Effects Of Enhanced Ultraviolet Radiation On Greenhouse Gases (CO₂, CH₄ And N₂O) Emission From Agroecosystems

Global warming caused mainly by increasing of greenhouse gas concentration in atmosphere, as well as enhanced ultraviolet radiation on the earth' surface caused by exhaustion of stratospheric ozone are two mostly concerned environmental problems. CO₂, CH₄ and N₂O are three most important greenhouse gases. CFCs and nitrogen oxide compound were discharged into atmosphere and induced the thinning of stratospheric ozone, then made ultraviolet radiation on the earth surface to enhance.Agroecosystem manipulated intensively by human played an important status in the global change. Enhanced ultraviolet radiation affected crop physiological characteristics and growth, affected the composition and activity of soil microorganism in top soil, thus would affect the product and emission of greenhouse gas (CO₂, CH₄ and N₂O) in agroecosystem. The quantitative investigation the influence of enhanced ultraviolet radiation on the emission of greenhouse gases from farmland will be helpful to understand the effect of enhanced ultraviolet radiation on greenhouse gas emissions under the scenario of ultraviolet radiation continues to strengthen in the future. Furthermore, this study will also provides the base to objective evaluate the ecological effect of ultraviolet radiation and to estimate the regional farmland greenhouse gas emission and its' long term tendency in farmland.The goal of research is to quantitatively investigate the influence of enhanced ultraviolet radiation on greenhouse gas (CO₂, CH₄ and N₂O) emission from farmland.Winter-wheat, soybean and paddy rice, conventional crops in our country, representing the monocotyledon crops and dicotyledon crops, the xerophil crops and paddy crops, the nitrogen fixation crops and non-nitrogen fixation crops, were selected as the experimental material. Outside pot experiments including two winter wheat growing seasons, three soybeans growing seasons and one paddy rice growth season were carried out. UV-B and UV-A experiments were established in outdoor. UV-B experiment had two horizontal conditions, which were control (CK, the natural light UV-B intensity) and treatment (T, UV-B intensity increased 20% compared to CK). And UV-A experiment had three horizontal conditions which were control (CK, the natural light UV-A intensity), treatment 1 (T₁, UV-A intensity increased 10% compared to CK) and treatment 2 (T₂, UV-A intensity increased 20% compared to CK). A "square wave system" method including UV-B lamps, UV-A lamps and Mylar membranes was used to simulate enhanced ultraviolet radiation. A static opaque chamber-gas chromatograph method was used to gather and analysis gas samples.Main results of this study are presented as follows:1. Enhanced UV-B did not change the seasonal variations of respiration rate of the winter wheat, soybean and rice-planted soil-crop system. However, enhanced UV-B reduced significantly average respiration rate of soil-soybean system compared with control, with the reduction of 49.03% (p=0.005) and 45.43% (p=0.002) respectively, during flowering-maturity stage in 2004 and 2006. The respiration rate for UV-B treatment of soil-winter wheat system was reduced by 21.39% (p=0.002) during turning-green stage and 16.36% (p=0.001) during elongation-pregnant stage, respectively, compared with control. Enhanced UV-B radiation had no significant effect on the respiration rate of soil-rice system.2. Enhanced UV-B radiation did not influnce the seasonal change patterns of N₂O emission fluxes from three soil-crop systems of soil-winter wheat, soil-soybean and soil-rice paddy. While enhanced UV-B radiation leaded to a sharp decrease of average N₂O emission flux from soil-soybean system compared with control, with 40.43% (p=0.009) and 44.93% (p=0.005) respectively, during flowering-maturity stage in 2004 and 2006. Average N₂O emission flux from winter-wheat system was reduced by 16.38% (0=0.015) in elongation-pregnant stage compared with control. Enhanced UV-B radiation did not change the seasonal patterns of CH4 fluxes from soil-rice system, and had no significant effect on N20 and CH4 emission flux (0=0.402, 0.324).3. Enhanced UV-B radiation had not changed CO2 and CH4 emission flux from bare soil, but enhanced UV-A radiation accelerated CO2 and CH4 emission. Enhanced 20% UV-A radiation made cumulative amount of CO₂ and N₂O increase 263.50% (p=0.038) and 186.48% (p=0.059), respectively, and made the cumulative amount of N20 of soil-winter wheat system increase 32.93% (0=01010), compared with control.4. Enhanced UV-B radiation reduced remarkably soybean's biomass. Harvested soybean biomass was reduced by 51.81% (p=0.020) in 2004 and 43.37% (p=0.000) in 2006, respectively, compared with control. Moreover, winter-wheat's biomass was reduced by 24.85% (0=0.037) during elongation-pregnant stage. However, enhanced UV-B radiation had no significant influence on paddy rice's biomass.5. Among the three kinds of crop types, the emission of greenhouse gas and crop's biomass that were influenced most significantly by enhanced UV-B radiation was soybean, next was the winter wheat, and the paddy rice was not been affected.