Effects of atmospheric ammonia (NH_3) on terrestrial vegetation: a review

摘要

At the global scale, among all N (nitrogen) species in the atmosphere and their deposition on to terrestrial vegetation and other receptors, NH_3 (ammonia) is considered to be the foremost. The major sources for atmospheric NH_3 are agricultural activities and animal feedlot operations, followed by biomass burning (including forest fires) and to a lesser extent fossil fuel combustion. Close to its sources, acute exposures to NH_3 can result in visible foliar injury on vegetation. NH_3 is deposited rapidly within the first 4―5 km from its source. However, NH_3 is also converted in the atmosphere to fine particle NH_4~+ (ammonium) aerosols that are a regional scale problem. Much of our current knowledge of the effects of NH_3 on higher plants is predominantly derived from studies conducted in Europe. Adverse effects on vegetation occur when the rate of foliar uptake of NH_3 is greater than the rate and capacity for in vivo detoxification by the plants. Most to least sensitive plant species to NH_3 are native vegetation > forests > agricultural crops. There are also a number of studies on N deposition and lichens, mosses and green algae. Direct cause and effect relationships in most of those cases (exceptions being those locations very close to point sources) are confounded by other environmental factors, particularly changes in the ambient SO_2 (sulfur dioxide) concentrations. In addition to direct foliar injury, adverse effects of NH_3 on higher plants include alterations in: growth and productivity, tissue content of nutrients and toxic elements, drought and frost tolerance, responses to insect pests and disease causing microorganisms (pathogens), development of beneficial root symbiotic or mycorrhizal associations and inter species competition or biodiversity. In all these cases, the joint effects of NH_3 with other air pollutants such as all-pervasive O_3 or increasing CO_2 concentrations are poorly understood. While NH_3 uptake in higher plants occurs through the shoots, NH_4~+ uptake occurs through the shoots, roots and through both pathways. However, NH_4~+ is immobile in the soil and is converted to NO_3~- (nitrate). In agricultural systems, additions of NO_3~- to the soil (initially as NH_3 or NH_4~+) and the consequent increases in the emissions of N_2O (nitrous oxide, a greenhouse gas) and leaching of NO_3~- into the ground and surface waters are of major environmental concern. At the ecosystem level NH_3 deposition cannot be viewed alone, but in the context of total N deposition. There are a number of forest ecosystems in North America that have been subjected to N saturation and the consequent negative effects. There are also heathlands and other plant communities in Europe that have been subjected to N-induced alterations. Regulatory mitigative approaches to these problems include the use of N saturation data or the concept of critical loads. Current information suggests that a critical load of 5―10 kg ha~(-1) year~(-1) of total N deposition (both dry and wet deposition combined of all atmospheric N species) would protect the most vulnerable terrestrial ecosystems (heaths, bogs, cryptogams) and values of 10―20 kg ha~(-1) year~(-1) would protect forests, depending on soil conditions. However, to derive the best analysis, the critical load concept should be coupled to the results and consequences of N saturation.