An assay system was evaluated for denitrification measurement with potted ornamental plants cultivated in peat substrate (Pelargonium zonale, Euphorbia pulcherrima). Flow-through chambers only enclosing the pot of the plants were considered best for denitrification measurement. Loss of N2O from the chambers by transport through the plant shoot was negligible with both species. To determine (N2O + N2)-N loss, C2H2 was applied to inhibit reduction of N2O. Experiments were conducted with unplanted substrate in closed incubation systems to determine optimum C2H2 concentration and pre-treatment duration. Complete inhibition of N2O reduction in peat substrate was achieved using 1 vol% C2H2. However, a concentration of 5 vol% C2H2 was chosen for further experiments because C2H2 concentrations in flow-through chambers varied. The duration of C2H2 pre-treatment (0, 2, 12, 24 h) showed no clear effect on (N2O + N2)-N accumulation. However, a pre-treatment duration of 2 h was chosen to guarantee immediate inhibition of N2O reductase at the start of experiments. Exposure to C2H2 gas proved to affect plants of both species. During C2H2 exposition in flow-through chambers, the leaves of P. zonale became chlorotic (48 h) and necrotic (72 h). E. pulcherrima showed no chlorosis but did exhibit leaf epinasty (24 h) and wilting (96 h). Transpiration of P. zonale and C availability in the growing medium of both species were not affected by 52 h and 24 h treatments with C2H2, respectively. As N emissions usually ended within 38 h of C2H2 treatment, it was concluded that side effects of C2H2 did not affect denitrification measurements.
