Studies On N₂O Emissions From Paddy Field Ecosystem In Sanjiang Plain

Nitrous Oxide (N₂O), which is a very important greenhouse gases in the atmosphere but known less at present, could produce the greenhouse effects and destroy the stratospheric ozone. The paddy field ecosystem is an important biological source of the atmospheric N₂O emissions. China is a country of the rice with a big amount of production, so the impacts on greenhouse effects of paddy field ecosystem have become one of the focuses which people pay attention to closely. Most of studies of the greenhouse effects on paddy field ecosystem are carried out in the South of China and less in the Northeast of China, especially, the region of paddy field of Sanjiang Plain in Heilongjiang Province, which is the very important grain producing region of the Northeast of China.In order to understand of N₂O emissions from the paddy field ecosystem of Sanjiang Plain in Heilongjiang Province of Northeast of China, a field experiment was conducted in situ for two years in Sanjiang Plain Experiment Station of Wetland Ecology of Chinese Academy of Sciences (CAS). In this study, the characteristic of N₂O emissions was investigated from the paddy field ecosystem, and the effects of N, crop plant, plant density and other environment factors on N₂O emissions were analyzed. Based on the area of paddy field in Sanjiang Plain, the amount of N₂O emissions was estimated from the paddy field ecosystem in this region, which offered the basis for estimating N₂O emissions accurately from farmland ecosystem and the scientific basis for sustainable development of agriculture and environment. The main conclusions of study are as follows:1. The paddy field ecosystem of Sanjiang Plain is the source of N₂O emissions, the range of N₂O fluxes is from -0.016 to 0.527 mg.m^-2.h^-1. The growth season is the main period of N₂O emissions from paddy field ecosystem and the average flux is 0.057±0.037 mg·m^-2·h^-1, which accounts for 69.2% of the total amount of N₂O emissions of the year. Compared with the growth period, N₂O flux reduced obviously in non-growth season and the average flux of N₂O is 0.016±0.007 mg·m^-2·h^-1, which is the 31.8% of the total amount of N₂O. Because of the long non-growth season, the N₂O emissions couldn't be ignored either.2. The N₂O emissions have obviously seasonal, yearly and annual variations from the paddy field ecosystem in this region. The N₂O emissions in summer and autumn are more than that in spring and winter. In the growth season, it has obvious emissions after using base manure, topdressing and in initial stage of draining off water. If there is no water on the surface of soil in flooding period, it will have the discharge peak of different intensity. The N₂O fluxes have certain differences in different years in flooding and drainage period, which mainly depend on the amounts of emission peak and the duration, but most of the time in rice growth season is flooded, the amount of N₂O emissions in flooding period is more than in drainage period. According to the growth stage of rice, the N₂O flux and the amount of N₂O emissions in tillering stage and in earring stage is relative higher than that in other stages, The N₂O flux and amount of N₂O emissions in ripe-drainage stage is the lowest. The N₂O flux is obviously low in non-growth season, it discharges faintly in icy and presents the phenomenon of absorbing N₂O while the snow covering. When the ice and snow is melted, The N₂O emissions were increased obviously, so it could not be ignored during freezing and thawing. For diurnal variation of N₂O fluxes, it is always higher in daytime than in nighttime, there is no rule in the flooding stage, but it is identical with the air temperature and soil temperature variation in drainage stage.3. Seasonal variation of N₂O fluxes was controlled by the water management from paddy field ecosystem, but there is no prominent relevance between N₂O emissions and water depth of paddy field. The temperature is a decisive factor of N₂O emissions in different month of the year and it is the main influence factor for diurnal variation of N₂O fluxes.4. Manuring is an important influence factor for N₂O emissions. Using fertilizer could not change the rule of N₂O emissions, but there are certain differences for intensity and quantity of the peak of N₂O emissions. There is little effect on N₂O emissions in the low nitrogen treatment, but the N₂O emissions were increased obviously in middle and high nitrogen treatments. According to the experimental data, the background value of N₂O emissions from paddy field ecosystem in Sanjiang Plain is 1.192kgN·hm^-2 in the growth period, 0.677kgN'hm^-2 in the non-growth period and 1.869kgN.hm^-2 for the year respectively. When the nitrogen consumption is 150kgN·hm^-2, the proportions of N₂O emissions of the total amount in growth period caused by nitrogen and Effect Factor (EF) are 41.29% and 0.59% respectively. When the nitrogen consumption is 250kgN.hm^-2, the proportions of N₂O emissions of the total amount in growth period caused by nitrogen and Effect Factor (EF) are 57.52% and 0.81%. Considering rice output and environmental protection synthetically, the nitrogen consumption of 150kgN·hm^-2 applied is better than the others.5. The N₂O emissions of paddy field ecosystem were influenced obviously by the rice plant. There is an apparent relevance among the leaf area indexes (LAL), over-ground biomass of rice and N₂O emissions. The N₂O emissions from bare soil are high in drainage period, and it presents the unimodal type. Compared with the bare soil, because of the nitrogen absorbed by the rice plant, the N₂O emissions were decreased obviously from paddy field ecosystem and its number is only one fourth of that from bare soil. There are obvious differences among different treatments of rice densities. Compared with normal planting density, the output and biomass of rice were increased and the N₂O emissions were decreased in appropriately sparse planting. Comprehensive analysis, the planting density of 16 caves per square meter is relatively better.6. According to the area of paddy field and the experimental data of N₂O emissions in Sanjiang Plain, the average N₂O flux and the total amount of N₂O emissions per year are 2.98 kg-hm^-2·a^-1 and 3.17×10^-3Tg·a^-1 respectively.