Study On Greenhouse Gases Fluxes Of Urban Lawn Ecosystems

Global warming caused by greenhouse gases is one of the most important environmental problems,which deeply influence the sustainable development of human society and survival.Research on greenhouse gases emission of the terrestrial ecosystem is a hot issue in recent years.As a complicated component of the terrestrial ecosystem,urban lawn has an extremely important effect on the exchange of energy and water,as well as the biogeochemical cycle of C and N between the atmosphere and the ground in urban region.However,very few studies have been conducted on greenhouse gases emissions/absorption fluxes of urban lawn ecosystem.This research investigates the emission/absorption fluxes of three primary types of greenhouse gases(CO₂,CH₄,N₂O)of urban lawn ecosystems in Shanghai,using static closed chamber sampling technique and gas chromatograph analysis.The species of the lawn are Zoysia japonica Steud,Ophiopogon japonicus and Fescue arrudinacea Schreh.The diurnal and seasonal variation of the greenhouse gasses fluxes and the influence of environmental factors are discussed,while a comparison of different types of city lawn as well as the natural grassland ecosystems are made.The main results are as follows:Diurnal and seasonal variation of CO₂ fluxes of Zoysia japonica Steud lawnIn dark chamber,CO₂ emission fluxes exhibited an obvious diurnal and seasonal variation.The values of the fluxes were all positive throughout the day and the lawns were sources of CO₂.Mean value and variability of the fluxes were higher during the daytime than that at night.The maximum values appeared at about noon when the temperature was high,while the minimum values appeared at night or dawn when the temperature was low.The diurnal mean values appeared between 10:00～12:00 AM. The maximum diurnal mean flux value was 1710.5 mg·m^-2·h^-1appeared in July,while the minimum was 78.0 mg·m^-2·h^-1appeared in January.During the observation,the average flux value was 619.4 mg·m^-2·h^-1,the maximum dally variation range was 1187.6 mg·m^-2·h^-1(1 July),and the minimum one was 89.2 mg·m^-2·h^-1(3 Jan.).The dally variation of the flux was different with season.It showed a double peak type in Spring(27 Apr.and 13 May),a single peak in summer(1 July and 30 Aug.),and multi peaks in winter(1 Jan.).The seasonal variation of the flux showed the maximum value appeared in summer and the minimum one in winter,which was caused by temperature and plant growth.CO₂ flux was different in light chamber with the dark one.In light chamber, during the growing season,the fluxes were negative during the daytime and the lawns were sinks of air CO₂.At night,the values were positive and the lawns were sources of CO₂ and the diurnal mean value of the flux was positive.The maximum of CO₂ absorption and emission of the lawns appeared at 10:00～14:00 and 18:00～20:00, respectively.The flux values changed from positive to negative at about 7 AM.,and from negative to positive at about 17 PM.During non-growth season(1 Jan.),the values of the fluxes were positive both in the daytime and at night,and hence the lawns were the sources of CO₂ during that period.Seasonal variations of the flux were consistent with that of dark chamber,but the mean value was much less than that of dark chamber because of photosynthesis of grasses.Diurnal and seasonal variation of N₂O fluxes of Zoysia japonica Steud lawnDiurnal variations of N₂O flux were different in dark and light chamber.During the growing season,the values of N₂O fluxes were positive in dark chamber and the lawns were the sources of N₂O.The mean value and the variation range of N₂O emission fluxes during the daytime were higher than that at night.The maximum values appeared at 10:00～14:00 AM,and the minimum values appeared at night or dawn when the temperature was low.The diurnal mean value appeared at 8:00～12:00 AM.The maximum daily variation range was 118.7μg·m^-2·h^-1(30 Aug.),the minimum was 59.2μg·m^-2·h^-1(3 Jan.).The diurnal patterns of the fluxes exhibited an obvious seasonal variation,that is,maximum value appeared in summer while minimum one in winter(3 Jan.).During the observation period,the fluxes fluctuated between -7.4μg·m^-2·h^-1and 152.9μg·m^-2·h^-1,and the mean value was 75.3μg·m^-2·h^-1.Diurnal and seasonal variation of CH₄ fluxes of Zoysia japonica Steud lawnCH₄ fluxes exhibited complicated diurnal and seasonal variations in both dark and light chambers.The diurnal variations and the daily average values were different in different days due to the influence of temperature and light intensity.During the observation period,the mean flux value of light and dark chamber was -13.85μg·m^-2·h^-1and -13.91μg·m^-2·h^-1respectively and the lawn was the weak sink of air CH₄. Influence of environmental factors on greenhouse gases fluxIn dark chamber,the diurnal variation of CO₂ fluxes correlated significantly with air and ground temperature,but weakly with air CO₂ concentration.It was concluded that air and ground temperature were the main factors that influenced the fluxes.Soil moisture showed less influence on the flux.While in light chamber,in addition to factors mentioned above,light intensity was another factor that affected the fluxes.For N₂O,temperature was the main factor that affected the fluxes in dark chamber,but the sensitivity of N₂O fluxes to temperature was different in different growing stages of lawns.In addition,the metabolic activity of plant,oxygen supply to the soil and the diffuse resistance of soil gas were the main factors in controlling the N₂O fluxes.While for light chamber,the diurnal variation of the fluxes correlated significantly with light intensity,air and ground temperature.With the increase of soil depth,the influence of ground temperature on the fluxes decreased.The relationship between the seasonal fluxes and temperature could be expressed by quadratic curve.For CH₄,correlation of diurnal variation of the fluxes with temperature in dark chamber reached the significant level except in some individual day.In light chamber, they were not significant at all.Relationship among the main greenhouse gases fluxesExcept for special weather process and in certain period of non-growth season, significant positive relationships were found between CO₂ fluxes and N₂O fluxes. 5the correlation coefficients were higher in dark chamber than that in light chamber. No significant correlations were existed between CO₂ and CH₄ fluxes and between CH₄ and N₂O fluxes as well.Greenhouse gases concentration above the surface of lawnsDiurnal variation of the greenhouse gases concentration above the urban lawn showed great differences from that of non-urban areas.The maximum value often appeared at rush hours.Seasonal change of gas concentration showed higher values in winter and lower values in summer,which was apparently influenced by human activity such as fossil fuel combustion.Comparison of the main greenhouse gases fluxes of different lawnsExcept special weather process,diurnal variation of CO₂ and N₂O fluxes was similar for different lawns.For CH₄,it was not the case.The order of CO₂ and N₂O flux values was:Ophiopogon japonicus lawn＞Zoysia japonica Steud lawn＞ Fescue arrudinacea Schreh lawn,and for CH₄ flux:Ophiopogon japonicus lawn＞Fescue arrudinacea Schreh lawn＞Zoysia japonica Steud lawn.Comparison of the fluxes between urban lawn and natural ecosystemUrban lawn in Shanghai was the source of CO₂ and N₂O and the emission intensity was slightly higher than natural grassland ecosystem in temperate zone. Urban lawn was a weak sink of CH₄,and the absorption intensity was slightly lower than similar natural ecosystem.