However, it is unclear how the difference in soil contact changes the effect of UV-B radiation on straw decomposition, nor is it known how UV-B radiation drives straw degradation. The effect of UV-B exposure on litter decomposition depends on the relative contribution of photodegradation and microbial decomposition. The proximity of litter to the soil surface promotes litter decomposition by offering enhanced microbial activity. Reports have suggested that litter placed on the soil surface may decompose faster than litter suspended aboveground without soil contact 23, 24. This may be due to the fact that soil acts as a vector for microbial colonization, while protecting the microorganisms from high temperatures and desiccation in this ecosystem. 22 found that the combination of litter and soil can reduce the direct effect of UV photodegradation and can ameliorate the development of soil-microbial films in the Santa Rita Experimental Range in SE Arizona, USA. UV-B radiation can directly influence litter decomposition on the soil surface by changing the microbial decomposer communities 20, 21 and biotic processes 18. The plant litter on the soil surface in an agroecosystem is exposed to solar radiation as a result, the microbes within the soil can also be influenced by UV-B exposure. Some studies have indicated that UV-B exposure does not accelerate litter decomposition rates 19, or even inhibits decomposition due to reduced microbial activity 8. ![]() Elevated UV-B radiation doses often increase the mass loss of litter 18, but not always. Photochemical mineralization is the direct breakdown of litter to CO 2 in the absence of microbial activity 11, 16 and microbial facilitation is the breakdown of large organic compounds by solar radiation into smaller compounds that can be utilized by microbes 5, 14, 17. Through photochemical mineralization, UV-B exposure can destroy the physical structure of the litter surface, while further facilitating microbial decomposition 9, 15. 14 estimated that 14–22% of the mass loss of leaf litter can be attributed to UV-B exposure based on a photodegradation experiment carried out over 4–5 months. A growing number of studies have suggested that UV-B exposure may contribute significantly to litter decomposition, resulting in increased loss of litter mass due to photodegradation 9, 10, 13. UV-B radiation, which is comprised of high-energy photons, can induce the photochemical mineralization of plant litter 11 and disrupt the structures of complex compounds in plant litter, especially those of lignin and polyphenols such as tannin 12. Ultraviolet-B (UV-B) irradiance is regarded as responsible for the vast majority of photodegradation 5, 6, 7, 8, 9, 10. However, photodegradation, which is the direct breakdown of litter by solar radiation, is generally known to contribute to litter decomposition rates and ultimately results in changes in C and nutrient cycling among the atmosphere, plants and soils 4. In terrestrial ecosystems, it is generally assumed that litter decomposition is driven solely by biotic processes 2, 3, while the role of abiotic processes in litter decomposition at the soil surface remains largely overlooked. ![]() Litter decomposition plays an important role in carbon (C) and nutrient cycling in the biosphere 1. These results indicate that we must take into account the effects of photodegradation when explaining the mechanisms of straw decomposition in mesic ecosystems. The difference in soil contact may influence the contribution of photodegradation to the overall straw decomposition process. Light exposure resulted in decreased O-alkyl carbons and increased alkyl carbons for both the wheat and maize straws compared with no-sunlight control. Elevated UV-B exposure decreased the decomposition rates of both wheat and maize straws when in contact with soil. Compared with the no-sunlight control, UV-B radiation increased the mass loss by 14–19% and the ambient radiation by 9–16% for wheat and maize straws without soil contact after 12 months. ![]() A block control with soil contact was not included. Wheat and maize straw samples with and without soil contact were exposed to three radiation levels: a no-sunlight control, ambient solar UV-B and artificially elevated UV-B radiation. We investigated the mass loss and chemical structures of straw decomposition in response to elevated UV-B radiation with or without soil contact over a 12-month litterbag experiment. The role of photodegradation, an abiotic process, has been largely overlooked during straw decomposition in mesic ecosystems.
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