy-scale discrimination (Δ_e) to better interpret and understand variations in NEE and ecosystem respiration (R_E). Values of (δ¹³C_R) prior to leaf emergence were approximately -27 per mil and increased rapidly (became more positive) following leaf emergence and reached a maximum of -16 per mil at full canopy. A mixing model was used to partition R_E into C₄ and C₃ contributions and indicated that respiration from corn accounted for greater than 60% of R_E at full canopy. Following senescence, the contribution of C₄ substrates to respiration rapidly decreased and respiration from C₃ substrates constituted the major fraction of R_E. The seasonal variation in the isotopic ratio of the surface layer showed the strong effect of canopy photosynthesis on the enrichment of ¹³CO₂. Variation in canopy-scale discrimination indicated that midsummer drought caused a significant reduction in photosynthesis and the net gain of CO₂. The combination of micrometeorological and continuous stable isotope measurements provides additional information at diurnal and seasonal timescales that can be used to better understand the processes controlling NEE.