Application of a bi-directional ammonia exchange model for optimization of input parameters at a fertilized crop system; validation by flux measurements

In parameterization of the bi-directional ammonia exchange models over vegetated surfaces there are three most crucial parameters: (1) the stomatal (χ_s) and (2) the soil (χ_g) compensation point concentrations as the function of Γ=[NH₄⁺]/[H⁺] in the apoplast and soil, as well as (3) the cuticular resistance R_w. These factors determine the direction and magnitude of the ammonia flux. Moreover, in the sophisticated models the soil (F_g) and litter (F_l) fluxes must be distinguished as well. Furthermore, the recapture of ammonia volatilized from the ground in the lower layer of the canopy should also be considered.

            For partitioning the measured ammonia flux into stomatal, cuticular and ground parts two-layer, bi-directional exchange models are generally used. However, the parameterization mostly based on empirical relationships involves uncertainties, resulting in disagreements among the applied models in the estimation of the stomatal/soil flux ratio.  

            The main reasons of the deviations may be the following: 

• a) Overestimation of the soil compensation-point (χ_g).The Γ_g calculated from the ammonium content of the soil and the pH of the soil solution is overestimated, because part of the ammonium content in the soil is bound in the solid phase hence Henry's law for the liquid phase cannot be applied for this fraction.
• b) Neglecting of the part of soil derived ammonia recaptured by leaves. For this reason, soil emissions may be underestimated.
• c) Uncertainty or lack of bioassay measurement for Γ_s and difficulties with the Γ_s determined by indirect way. Instead of complicated bioassay measurements, the models generally use empirical approximations to calculate the stomatal compensation point concentration or infer it from the bulk ammonium content of the leaf tissue. Both methods can be a source of error.
• d) Inaccurate or rough estimate of cuticular resistance. Beside the temperature and humidity, the ratio of acidic air components and ammonia determines the R_w. Models often consider a constant site-specific average for this parameter, even though the ratio of acidic substances to ammonia varies from day to day.

            Due to these uncertainties, the estimation of the share of fluxes controlled by soil and vegetation is often uncertain. Furthermore, the uncertainty of the parameterization limits the applicability of the model and reduces its robustness.

            As a conception, we are aiming the use of the following measurement and parameterisation protocol:

• a) Measurement of the flux above bare soil and above the litter covered soil separately, by soil chambers using the PICARRO-G2103 NH₃ Hence, the F_g and F_l and the ratio of compensation point concentrations (χ_g/χ_l) can be estimated separately. Comparison of the χ_g calculated from soil NH₄⁺ and pH with the measured values.
• b) Calculation of the recaptured ammonia by the model as the residual term among soil-cuticular-stomatal exchange.
• c) Performing bioassay measurements.
• d) Use of daily acid/base gas ratio from the nearby regional background air pollution station.

            Model conception based on previously developed models. Bulk fluxes above the canopy will be measured by the relaxed eddy accumulation technique (REA) with a newly designed photoacoustic system using a QCL as light source.