[show abstract][hide abstract] ABSTRACT: Although ammonia-oxidizing bacteria (AOB) are likely to play a key role in the soil nitrogen cycle, we have only a limited understanding of how the diversity and composition of soil AOB communities change across ecosystem types. We examined 23 soils collected from across North America and used sequence-based analyses to compare the AOB communities in each of the distinct soils. Using 97% 16S rRNA sequence similarity groups, we identified only 24 unique AOB phylotypes across all of the soils sampled. The majority of the sequences collected were in the Nitrosospira lineages (representing 80% of all the sequences collected), and AOB belonging to Nitrosospira cluster 3 were particularly common in our clone libraries and ubiquitous across the soil types. Community composition was highly variable across the collected soils, and similar ecosystem types did not always harbor similar AOB communities. We did not find any significant correlations between AOB community composition and measures of N availability. From the suite of environmental variables measured, we found the strongest correlation between temperature and AOB community composition; soils exposed to similar mean annual temperatures tended to have similar AOB communities. This finding is consistent with previous studies and suggests that temperature selects for specific AOB lineages. Given that distinct AOB taxa are likely to have unique functional attributes, the biogeographical patterns exhibited by soil AOB may be directly relevant to understanding soil nitrogen dynamics under changing environmental conditions.
[show abstract][hide abstract] ABSTRACT: The relationship between ecosystem processes and species richness is an active area of research and speculation. Both theoretical and experimental studies have been conducted in numerous ecosystems. One finding of these studies is that the shape of the relationship between productivity and species richness varies considerably among ecosystems and at different spatial scales, though little is known about the relative importance of physical and biological mechanisms causing this variation. Moreover, despite widespread concern about changes in species' global distributions, it remains unclear if and how such large-scale changes may affect this relationship. We present a new conceptual model of how invasive species might modulate relationships between primary production and species richness. We tested this model using long-term data on relationships between aboveground net primary production and species richness in six North American terrestrial ecosystems. We show that primary production and abundance of non-native species are both significant predictors of species richness, though we fail to detect effects of invasion extent on the shapes of the relationship between species richness and primary production.
American Midland Naturalist 01/2009; · 0.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: Increased carbon storage in ecosystems due to elevated CO(2) may help stabilize atmospheric CO(2) concentrations and slow global warming. Many field studies have found that elevated CO(2) leads to higher carbon assimilation by plants, and others suggest that this can lead to higher carbon storage in soils, the largest and most stable terrestrial carbon pool. Here we show that 6 years of experimental CO(2) doubling reduced soil carbon in a scrub-oak ecosystem despite higher plant growth, offsetting approximately 52% of the additional carbon that had accumulated at elevated CO(2) in aboveground and coarse root biomass. The decline in soil carbon was driven by changes in soil microbial composition and activity. Soils exposed to elevated CO(2) had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme, which led to more rapid rates of soil organic matter degradation than soils exposed to ambient CO(2). The isotopic composition of microbial fatty acids confirmed that elevated CO(2) increased microbial utilization of soil organic matter. These results show how elevated CO(2), by altering soil microbial communities, can cause a potential carbon sink to become a carbon source.
Proceedings of the National Academy of Sciences 04/2007; 104(12):4990-5. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: There is growing interest in understanding the linkages between above- and belowground communities, and very little is known about these linkages in tropical systems. Using an experimental site at La Selva Biological Station, Costa Rica, we examined whether plant diversity, plant community composition, and season influenced microbial communities. We also determined whether soil characteristics were related to differences in microbial communities. Phospholipid fatty acid (PLFA) composition revealed that microbial community composition differed across a plant diversity gradient (plots contained 1, 3, 5, or over 25 species). Plant species identity also was a factor influencing microbial community composition; PLFA composition significantly varied among monocultures, and among three-species combinations that differed in plant species composition. Differences among treatments within each of these comparisons were apparent in all four sampling dates of the study. There was no consistent shift in microbial community composition between wet and dry seasons, although we did see significant changes over time. Of all measured soil characteristics, soil C/N was most often associated with changes in microbial community composition across treatment groups. Our findings provide evidence for human alteration of soil microbial communities via the alteration of plant community composition and diversity and that such changes are mediated in part by changes in soil carbon quality.
[show abstract][hide abstract] ABSTRACT: Soil microorganisms mediate many critical ecosystem processes. Little is known, however, about the factors that determine
soil microbial community composition, and whether microbial community composition influences process rates. Here, we investigated
whether aboveground plant diversity affects soil microbial community composition, and whether differences in microbial communities
in turn affect ecosystem process rates. Using an experimental system at La Selva Biological Station, Costa Rica, we found
that plant diversity (plots contained 1, 3, 5, or > 25 plant species) had a significant effect on microbial community composition
(as determined by phospholipid fatty acid analysis). The different microbial communities had significantly different respiration
responses to 24 labile carbon compounds. We then tested whether these differences in microbial composition and catabolic capabilities
were indicative of the ability of distinct microbial communities to decompose different types of litter in a fully factorial
laboratory litter transplant experiment. Both microbial biomass and microbial community composition appeared to play a role
in litter decomposition rates. Our work suggests, however, that the more important mechanism through which changes in plant
diversity affect soil microbial communities and their carbon cycling activities may be through alterations in their abundance
rather than their community composition.
[show abstract][hide abstract] ABSTRACT: Ecologists have long sought to understand the relationships among species diversity, community productivity and invasion by non-native species. Here, four long-term observational datasets were analyzed using repeated measures statistics to determine how plant species richness and community resource capture (i.e. productivity) influenced invasion. Multiple factors influenced the results, including the metric used to quantify invasion, interannual variation and spatial scale. Native richness was positively correlated with non-native richness, but was usually negatively correlated with non-native abundance, and these patterns were stronger at the larger spatial scale. Logistic regressions indicated that the probability of invasion was reduced both within and following years with high productivity, except at the desert grassland site where high productivity was associated with increased invasion. Our analysis suggests that while non-natives were most likely to establish in species rich communities, their success was diminished by high resource capture by the resident community.
[show abstract][hide abstract] ABSTRACT: Ammonia-oxidizing bacteria (AOB) perform the rate-limiting step of nitrification, a key ecosystem process that in part determines the fate of nitrogen in ecosystems. However, little is known about the factors that determine soil AOB diversity or composition, especially in tropical systems. Using a set of study systems in Costa Rica, we examined whether plant diversity or land-use influenced AOB diversity or composition and whether AOB diversity or composition were associated with nitrification rates. We characterized the molecular diversity and composition of AOB via polymerase chain reaction amplification, cloning, and sequencing of 16S rDNA. We found that AOB diversity or composition did not change significantly across plant diversity treatments. In contrast, AOB differed among land-use types in some measures of diversity and in composition, and differences in AOB composition among land-use types were correlated with potential rates of nitrification. Our results suggest that anthropogenic changes of ecosystems can alter microbial communities in ways that may affect the processes they mediate.
[show abstract][hide abstract] ABSTRACT: Bacteria may be one of the most abundant and species-rich groups of organisms, and they mediate many critical ecosystem processes. Despite the ecological importance of bacteria, past practical and theoretical constraints have limited our ability to document patterns of bacterial diversity and to understand the processes that determine these patterns. However, recent advances in molecular techniques that allow more thorough detection of bacteria in nature have made it possible to examine such patterns and processes. Here, we review recent studies of the distribution of free-living bacterial diversity and compare our current understanding with what is known about patterns in plant and animal diversity. From these recent studies a preliminary picture is emerging: bacterial diversity may exhibit regular patterns, and in some cases these patterns may be qualitatively similar to those observed for plants and animals.
Proceedings of the Royal Society B: Biological Sciences 02/2004; 271(1535):113-22. · 5.68 Impact Factor