Diversity-function relationship of ammonia-oxidizing bacteria in soils among functional groups of grassland species under climate warming

Although warming and plant diversity losses have important effects on aboveground ecosystem functioning, their belowground effects remain largely unknown. We studied the impact of a 3 °C warming and of three plant functional groups (forbs, grasses, legumes) on ammonia-oxidizing bacteria (AOB) diversity (polymerase chain reaction-denaturing gradient gel electrophoresis, PCR-DGGE) and their function (potential nitrification) in artificial grasslands. Warming did not influence AOB diversity and function. Sequencing of 16S rRNA gene fragments retrieved from DGGE gel revealed that they were all related to Nitrosospira-like sequences. Clustering analysis of DGGE profiles resulted in two nodes, separating AOB community structure under legumes from all other samples. Decreased AOB richness (number of DGGE bands) and concurrent increased potential nitrification were also observed under legumes. We hypothesized that ammonium availability was the driving force regulating the link between aboveground and belowground communities, as well as the AOB diversity and function link. The results document that the physiology of AOB might be an important regulator of AOB community structure and function under plant functional groups. This study highlights the major role of the microbial community composition in soil process responses to changes in the functional composition of plant communities

How do climate warming and plant species richness affect water use in experimental grasslands?

Climate warming and plant species richness loss have been the subject of numerous experiments, but studies on their combined impact are lacking. Here we studied how both warming and species richness loss affect water use in grasslands, while identifying interactions between these global changes. Experimental ecosystems containing one, three or nine grassland species from three functional groups were grown in 12 sunlit, climate-controlled chambers (2.25 m(2) ground area) in Wilrijk, Belgium. Half of these chambers were exposed to ambient air temperatures (unheated), while the other half were warmed by 3 degrees C (heated). Equal amounts of water were added to heated and unheated communities, so that warming would imply drier soils if evapotranspiration (ET) was higher. After an initial ET increase in response to warming, stomatal regulation and lower above-ground productivity resulted in ET values comparable with those recorded in the unheated communities. As a result of the decreased biomass production, water use efficiency (WUE) was reduced by warming. Higher complementarity and the improved competitive success of water-efficient species in mixtures led to an increased WUE in multi-species communities as compared to monocultures, regardless of the induced warming. However, since the WUE of individual species was affected in different ways by higher temperatures, compositional changes in mixtures seem likely under climatic change due to shifts in competitiveness. In conclusion, while increased complementarity and selection of water-efficient species ensured more efficient water use in mixtures than monocultures, global warming will likely decrease this WUE, and this may be most pronounced in species-rich communities

Combined effects of climate warming and plant diversity loss on above- and below-ground grassland productivity

Projections of global change predict both increases of the surface temperature and decreases of biodiversity, but studies on the combined impact of both on terrestrial ecosystems are lacking. We assessed the impact of these two global changes on above- and below-ground productivity of grassland communities. Experimental ecosystems containing one, three or nine grassland species were grown in 12 sunlit, climate-controlled chambers in Wilrijk, Belgium. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3 degrees C. Equal amounts of water were added to heated and unheated communities, so that any increases in evapotranspiration due to warmer conditions would result in a drier soil. Warming led to a decreased productivity of both above-ground plant parts (-18%) and roots (-23%), which coincided with a significantly lower soil water content. Complementarity in resource use and/or facilitation slightly enhanced above-ground productivity in multi-species communities, regardless of the induced warming. Interactive effects between temperature treatment and species richness level were found below-ground, however, where warming nullified the positive effect of richness on root productivity. Future warmer conditions could further increase losses of productivity associated with declining species numbers. (c) 2006 Elsevier B.V. All rights reserved.Effects of biodiversity loss and climate warming on carbon sequestration mechanisms in terrestrial ecosystem

End-of-season effects of elevated temperature on ecophysiological processes of grassland species at different species richness levels

The combined effect of declining diversity and elevated temperature is a less-studied aspect of global change. We investigated the influence of those two factors and their possible interactions oil leaf ecophysiological processes in artificial grassland communities. Changes at the leaf level are at the basis of changes at the community level (and vice versa) but have remained largely unexplored in biodiversity experiments. We focused on end-of-season responses to assess whether species richness and air temperature affect the duration of the growing season. Grassland model ecosystems were used in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were Nvarnied 3 degrees C. Each chamber contained 24 plant communities, created with nine grassland species: three grass species. three nitrogen (N) fixers and three non-N-fixing dicots. Each plant community consisted of either one, three or nine species in order to create different species richness levels. Various ecophysiological variables (processes and characteristics) and above ground biomass were influenced by temperature. In several variables, the effects of temperature and species richness varied with species. No single-factor species richness effect was found due to opposite responses of the species canceling out the effect. We expect that these interactions may increase with time. (c) 2005 Elsevier B.V. All rights reserved

Biomass production in experimental grasslands of different species richness during three years of climate warming

Here we report on the single and combined impacts of climate warming and species richness on the biomass production in experimental grassland communities. Projections of a future warmer climate have stimulated studies on the response of terrestrial ecosystems to this global change. Experiments have likewise addressed the importance of species numbers for ecosystem functioning. There is, however, little knowledge on the interplay between warming and species richness. During three years, we grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers in Wilrijk, Belgium. Half of these chambers were exposed to ambient air temperatures (unheated), while the other half were warmed by 3 degrees C (heated). Equal amounts of water were added to heated and unheated communities, so that warming would imply drier soils if evapotranspiration was higher. Biomass production was decreased due to warming, both aboveground (-29%) and belowground (-25%), as negative impacts of increased heat and drought stress in summer prevailed. Complementarity effects, likely mostly through both increased aboveground spatial complementarity and facilitative effects of legumes, led to higher shoot and root biomass in multi-species communities, regardless of the induced warming. Surprisingly, warming suppressed productivity the most in 9-species communities, which may be attributed to negative impacts of intense interspecific competition for resources under conditions of high abiotic stress. Our results suggest that warming and the associated soil drying could reduce primary production in many temperate grasslands, and that this will not necessarily be mitigated by efforts to maintain or increase species richness