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Environmental context. Methane is an important greenhouse gas and its atmospheric concentration has drastically increased since pre-industrial times. Until recently biological methane formation has been associated exclusively with anoxic environments and microbial activity. In this article we discuss several alternative formation pathways of methane in aerobic environments and suggest that non-microbial methane formation may be ubiquitous in terrestrial and marine ecosystems. Abstract. Methane (CH4), the second principal anthropogenic greenhouse gas after CO2, is the most abundant reduced organic compound in the atmosphere and plays a central role in atmospheric chemistry. Therefore a comprehensive understanding of its sources and sinks and the parameters that control emissions is prerequisite to simulate past, present and future atmospheric conditions. Until recently biological CH4 formation has been associated exclusively with anoxic environments and methanogenic activity. However, there is growing and convincing evidence of alternative pathways in the aerobic biosphere including terrestrial plants, soils, marine algae and animals. Identifying and describing these sources is essential to complete our understanding of the biogeochemical cycles that control CH4 in the atmospheric environment and its influence as a greenhouse gas.

Quercus robur L. saplings were exposed in an outdoor experiment to supplemental levels of UV-8 (280-315 nm) radiation using treatment arrays of cellulose diacetate-filtered fluorescent lamps that also produce UV-A (315-400 nm) radiation. Saplings were also exposed to UV-A radiation alone using control arrays of the same lamps filtered with polyester and to ambient levels of radiation, using arrays of unenergized lamps. The UV-B treatment was modulated to maintain a 30% elevation above the ambient level of UV-B radiation, measured by a broad-band sensor weighted with an erythemal action spectrum. Saplings exposed to UV-B radiation beneath treatment arrays developed thicker leaves than those beneath ambient and control arrays. Despite the fact that supplemental levels of UV-A radiation were only a small percentage of ambient levels, apparent UV-A effects were also recorded. Significant increases in sapling height, lammas shoot length and herbivory by chewing insects were observed under treatment and control arrays, relative to ambient, but there were no differences between the responses of saplings under treatment and control. These data imply that supplemental UV-A radiation or other effects associated with energised lamps can significantly affect plant growth parameters and herbivory in outdoor studies. We conclude that the results from current outdoor UV-B supplementation experiments that lack control exposures using polyester-filtered lamps need to be interpreted with caution and that future supplementation experiments should include appropriate controls.

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