Compensation point of NOx exchange: Net result of NOx consumption and production

The compensation point of bidirectional NOx fluxes of plants is often determined in chamber measurements by varying the NOx concentration and searching for the point where the net flux is zero. We hypothesized that the compensation point can vary with changing conditions so much that one constant value is not sufficiently accurate. The compensation point for NOx fluxes of Scots pine was analysed with a model describing the NOx consumption and production processes. Consumption was assumed to occur via the stomata and on needle surfaces, with the rate depending on ambient concentration and degree of stomatal opening. Production was assumed to occur on the needle surfaces depending on solar UV-A irradiance. We used NOy flux data measured with two gas-exchange chambers in southern Finland to parameterize the model. The estimated compensation points increased with increasing UV-A irradiance and decreased with increasing stomatal conductance. With open stomata, it varied several ppb even under field conditions.