Mechanism Of N₂O Emissions And Respiration In Soil-Crop Systems Under UV-B Radiation

The increasing concentration of greenhouse gases in the atmosphere contributes to global wanning and ozone depletion that increase the intensity of UV-B radiation at the ground. It is well documented that enhanced UV-B radiation has impacts on crop physiological and ecological characteristics and soil microbial activities, which is closely associated with N₂O emissions and soil respiration. Hence, enhanced UV-B radiation may also have an indirect effect on N₂O emissions and respiration from soil-crop systems. To test this hypothesis, different experimental approaches (outdoor experiments and indoor simulation experiments, laboratory incubation) were used to explore the mechanism of UV-B radiation effects on N₂O emissions and respiration from various soil-crop systems.The results of this study are presented as follows:1. Effects of UV-B radiation on N₂O emissions and respiration from soil-crop systems(1). UV-B radiation had no significant impact on N₂O emission and respiration of soil-winter wheat system during the turning-green stage. While enhanced UV-B radiation reduced both respiration rate and N₂O emissions during the elongation-pregnant stage. From latter of elongation-pregnant to heading, there was a significant difference in the respiration rate but no significant difference in N₂O emissions under enhanced UV-B radiation compared to ambient radiation.(2). UV-B radiation had no significant impact on N₂O emissions from soil-soybean systems during growing season. However, during earlier stages of UV-B treatment, there was no significant difference in respiration rate under UV-B radiation; from latter of branching stage to flowering stage, respiration rate was significantly decreased by UV-B radiation; but in pod period of soybean, the influence of UV-B on respiration rate was not pronounced. (3). UV-B radiation had no significant impact on N₂O emission and respiration rate of soil.2. The mechanism of UV-B radiation effects on N₂O emission and respiration rate from soil-crop systems(1). An increase in UV-B radiation could significantly decrease the aboveground dry matter weight of winter wheat during elongation-pregnant and heading stages and the plant-mediated respiration rate in the former stage, but it had no impact on respiratory coefficient during the both stages. There was less content of chlorophyll a and more carotenoid content in UV-treated leaves than in the untreated ones during both stages. Also, the UV-treated leaves contained less soluble sugar content during the elongation-pregnant stage. Although there was no change in nitrate contents and activities Of nitrate reductase in the UV-treated leaves during the heading stage, but the UV treatment could increase glutamine synthetase activity in leaves significantly (P<0.05). Furthermore, the UV treatment increased concentrations of total phosphorus, total nitrogen during the elongation-pregnant stage, and increased soluble protein and free amino acid in leaves during both stages (P<0.1). Therefore, N metabolism of winter wheat could have been accelerated by an increase in UV-B radiation.(2). the effects of UV-B radiation on soil N₂O emissions may be through an indirect pathway. The results of this study showed that wheat plant growth under enhanced UV-B treatments significantly increased the soil available nitrogen, soil microbial biomass C and N, and also changed the ratio of soil microbial C:N (from5.0 to 6.8) in winter-wheat rhizosphere.3. To further study the decomposition of spring wheat straw affected by UV-B radiation from turning-green to maturity stage, laboratory incubations were carried out. We found that ADS, total phosphorus, total carbon and C:N were decreased but total nitrogen was increased by enhanced UV-B radiation in spring wheat straw.A significant increase in N₂O emissions was observed among fresh upland soils with wheat straw application whether the soils were fertilized or not; but it had no such impact in flooding treatments. In contrast, no changes in respiration rate were found in fresh upland soil and flooding treatments.In conclusion, the impacts of UV-B radiation on the emission of N₂O and respiration rate from soil-crop system were changed during growing periods. The impact mechanisms of enhanced UV-B radiation on N₂O emission and respiration rate from soil-wheat system may depend on changes of the nitrogen and carbon metabolism process of wheat. It may also be related with soil microbes in winter-wheat rhizosphere. UV-B could change chemical constituents of spring wheat straw under UV-B radiation from turning-green to maturity stage, and could promote N₂O emissions from fresh upland soils with spring wheat straw incorporation.