A decline in the availability of nitrogen (N) for plant growth (progressive nitrogen limitation or PNL) is a feedback that could constrain terrestrial ecosystem responses to elevated atmospheric CO 2 . Several long-term CO 2 enrichment experiments have measured changes in plant and soil pools and fluxes consistent with PNL but evidence for PNL in grasslands is limited. In an 11 year Free Air CO 2 Enrichment (FACE) experiment on grazed grassland we found the amount of N harvested in aboveground plant biomass was greater at elevated CO 2 but declined over time to be indistinguishable from ambient after 5 years. Re-wetting after a major drought resulted in a large input of N from mineralisation and a return to a higher N harvested under elevated CO 2 followed by a further decline. Over these two periods the amount of N in soil significantly increased at elevated CO 2 . Data from mesocosms introduced into the rings at intervals, and therefore having different lengths of exposure to CO 2 , showed plant N availability declined at elevated CO 2 reaching a new equilibrium after 6 years of exposure. We conclude that the availability of N for plants in this grassland is dynamic but the underlying trend at elevated CO 2 is for PNL.

Our limited understanding of terrestrial ecosystem responses to elevated CO2 is a major constraint on predicting the impacts of climate change. A change in botanical composition has been identified as a key factor in the CO2 response with profound implications for ecosystem services such as plant production and soil carbon storage. In temperate grasslands, there is a strong consensus that elevated CO2 will result in a greater physiological stimulus to growth in legumes and to a lesser extent forbs, compared with C3 grasses, and the presumption this will lead in turn to a greater proportion of these functional groups in the plant community. However, this view is based on data mainly collected in experiments of three or less years in duration and not in experiments where defoliation has been by grazing animals. Grazing is, however, the most common management of grasslands and known in itself to influence botanical composition. In a long-term Free Air Carbon Dioxide Enrichment (FACE) experiment in a temperate grassland managed with grazing animals (sheep), we found the response to elevated CO2 in plant community composition in the first 5 years was consistent with the expectation of increased proportions of legumes and forbs. However, in the longer term, these differences diminished so that the proportions of grasses, legumes and forbs were the same under both ambient and elevated CO2 . Analysis of vegetation before and after each grazing event showed there was a sustained disproportionately greater removal ('apparent selection') of legumes and forbs by the grazing animals. This bias in removal was greater under elevated CO2 than ambient CO2 . This is consistent with sustained faster growth rates of legumes and forbs under elevated CO2 being countered by selective defoliation, and so leading to little difference in community composition.

Anaerobic enrichment cultures, isolated from arsenic-contaminated lake sediment in the Canadian sub-arctic and grown in five selective media, methylated arsenatef arsenite to produce mono-, di-and tri-methyl arsenicals. The extent of methylation and methylarsenic species produced varied with the type of enrichment. Iron-reducing, manganese-reducing, sulfate-reducing and broadspectrum anaerobic heterotrophic mixed cultures all produced methylarsenicals. Sulfate-reducing cultures produced higher concentrations of methylarsenicals (especially trimethyl species) than ironor manganese-reducers. There is evidence that several of the methylarsenicals, which were hydride-reactive at pH 6, were methylarsenic(II1) thiols. The organoarsenicals produced by enrichment cultures were the same as those detected in the porewater of the lake sediments used to initiate the enrichment cultures. Overall, this study demonstrates that microbes from anaerobic lake sediments can methylate (and demethylate) arsenic, a capability shared by manganese-, iron-, and sulfate-reducing microbial consortia.

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