Ecosystem-level functional differences between C3 soybean and C4 maize inferred from eddy covariance data

Physiological differences between C3 and C4 plants are well established at the leaf level, but whether these differences can be observed at the ecosystem scale remains poorly characterised. The big-leaf framework, when combined with eddy covariance observations, provides a parsimonious approach to infer key aspects of physiological behaviour from flux measurements. In this study, we assess whether ecosystem-scale functional properties inferred from eddy covariance data are consistent with known leaf-level physiological differences between C3 and C4 plants. We derive key ecosystem functional properties from a 13-year time series of eddy covariance data measured at the US-Ro1 site, a maize (C4) and soybean (C3) rotation site in the temperate United States. This site provides a valuable testbed for comparing ecosystem functional properties between maize and soybean under otherwise similar eco-climatic conditions. Our analysis reveals fundamental physiological differences between the C4 and C3 crops at the ecosystem level. Specifically, maize exhibited higher water-use efficiency, greater ecosystem apparent quantum yield, and a higher temperature optimum than soybean. These functional contrasts are qualitatively consistent with established theory and leaf-level observations, indicating that the big-leaf approach can effectively capture key aspects of ecosystem-scale physiological behaviour. We conclude by discussing the strengths and limitations of the big-leaf framework for inferring ecosystem functional properties from eddy covariance observations.