Three approaches to define the ecotoxicity threshold for atmospheric ammonia

Atmospheric NH 3 is potentially toxic to terrestrial plants, and there are other environmental effects such as haze from (NH 4 ) 2 SO 4 aerosols and N enrichment of ecosystems that are normally very low in N. Toxic levels can arise from atmospheric releases from livestock operations. Plants detoxify NH 3 by incorporation of the N into the plant, so direct toxic effects are most likely when plants are growing slowly, as in cold weather. Indirect effects include leaching of nitrates and cations, and cation imbalances, and are most likely in N-poor, non-agronomic settings. The concentrations in air that cause direct toxic effects were summarized, and the corresponding "expected no effect value" (ENEV) was established at 35 nmol NH 3 mol -1 (48 µg m -3 ). Critical loads, the threshold flux density of NH x -N to the landscape that causes detrimental effects, were also summarized. The critical load was 10 kg N ha -1 yr -1 . Using deposition velocity values from the literature (0.3 to 3.6 cm s -1 ), the ENEV implies a load flux of 24 to 290 kg N ha -1 yr -1 . This is above the critical load, suggesting that critical loads to the landscape are more limiting than are direct toxic effects on plants. Another novel approach was to define an ENEV as the mean compensation point (ENEV cp ). Compensation point is the atmospheric concentration below which plants emit NH 3 and above which they absorb NH 3 . Compensation point data were summarized and the ENEV cp was established at 3.7 nmol mol -1 (5.1 µg m -3 ). This coincides with a load flux of 4.8 to 58 kg N ha -1 yr -1 , more consistent with the critical loads.