Seasonal Variation of the Net CO2 Ecosystem Exchange Isotopic Signature Above a Corn Canopy.

Stable isotopes of CO₂ are powerful tools to study the carbon dioxide exchange between vegetation and the atmosphere. The development of optical sensors has allowed measurements of CO₂ isotopes in the atmosphere at an unprecedented temporal resolution. The objective of this study was to study the ¹³CO₂ composition of the net CO₂ exchange (NEE) above a corn canopy. Measurements of ¹²CO₂ and ¹³CO₂ above and below the corn canopy were taken using a multiport sampling system connected to a tunable diode laser trace gas analyzer. The flux signature of NEE was calculated using isotopic measurements and the flux gradient technique and a Lagrangian dispersion analysis. The use of a gradient of a concentration threshold to screen half hourly period improved the estimates of flux signatures by the isotope flux ratio approach. The Langrangian dispersion analysis and the isotope flux ratio method estimates showed good agreement above the corn canopy, indicating that the former method can be a viable alternative to study the isotopic exchange within plant canopies. The ¹³CO₂ composition of NEE showed a downward trend near the end of the growing season, which may be related to a reduction of autotrophic respiration in the soil.