Influence of rewetting on microbial communities involved in nitrification and denitrification in a grassland soil after a prolonged drought period.

The frequency of extreme drought and heavy rain events during the vegetation period will increase in Central Europe according to future climate change scenarios, which will affect the functioning of terrestrial ecosystems in multiple ways. In this study, we simulated an extreme drought event (40 days) at two different vegetation periods (spring and summer) to investigate season-related effects of drought and subsequent rewetting on nitrifiers and denitrifiers in a grassland soil. Abundance of the microbial groups of interest was assessed by quantification of functional genes (amoA, nirS/nirK and nosZ) via quantitative real-time PCR. Additionally, the diversity of ammonia-oxidizing archaea was determined based on fingerprinting of the archaeal amoA gene. Overall, the different time points of simulated drought and rewetting strongly influenced the obtained response pattern of microbial communities involved in N turnover as well as soil ammonium and nitrate dynamics. In spring, gene abundance of nirS was irreversible reduced after drought whereas nirK and nosZ remained unaffected. Furthermore, community composition of ammonia-oxidizing archaea was altered by subsequent rewetting although amoA gene abundance remained constant. In contrast, no drought/rewetting effects on functional gene abundance or diversity pattern of nitrifying archaea were observed in summer. Our results showed (I) high seasonal dependency of microbial community responses to extreme events, indicating a strong influence of plant-derived factors like vegetation stage and plant community composition and consequently close plant-microbe interactions and (II) remarkable resistance and/or resilience of functional microbial groups involved in nitrogen cycling to extreme weather events what might indicate that microbes in a silty soil are better adapted to stress situations as expected.

Management versus site effects on the abundance of nitrifiers and denitrifiers in European mountain grasslands.

It is well established that the abundances of nitrogen (N) transforming microbes are strongly influenced by land-use intensity in lowland grasslands. However, their responses to management change in less productive and less fertilized mountain grasslands are largely unknown. We studied eight mountain grasslands, positioned along gradients of management intensity in Austria, the UK, and France, which differed in their historical management trajectories. We measured the abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) as well as nitrite-reducing bacteria using specific marker genes. We found that management affected the abundance of these microbial groups along each transect, though the specific responses differed between sites, due to different management histories and resulting variations in environmental parameters. In Austria, cessation of management caused an increase in nirK and nirS gene abundances. In the UK, intensification of grassland management led to 10-fold increases in the abundances of AOA and AOB and doubling of nirK gene abundance. In France, ploughing of previously mown grassland caused a 20-fold increase in AOA abundance. Across sites the abundance of AOB was most strongly related to soil NO3--N availability, and AOA were favored by higher soil pH. Among the nitrite reducers, nirS abundance correlated most strongly with N parameters, such as soil NO3--N, microbial N, leachate NH4+-N, while the abundance of nirK-denitrifiers was affected by soil total N, organic matter (SOM) and water content. We conclude that alteration of soil environmental conditions is the dominant mechanism by which land management practices influence the abundance of each group of ammonia oxidizers and nitrite reducers.

Using plant traits to explain plant-microbe relationships involved in nitrogen acquisition.

It has long been recognized that plant species and soil microorganisms. are tightly linked, but understanding how different species vary in their effects on soil is currently limited. In this study, we identified those. plant characteristics (identity, specific functional traits, or resource acquisition strategy) that were the best predictors of nitrification and denitrification processes. Ten plant populations representing eight species collected from three European grassland sites were chosen for their contrasting plant trait values and resource acquisition strategies. For each individual plant, leaf and root traits and the associated potential microbial activities (i.e., potential denitrification rate [DEA], maximal nitrification rate [NEA], and NH4+ affinity of the microbial community [NHScom]) were measured at two fertilization levels under controlled growth conditions. Plant traits were powerful predictors of plant-microbe interactions, but relevant plant traits differed in relation to the microbial function studied. Whereas denitrification was linked to the relative growth rate of plants, nitrification was strongly correlated to root trait characteristics (specific root length, root nitrogen concentration, and plant affinity for NH4+) linked to plant N cycling. The leaf economics spectrum (LES) that commonly serves as an indicator of resource acquisition strategies was not correlated to microbial activity. These results suggest that the LES alone is not a good predictor of microbial activity, whereas root traits appeared critical in understanding plant-microbe interactions.

Nitrogen turnover in soil and global change.

Nitrogen management in soils has been considered as key to the sustainable use of terrestrial ecosystems and a protection of major ecosystem services. However, the microorganisms driving processes like nitrification, denitrification, N-fixation and mineralization are highly influenced by changing climatic conditions, intensification of agriculture and the application of new chemicals to a so far unknown extent. In this review, the current knowledge concerning the influence of selected scenarios of global change on the abundance, diversity and activity of microorganisms involved in nitrogen turnover, notably in agricultural and grassland soils, is summarized and linked to the corresponding processes. In this context, data are presented on nitrogen-cycling processes and the corresponding microbial key players during ecosystem development and changes in functional diversity patterns during shifts in land use. Furthermore, the impact of increased temperature, carbon dioxide and changes in precipitation regimes on microbial nitrogen turnover is discussed. Finally, some examples of the effects of pesticides and antibiotics after application to soil for selected processes of nitrogen transformation are also shown.