Model of stomatal ammonia compensation point (STAMP) in relation to the plant nitrogen and carbon metabolisms and environmental conditions

Abstract Agricultural crops can be either a source or a sink of ammonia (NH 3 ). Most NH 3 exchange models developed so far do not account for the plants nitrogen (N) metabolism and use prescribed compensation points. We present here a leaf-scale simplified NH 3 stomatal compensation point model related to the plants N and carbon (C) metabolisms, for C 3 plants. Five compartments are considered: xylem, cytoplasm, apoplasm, vacuole and sub-stomatal cavity. The main processes accounted for are the transport of ammonium (NH 4 + ), NH 3 and nitrate (NO 3 − ) between the different compartments, NH 4 + production through photorespiration and NO 3 − reduction, NH 4 + assimilation, chemical and thermodynamic equilibriums in all the compartments, and stomatal transfer of NH 3 . The simulated compensation point is sensitive to paramaters related to the apoplastic compartment: pH, volume and active transport rate. Determining factors are leaf temperature, stomatal conductance and NH 4 + flux to the leaf. Atmospheric NH 3 concentration seem to have very little effect on the compensation point in conditions of high N fertilization. Comparison of model outputs to experimental results show that the model underestimates the NH 3 compensation point for high N fertilization and that a better parametrisation of sensitive parameters especially active trasport rate of NH 4 + may be required.