Effects of enhanced UV-B radiation on nitrogen transformation and functional biological properties in the rice rhizosphere

To clarify the effect of enhanced UV-B radiation on soil nitrogen transformation and its biological processes in paddy fields, the effects of enhanced ultraviolet-B radiation (0, 2.5, 5.0 and 7.5 kJ m(-2)) on soil nitrogen transformation and N2O emission, functional microbial quantity, enzyme activity, and gene expression during the growth period (tillering, jointing, booting and maturity stage) of rice were investigated in situ in a rice paddy field in the Yuanyang terraces, Yunnan Province, southwest China. The results show that UV-B radiation decreased the NH4+-N content, increased the NO3-N content in the rhizosphere, decreased N2O emissions from paddy fields, and significantly affected the properties of soil nitrogen transforming microorganisms. The 5.0 kJ m(-2) UV-B radiation increased the quantity of ammonifying bacteria and ammonia oxidizing bacteria by 18-160% and 34-151%, respectively, decreased the urease activity in the rice rhizosphere by 11.9-27%; and significantly increased the expression of AOB-amoA and nosZ genes. Thus, the enhanced UV-B radiation increased the ammonia-oxidizing bacteria and AOB-amoA and nosZ gene expression, promoted the transformation of NH4+-N to NO3-N in the rice rhizosphere, and reduced N2O emissions in the paddy field.