Soil–plant–atmosphere ammonia exchange associated with calluna vulgaris and deschampsia flexuosa

Ammonia fluxes and compensation points at atmospheric NH3 concentrations corresponding to those occurring under natural growth conditions (0–26 nmol NH3 mol air-1) were measured for canopies of two species native to heathland in N.W. Europe, viz. Calluna vulgaris (L.) Hull and Deschampsia flexuosa (L.) Trin. The NH3 compensation point in 2 yr-old C. vulgaris plants, in which current year’s shoots had just started growing, was below the detection limit (0.1 nmol mol-1 at 8°C). Fifty days later, when current year’s shoots were elongating and flowers developed, the NH3 compensation point was approximately 6±2.0 nmol mol-1 at 22°C (0.8±0.3 nmol mol-1 at 8°C). The plants in which the shoot tips had just started growing were characterized by a low N concentration in the shoot dry matter (5.8 mg N g-1 shoot dry weight) and a low photosynthetic CO2 assimilation compared to the flowering plants in which the average dry matter N concentration in old shoots and woody stems was 7.4 and in new shoots 9.5 mg N g-1 shoot dry weight. Plant-atmosphere NH3 fluxes in C. vulgaris responded approximately linearly to changes in the atmospheric NH3 concentration. The maximum net absorption rate at 26 nmol NH3 mol-1 air was 12 nmol NH3 m-2 ground surface s-1 (equivalent to 13.3 pmol NH3 g-1 shoot dry matter s-1). Ammonia absorption in Deschampsia flexuosa plants increased approximately linearly with increasing NH3 concentrations up to 20 nmol mol-1. The maximum NH3 absorption was 8.5 nmol m-2 ground surface s-1 (30.4 pmol g-1 shoot dry weight s-1). The NH3 compensation point at 24°C was 3.0±1.1, and at 31°C 7.5±0.6 nmol mol air-1. These values correspond to a NH3 compensation point of 0.45±0.15 at 8°C. The soil used for cultivation of C. vulgaris (peat soil with pH 6.9) initially adsorbed NH3 at a rate which exceeded the absorption by the plant canopy. During a 24 d period following the harvest of the plants soil NH3 adsorption declined and the soil NH3 compensation point increased from below the detection limit to 8.0±1.8 nmol NH3 mol air-1 (22°C). No detectable NH3 exchange took place between the D. flexuosa soil (sandy soil with pH 6.8) and the atmosphere.