Browsing by Subject "Ammoniakoxidierer (AOB)"

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    Linking microbial abundance and function to understand nitrogen cycling in grassland soils
    (2017) Regan, Kathleen Marie; Kandeler, Ellen
    This thesis characterized spatial and temporal relationships of the soil microbial community, the nitrogen cycling microbial community, and a subset of the nitrogen cycling community with soil abiotic properties and plant growth stages in an unfertilized temperate grassland. Unfertilized perennial grasslands depend solely on soil-available nitrogen and in these environments nitrogen cycling is considered to be both highly efficient and tightly coupled to plant growth. Unfertilized perennial grasslands with high plant diversity, such as ours, have also been shown to have higher soil organic carbon, total nitrogen, and microbial carbon; greater food web complexity; and more complex biological communities than more intensively managed grasslands or croplands. This made the choice of study plot especially well-suited for characterizing the relationships we sought to identify, and made it possible to detect spatial and temporal patterns at a scale that has heretofore been under-examined. The first study used a combination of abiotic, plant functional group, and PLFA measurements together with spatial statistics to interpret spatial and temporal changes in the microbial community over a season. We found that its overall structure was strongly related to the abiotic environment throughout the sampling period. The strength of that relationship varied, however, indicating that it was not constant over time and that other factors also influenced microbial community composition. PLFA analysis combined with principal components analysis made it possible to discern changes in abundances and spatial distributions among Gram-positive and Gram-negative bacteria as well as saprotrophic fungi. Modeled variograms and kriged maps of the changes in distributions of exemplary lipids of both bacterial groups also showed distinct differences in their distributions on the plot, especially at stages of most rapid plant growth. Although environmental properties were identified as the main structuring agents of the microbial community, components of those environmental properties varied over the season, suggesting that plant growth stage had an indirect influence, providing evidence of the complexity and dynamic nature of the microbial community in a grassland soil. The second study took the same analytical approach, this time applying it to abundances of key members of the soil nitrogen cycling community. Marker genes for total archaea and bacteria, nitrogen fixing bacteria, ammonia oxidizing archaea and bacteria, and denitrifying bacteria were quantified by qPCR. Potential nitrification activity and denitrifying enzyme activity were also determined. We found clear seasonal changes in the patterns of abundance of the measured genes and could associate these with changes in substrate availability related to plant growth stages. Most strikingly, we saw that small and ephemeral changes in soil environmental conditions resulted in changes in these microbial communities, while at the same time, process rates of their respective potential enzyme activities remained relatively stable. This suggests both short term niche-partitioning and functional redundancy within the nitrogen cycling microbial community. The seasonal changes in abundances we observed also provided additional evidence of a dynamic relationship between microorganisms and plants, an important mechanism controlling ecosystem nitrogen cycling. The third study determined spatial and temporal interactions between AOA, AOB and NOB. These steps are related in both space and time, as the ammonia-oxidizers provide the necessary substrate for nitrite-oxidizers. Using a combination of spatial statistics and phylogenetic analysis, our data indicated seasonally varying patterns of niche differentiation between the two bacterial groups, Nitrospira and Nitrobacter in April, but more homogeneous patterns by August which may have been due to different strategies for adapting to changes in substrate concentrations resulting from competition with plants. We then asked a further question: was the microbial structure at sampling sites with high NS gene abundances fundamentally different from those with low NS gene abundances? Using a phylogenetic approach, the operational taxonomic unit composition of NS was analyzed. Community composition did not change over the first half of the season, but by the second half, the relative proportion of a particular OTU had increased significantly. This suggested an intraspecific competition within the NS and the possible importance of OTU 03 in nitrite oxidation at a specific period of time. Observed positive correlations between AOA and Nitrospira further suggested that in this unfertilized grassland plot, the nitrification process may be predominantly performed by these groups, but is restricted to a limited timeframe.