Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.
The speciation of hexavalent uranium at the solid−solution interface was investigated. To experimentally identify the sorption equilibria, we characterized the structure of the surface complex formed by binding between uranyl ions and surface groups of solid matrixes. To understand the sorption mechanisms at a molecular level, we performed optical and X-ray photoelectron spectroscopies and X-ray absorption spectroscopy on uranyl ion loaded phosphate solids (LaPO4 and La(PO3)3). Two lanthanum phosphates were synthesized. The samples were contacted with aqueous uranyl solutions of pH values ranging from 1.0 to 4.0. Whatever the conditions, uranium surface coverage was always lower than 30% of the monolayer as measured by the proton-induced X-ray emission technique. The U 4f X-ray photoelectron spectra and the lifetime values of uranyl ions sorbed on the lanthanum monophosphate compound clearly evidence that this solid exhibits two different types of sorption sites, as well as lanthanum polytrioxophosphate. The nature of the site which interacts with uranyl ions also depends on the pH value for both solids. Moreover, the interaction of the uranyl ions and the phosphate solids in a nitrate medium leads to two different sorbed species: free aquo UO2 2+ ions and UO2(NO3)+ ions. The X-ray absorption spectroscopy performed on the sorbed samples gives evidence of the presence of an inner-sphere mononuclear polydentate surface complex.
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