Using continuous stable isotope measurements to partition net ecosystem CO2 exchange into photosynthesis and respiration of a corn-soybean rotation ecosystem

Field-scale estimates of gross ecosystem photosynthesis (P_g) and respiration (R_e) using micrometeorological techniques remains an important yet difficult challenge. Here, we combined micrometeorological and stable isotope techniques to partition net ecosystem CO₂ exchange (NEE) into P_g and R_e in a corn-soybean rotation ecosystem in the Upper Midwest, United States during the summer 2003 corn phase. Mixing ratios of the isotopomers, ¹²CO₂ and ¹³CO₂, were measured continuously from May 21 to October 14 at three heights (two above the canopy and one within the canopy) using tunable diode laser absorption spectroscopy (TGA100, Campbell Scientific Inc.). Half-hour fluxes of NEE were measured using the eddy covariance technique and isotopic fluxes were measured using the gradient method. The diurnal and seasonal variations of the critical partitioning parameters (isotope ratio of R_e (δ¹³C_R) and isotope ratio of NEE (δ_b)) were examined. The variation of δ¹³C_R and δ_b showed a strong seasonal pattern related to changes in phenology. Half-hour values of NEE were partitioned into P_g and R_e based on the principle of mass conservation by using a set of equations that constrain the canopy conductance and plant discrimination. The partitioned P_g and R_e, based on the isotopic approach, were in close agreement with the more traditional nighttime temperature regression methods at full canopy, but differed significantly during other growth stages. A sensitivity analysis was performed to evaluate the uncertainty in the isotope partitioning method. Partitioning was most sensitive to the estimates of δ¹³C_R and δ_b, which were derived from the gradient measurements of the mixing ratios of ¹²CO₂ and ¹³CO₂.