Emission Process Of Greenhouse Gases And Its Mechanism From The Paddy-wheat Rotation Agro-ecosystem

Nowadays, researchers pay more attention on the study of greenhouse gases emission which results in global change directly. Agro-ecosystem acts as an important source of greenhouse gases in both global and local view. Methane (CH4), nitrous oxide (N₂O) and carbon dioxide (CO₂) are three of the typical greenhouse gases. In paddy wetland ecosystem, CH4 emission is significant, and alternation of wetting and drying in the system will enhance the N₂O emission. In winter wheat season, N₂O emits remarkably. Decomposition of organic matter in the field soil will produce CO₂ into the atmosphere. This study sets a test field in paddy-wheat rotation fields in Shanghai suburb. The study was conducted to observe the dynamic characteristics of greenhouse gases in one year scale, elucidate the effect factors of emission especially the field management (irrigation and fertilizer application), analyze the C/N cycle in agro-ecosystem with greenhouse gases emission, estimate total amount of emissions in each crop growing season, and build the semi-empirical model to simulate the fluxes.In seasonal scale, paddy field was the source of CH4, N₂O and CO₂. Wheat field emitted N₂O and CO₂, while the CH4 flux was slight. In common field management (fertilizing plot) in paddy season, three gases emitted with a value of 4.21 mg·m^-2·h^-1 (mean value, the same below) for CH4, 84.77 μg·m^-2·h^-1 for N₂O, and 602.73 mg·m^-2·h^-1 for CO₂, and in wheat season, 0.14 mg·m^-2·h^-1 for CH₄, 94.24 μg·m^-2·h^-1 for N₂O, and 265.31 mg·m^-2·h^-1 for CO₂. In one year scale, there was a significant positive relationship between CH4 and CO₂, and CH4 was found a negative correlation with N₂O.The effect factors of greenhouse gases are temperature, water condition, physical and chemical characters of soil, plant condition, as well as field management, etc. Temperature controlled the fluxes of CH4 and CO₂ linearly. The extremum of CH₄ occurred while soil temperature at 5 cm was 30-33℃, and the extremum of N₂O 18～30℃. The thickness of surface water and water temperature significantly positive correlated with CH₄, and significantly negative correlated with N₂O during the paddy season. Because of the longtime flush irrigation in paddy season, the paddy soil in 4～12 cm kept a reduction state. In wheat season without irrigation, the soil became the oxidation state. The plant height remarkably correlated with CO₂ in paddy season,and remarkably correlated with CO₂ and N₂O in wheat season. Analyzing the data of fertilizing and control plot with the paired t-test method, there was a significant distinctness of CH₄ and CO₂ emission in different fertilizer application.The total amount of greenhouse gases emissions of paddy-wheat rotation agro-ecosystem in Shanghai suburb in common field management (fertilizing plot) was 1.5×10⁵ g·hm^-2 for CH₄, 3.0× 10³g·hm^-2 for N₂O, and 2.1 × 10⁷ g·hm^-2 for CO₂ in paddy season. And the total amount in wheat season was 7.1 × 10³ g·hm^-2 for CH₄, 4.8 × 10³ g·hm^-2 for N₂O, and 1.3 × 10⁷ g·hm^-2 for CO₂. Paddy season produced most of the CH₄ emission, and the CH₄ amount of wheat season was only 1.5-4.8% of paddy season. N₂O amount of wheat season was higher than of paddy season. CO₂ emission amount in two seasons were close in control plot, and in fertilizing plot the amount of paddy season was higher than of wheat season. N lost due to the N₂O emission was only 0.50% of the N fertilizer amount in paddy season and 0.48% in wheat season, which indicated that the N₂O emission contributed slightly in the N cycle of the agro-ecosystem.Based on principal component analysis of the effect factors of gas emission, we build three semi-empirical equations of CH₄, N₂O and CO₂ in paddy season, and two equations of N₂O and CO₂ in wheat season. The simulated greenhouse gases fluxes based on these models accorded with the measured values in the same order of magnitude and the same dynamic characteristics.