Ammonia exchange between plant canopy and the atmosphere - a review

The research advancement of NH_3 exchange between plant canopy and the atmosphere was reviewed in this paper.A lot of researchers have indicated that the amount and direction of NH_3 exchange of plant canopy and the atmosphere is depended on the difference of ammonia partial pressure between intercellular and the atmosphere - NH_3 compensation point.The NH_3 compensation point of most crops ranges from 1 nmol NH_3/mol air to 6 nmol NH_3/mol air;under the natural conditions,the NH_3 compensation point may be near to the ammonia partial pressure of the substomatal cavity,and may be equal to the NH_3 content in the atmosphere.Plants growing in low N input ecosystems such as moorlands and forests generally seem to have NH_3 compensation points to zero.When the NH_3 content in the atmosphere is lower than the NH_3 compensation,the growing vegetation can release NH_3,in reverse,it can be absorb NH_3.Factors influencing the NH_3 compensation point mainly include the environmental factors such as leave temperature,light density and the air humidity,and the internal factors such as apoplastic pH,photorespiration,and glutamine synthetase activity.NH_3 exchange between plant and the atmosphere is dominated by absorption in the early growing stage and by release in the late growing stage.The amount of plant absorbing NH_3 and releasing NH_3 is different from the researchers,experimental materials and experimental conditions.The aboveground parts of plant absorb NH_3 mainly by two ways:one is by cortex,and the other is by stomata.NH_3 absorbed by cortex is about 3% of gaseous NH_3 absorption,so this way is not dominant.NH_3 absorption is mainly by stomata,and its amount and rate are up to the stomatal conductance,temperature and light etc.At present,the explanation about NH_3 emission mechanisms from plant is concentrated on the path of photorespiration nitrogen cycle,and of protein decomposition during plant senescence,but the photorespiration nitrogen cycle in plant NH_3 emission is not known completely.Now the researchers at home and abroad are emphasized on the influence of NH_3 content increase in the atmosphere on plant metabolism and physiological characters,they look down on the influence on the plant ecological characters,source-sink relationship,root exudates,nutrient use efficiency and the competition plant community.Therefore,further research of NH_3 exchange between plant and the atmosphere can not only enrich the nitrogen nutrition theory in plant,but also has the very important environmental and ecological meanings.