Ultraviolet-B radiation (UV-B, 280-315 nm) reaching the earth's surface has been increasing due to stratospheric ozone depletion during the last several decades. Elevated UV-B radiation influenced ecosystem properties and functional processes such as plant litter decomposition and the subsequent nutrient release. As a key process in nutrient and carbon cycling of terrestrial ecosystems, plant litter decomposition converts the products of photosynthesis to inorganic compounds, representing one of the primary sources of nutrients for plants, and both nutrients and energy for microbes. At the same time, stable soil organic matter is formed as part of the litter decomposition processes. Therefore, UV-B induced changes in litter decomposition could profoundly influence primary production, carbon storage, and carbon and nutrient fluxes between the soil and atmosphere. UV-B radiation has direct and indirect effects on plant litter decomposition. Direct effects of UV-B radiation result from UV-B exposure during litter decomposition while indirect effects are caused by UV-B exposure during plant growth that changes the litter quality and the subsequent decomposition of the litter. Elevated UV-B radiation may directly increase litter decomposition via enhanced lignin photodegradation or decrease litter decomposition through reducing the abundance and altering the community composition of decomposers, as well as indirectly accelerate or slow the rate of decomposition through changing the litter chemistry during plant growth, even though some studies have found no pronounced indirect effects. Currently, the majority of decomposition studies on the effect of elevated UV-B radiation focused on the indirect effects on the chemical composition and decomposition of herbaceous plant leaf litter. In contrast, the direct effect of UV-B radiation on the decomposition of and nutrient release from leaf litter of woody plants has been less studied.
Providing supplemental UV-B radiation with UV-B lamps and removing most of the solar UV-B reaching sample through the mylar filters are the two most common approaches when attempting to simulate and test the effects of enhanced UV-B levels associated with ozone depletion. Most of the studies were conducted in the Northern Hemisphere using UV-B lamps systems to stimulate enhanced UV-B conditions. But an unrealistic balance between UV-B, UV-A (315-400 nm), and visible (400-700 nm) radiation may exaggerate the effects of UV-B. Exclusion of solar UV-B through the use of mylar filters that absorb most of the incident solar UV-B provides a relatively simple mean to compare the influences of the current enhanced UV-B level vs. lower levels in the past.
The ozone change is also taking place in subtropical China with an average annual depletion of 0.27%. The effect of UV-B radiation on litter decomposition of woody plant was poorly understood especially in China. The leaf litter decomposition of Pinus massoniana, which is a common coniferous species in subtropical China, under ambient and reduced (by 22% under ambient) UV-B radiation conditions by filtering existing solar radiation with mylar filters was investigated with litterbag method during 12 months experiment in this study. The aim of our study is to assess the effect of the changes in UV-B radiation on leaf litter decomposition, lignin degradation, and nutrient (N, P and K) release from P. massoniana. The results showed that the leaf litter decomposition rate slowed 47.74% under reduced UV-B radiation than that under ambient UV-B. UV-B radiation is very significantly accelerated the leaf litter decomposition of P. massoniana (p<0.01), and promoted the release of P, K, C and lignin degradation, but had no obvious effect on release of N. The results indicated that UV-B radiation would speed up the nutrient cycle in P. massoniana forest and decrease the carbon store in litter layer of P. massoniana forest.