Applicability of a closed-path analyser for simultaneous eddy covariance measurement of carbon dioxide, water vapor, nitrous oxide, methane, ozone and ammonia fluxes in a horticultural crop.

The performance of a dual Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) was investigated for evaluating its applicability as close path analyser in eddy covariance monitoring of CO2, O3, NH3 H2O, CH4, N2O. The field test was carried out over a mulched watermelon crop during a whole growth season, in south Italy. Air was sampled by an inertial inlet" suitable for NH3 detection without mechanical filter. The synchronization issue between the sonic anemometer and close path signals was solved looking for the actual time lag between vertical wind speed (w') and concentration (c') fluctuations by a procedure based on the assessment of the cross-correlation function between w' and c' after a preliminary filtering pre-whitening procedure, followed by block-bootstrapping resampling. The performance of the QC-TILDAS in terms of limit of detection for mixing ratio values and fluxes demonstrated this detector suitable for monitoring low eddy covariance fluxes of all the gases (i.e. 0.1966, 1.2169, 0.0019, 0.0001, 0.0001, 0.0015 μmolm-2 s-1 for H2O, CO2, CH4, N2O, NH3 and O3, respectively). The dynamics of eco-physiological exchange of H2O, CO2 was accurately detected throughout all the growing season and the comparison with data by a second eddy covariance tower equipped with an open-path fast H2O, CO2 analyser showed a low underestimation by the close path EC system. The low N2O and NH3 fluxes following the fertilization events have been detected, responding to the slow release by the adopted fertilizer. The CH4 and O3 fluxes have been correlated, suggesting further investigation on their relationships.