Mechanism Of N₂O Emissions From Terrestrial Plant Leaves

Global warming is one of the biggest environmental problems which the human beings face.N₂O is the third greenhouse gas after CO₂ and CH₄,but its potential for warming is huge.Therefore,identifying the sources of N₂O emission and clearfying the mechanisms for reducing its emission have gradually become a research hotspot and chanllege in the field of natural science.Plants are the most important part of the earth's ecological environment.Plant can absorb or emit CH₄,CO₂ and N₂O during their respiration and metabolism.Plant leaves are the most important components and metabolic organs of plants.However,there are few studies on whether and via what pathways the plant leaves emit N₂O.Based on the"China famous and excellent botanical garden"of Fujian Agricultural and Forestry University,we firstly observed N₂O emission fluxes of in situ leaves and excised leaves of more than ten terrestrial plants.Then we used sterile seedlings together with N₂O electrodes to test whether the plant leaves can emission N₂O by itselves,and detected the pathways of N₂O emission from plant leaves using the stable isotope tracer method.Finally,we explored the influences of two important environmental factors,i.e.,light and temperature,on the N₂O emission from plant leaves.We verified whether plant leaf can produce and emit N₂O by observing the difference of N₂O emission rate between in situ leaf and excised leaf.The results showed thatall of the in situ leaves of the 11plants emitted N₂O and N₂O emission rates ranged from-3.77 pg?g^-1?h^-1 to194.86 pg?g^-1?h^-1.The N₂O emission rates of Mountain cherry,Bauhinia,Creepers,Phyllostachys pubescens,Hibiscus,Purple maple,Jiulixiang,Dalbergia odorifera leaves reached the maximum within the first 6 hours in in situ monitoring,and the maximum emission rate decreased in the order above.The N₂O emissions of the leaves of Plum,Gardenia and Banana shrub reached the maximum withinthe first 2 hours in situ monitoring?gardenia was the highest,plum was the second and banana shrub was the lowest?,and then the N₂O emission rates of the plants leaves slowed down with the increasing time.The excised leaves can also emit N₂O within 48 hours,and the emission rates ranged from-16.36pg?g^-1?h^-1 to 103.37 pg?g^-1?h^-1,and both were lower than those of the in situ leaves.The rates of N₂O emissions differed significantly amoungof the 16plant leaves and decreased with the following order:Dalbergia odorifera,Fuso,Elm,Chinese redbud,Crape myrtle,Rose-acacia,Apple,Lychee,Wenge,Weepingwillow,Bamboo,Creepers,Jiulixiang,Fragrans and Ginkgo.After excluding the transportation of N₂O through the transpiration flow from soil,a large amount of N₂O emissions were still observed from palnt leaves,which proved that plant leaves can produce and emit N₂O themselves.After proving that plant leaves can produce and emit N₂O themselves,we further verified whether the leaves would produce N₂O after removing the microorganisms on the surface of the leaves,using the sterile seedling experiment.The results show that the sterile seedlings of Mao bamboo,Fir and Poplar can emit N₂O,which proved that the N₂O emitted from plant leaves was no only originated from the microbes on the plant leaves.The N₂O concentration of the sterile seedlings of Mao bamboo increased from 0.3917?g?g^-1 to 0.4093?g?g^-1 within 24 hour,and further increased to 0.4647?g?g^-1 at 120 hour.The sterile seedlings of Mao bamboo emited more N₂O than the sterile seedlingses of Poplar and Fir.Using the Unisense N₂O electrode technology,the protoplasts of 9plant leaves were detected to contain N₂O.The N₂O content of the Green radish protoplasts was 7.278?mol?L^-1,which was the highest among the 9plant leaves investigated.The protoplast of Mao bamboo has the lowest N₂O content of 1.972?mol?L^-1.These results indicated that the vacuoles inside the plant leaf contain N₂O,which may be responsible for the plant itself to produce and emite N₂O.The dissimilatory reduction of nitrate is the most important pathway for the production of N₂O in soil.In order to verify whether the plant can produce N₂O in the process of assimilatory reduction of nitrate,nitrate solution was injected into the Green radish and Oil eucalyptus branches by drip injection method.The nitrate contents in the plant leaves increased significantly after injection of nitrate solution,while N₂O emissions increased sharply?14.2%-54.4%?.Using ¹⁵N-KNO₃ tracing method,it is proved that plant branches can absorb the traced nitrate,and the nitrate can be stored in the leaf cells by photosynthesis and transpiration,and then the nitrate in the plant leaf cells was reduced to N₂O through a series of metabolic reactions and emitted into the environment.The branches of Plum,Mao bamboo,Elm and Dalbergia odorifera were cultured in the controllable light incubator,and placed under light and dark conditions at the temperature of 25?,30?,35?and 40?,respectively.Then,we observed the variation of N₂O emissions from plant leaves and studied the response of plant leaf N₂O emissions to temperature and illumination.The results showed that the plant branches emitted most N₂O at 35?.When temperature less than 30?,plants branches tend to absorbed N₂O from the environment.Dark conditions can promote N₂O emission from plant branches.