CO2 and H2O flux partitioning in a Mediterranean cropping system

The flux-variance similarity (FVS) partitioning method to estimate transpiration/photosynthesis and evaporation/respiration, using only high frequency Eddy Covariance (EC) measurements, was studied. The FVS method was applied to two crops in successive growing seasons of a typical cropping system in Mediterranean region: fava bean followed by winter wheat. FVS applies the flux-variance arguments derived from Monin-Obukhov similarity theory separately to the stomatal (photosynthesis and transpiration) and non-stomatal (respiration and evaporation) processes. The leaf-level water use efficiency, an input of FVS method, is here calculated by using EC outputs and estimated humidity after the direct measurement of surface infrared temperature. The two experimental seasons were characterized by exceptionally rainy periods (262 mm in 85 days in 2014, 344 mm in 149 days in 2015). Nevertheless, the partitioning results are consistent with expected trends and absolute values of flux components throughout both seasons. In particular, heterotrophic soil respiration measured by chamber systems and modelled FVS respiration components are comparable at the beginning and at the end of the growing crop cycles, when crop biomass and leaf area are very low. Furthermore, the coupling of FVS method and root exclusion method for measuring soil respiration is able to give the heterotrophic and autotrophic components of respiration. Finally, the results showed that the succession fava bean to wheat permits a mitigation of the loss of carbon to the atmosphere and increases carbon sequestration.