Maize Crops Face High Stomatal Uptake During High Exposure to Ozone in an Agroecosystem in the United States Corn Belt.

Tropospheric ozone (O3) is a phytotoxic air pollutant. The stomatal uptake of O3 is a substantial sink of O3, but it can create oxidative stress within plants, reducing photosynthesis and crop yields. In seasonally dry climates, stomatal regulation causes temporal decoupling between stomatal conductance and atmospheric O3 concentrations protecting vegetation from high stomatal uptake of O3 during peak ambient concentrations in the afternoon, but less is known about this temporal decoupling over agricultural fields in continental humid climates. Here, we investigate how maize (Zea mays L.) ecophysiology impacts O3 dry deposition and diurnal O3 exposure-dose dynamics at an agricultural field in the central Corn Belt of the United States. We measured the field-scale eddy covariance flux of O3 using a UV-absorption based instrument, the NASA Rapid Ozone Experiment (ROZE). The stomatal component of total O3 flux was estimated with an inversion of the Penman-Monteith equation using observed latent heat flux and a model of stomatal conductance using gross primary productivity. We found that maize stomatal conductance remained high as vapor pressure deficit increased during the afternoon. The diurnal synchrony between O3 concentrations and stomatal conductance resulted in high stomatal uptake of O3 during peak O3 concentrations. The monthly mean stomatal flux reached > 70% of the total O3 flux to the land surface during high leaf area index. Furthermore, the total deposition velocity of O3 was tightly coupled with stomatal conductance. Our findings suggest that maize ecophysiology in our field in the Corn Belt of the United States couples high O3 stomatal uptake with high O3 exposure. Furthermore, our study demonstrates the first use of NASA ROZE to measure growing season O3 flux over an agricultural field, and NASA ROZE will be crucial for expanding O3 flux measurements necessary for studying O3 dry deposition and field-scale phytotoxic dose across other fields.