Emission Regular And Estimation Of Greenhouse Gases From Paddy-wheat Rotation Ecosystem In Chongming Island

In recent years, global warming has become a great threat, which also has become a major challenge to all human beings. The majority eminent scholars believe that greenhouse effect is an extremely significant reason of global warming, and the enhanced greenhouse effect is mainly due to the increasing concentrations of the global greenhouse gas caused by human activities. It is estimated that5%～20%CO2,15%～30%CH4,80%～90%N2O of the atmosphere are from soil each year, so farmland is the important source of greenhouse gas emissions. Above all, the emissions and absorption of CO2, CH4and N2O from farmland ecosystem, which has long been widespread concern at home and abroad as a key source of CH4and N2O, is very important to the research on greenhouse gas emission and global warming.This paper was financially supported by the Chongming special project of Shanghai Science and Technology (No.10DZ1200602). The paddy-wheat rotation ecosystem was selected as the research object and the experimental plots were located in perennial paddy-wheat rotation field of Chongming Island, Shanghai. Using static chamber technique to investigate the emission and consumption of CO2, CH4and N2O, this paper was aimed to observe the temporal variabilities of these three gases, explore the relationship between influence factors and greenhouse gas emissions. Based on actual measurement, the total emissions of greenhouse gases were preliminary estimated and the greenhouse effect of these three gases were assessed using global warming potentials (GWPs). Greenhouse gas emissions from paddy-wheat rotation ecosystem were simulated and calculated by DNDC model to discuss the applicability of DNDC model in Chongming. Main conclusions were as followings.(1) The significant emissions of CH4and N2O only appeared in paddy season, Interannual variation trend of CH4and N2O fluxes, which measured by opaque chamber and transparent chamber, was highly consistent. Generally paddy-wheat rotation ecosystem was the source of atmospheric N2O. And paddy field was atmospheric CH4source and wheat field was the sink of atmospheric CH4. Considering the effect of light (transparent chamber), paddy field also was the sink of atmospheric CO2, but wheat field was the source.(2) During the period of rice growth, the greenhouse gases fluxes had obvious seasonal variation. Considering the influence of light (transparent chamber), rice field was the weak emission source of atmospheric CO2in rice seedling stage, then with crop growth and environmental factors change, it gradually transformed into CO2absorption, and the peak of CO2absorption was observed in rice jointing stage. For CO2respiration emission (opaque chamber), the maximum flux and the minimum values respectively appeared in the early stages of seedling and jointing interim, and the CO2flux in rice jointing stage was higher than other growth stages.Except ripening stage, paddy fields were sources of CH4emission through the whole study period, and the peak appeared at tillering stage. Rice field was the sink of atmospheric N2O in seedling stage, but changed into the sources in other stages and the peak of N2O emission was observed at elongation stage.(3) In the period of winter-wheat growth, the net exchange of CO2emission (transparent chamber) was the weak source of atmospheric CO2in wheat seedling stage. Then it gradually transformed into atmospheric CO2sink in wheat earing-flowering stage, and the peak of CO2absorption was observed in rice jointing stage. Along with the growth of wheat into the milk-ripe stage the field became the emission source again. For CO2respiration emission (opaque chamber), the CO2emission was low in early growth period and CO2flux increased rapidly from wheat jointing stage to earing stage. The peak of CO2respiration emission was observed in wheat milk-ripe stage. Wheat fields were sources of CH4emission from wheatsowing stage to wheat reviving period, other stages were CH4sinks. The N2O emission was high in wheat early growth period, and the minimum values appeared in wheat earing-flowering stage. Overall, the emission and absorption of CH4and N2O were very small in wheat field, so they had little effect for the total greenhouse effect of farmland greenhouse gas emissions.(4) The CO2flux had obvious daily variation during the crop growth exuberant phase, in the daytime the ecosystem absorbed CO2from the atmosphere, while at night, it let out CO2to the atmosphere, and the fux varied according to the temperature and the intensity of the light radiation, which were different in the different growth stage of the crop. It’s mostly cloudy and rainy in Chongming, and the enviromental factors of the farmland were rather changeable, which caused that the diurnal variation of the CH4flux and the N2flux was really random, and the flux varied with non-regular waveform.(5) The temperature, soil moisture content, SOM, C/N, NH4+-N, NO3--N all in kind of affected the green house gas flux, except the pH. The correlation between the CH4、N2O、CO2respiration emission flux (opaque chamber) and the temperature of the environment, the different depth of the soil and the chamber was significant or extremely significant. Significant positive correlation between annual CH4flux variation and soil water content and N2O flux during wheat-growing period was found. Extremely significant positive correlation between CH4flux and the value of C/N was found during the period of crop-rotation of the grain (p<0.01); which also was found between the NO3--N content and the N2O flux value of the paddy field; moreover, there was significant positive correlation between the SOM, NH4+-N of plowing layer of the farmland and CH4flux (the former p<0.01, the latter p<0.05).(6) Both of the straw direct-application and burning-application to the field had obvious significance to the CO2emission during the growing season of the following winter wheat, while the significance to the N2O emission was not obvious. Whatever the absorption or emission of CH4of the winter wheat, the average flux value was on the small side and not much affected the total emission of the greenhouse gas during the growing season of the winter wheat. As known, the main source of the greenhouse gas emission was CO2, and the straw burning-application to the field can promote the CO2emissio, which means this kind of application made over1.59times more contribution than the direct-application, but the latter can promote the emission of CH4and N2O of the farmland.(7) The total emissions of greenhouse gases (CO2, CH4and N2O) were preliminary estimated. In the period of rice growth, the field was a sink of atmospheric CO2, and CH4and N2O were the main greenhouse gases in this study; while CO2was the dominant during the wheat growing season. The GWPs (Global Warming Potentials) of these three gases during periods of paddy and wheat were793kgCO2·hm-2and994.424kgCO2·hm-2, respectively. The applicability of DNDC model to simulate the greenhouse gas emission from paddy-wheat rotation ecosystem in our study area is still need to be validated by a great mass of experimental data. The results showed that the applicability of DNDC model in simulating the greenhouse gas emission fluxes from paddy-wheat rotation ecosystem in this study area should be validated in future study. On the bases of this study, DNDC model was simulated reasonably during the period of paddy, but it was not good enough during the period of winter-wheat.