Oliver Spott

Nitrogen transforming community in a horizontal subsurface-flow constructed wetland

Constructed wetlands are important ecosystems with respect to nitrogen cycling. Here we studied the activity and abundance of nitrogen transforming bacteria as well as the spatial distribution of nitrification, anaerobic ammonium oxidation (anammox), and denitrification processes in a horizontal subsurface-flow constructed wetland. The functional genes of the nitrogen cycle were evenly distributed in a linear way along the flow path with prevalence at the superficial points. The same trend was observed for the nitrification and denitrification turnover rates using isotope labeling techniques. It was also shown that only short-term incubations should be used to measure denitrification turnover rates. Significant nitrate consumption under aerobic conditions diminishes nitrification rates and should therefore be taken into account when estimating nitrification turnover rates. This nitrate consumption was due to aerobic denitrification, the rate of which was comparable to that for anaerobic denitrification. Consequently, denitrification should not be considered as an exclusively anaerobic process. Phylogenetic analysis of hydrazine synthase (hzsA) gene clones indicated the presence of Brocadia and Kuenenia anammox species in the constructed wetland. Although anammox bacteria were detected by molecular methods, anammox activity could not be measured and hence this process appears to be of low importance in nitrogen transformations in these freshwater ecosystems.

Microbial communities along biogeochemical gradients in a hydrocarbon-contaminated aquifer

Micro-organisms are known to degrade a wide range of toxic substances. How the environment shapes microbial communities in polluted ecosystems and thus influences degradation capabilities is not yet fully understood. In this study, we investigated microbial communities in a highly complex environment: the capillary fringe and subjacent sediments in a hydrocarbon-contaminated aquifer. Sixty sediment sections were analysed using terminal restriction fragment length polymorphism (T-RFLP) fingerprinting, cloning and sequencing of bacterial and archaeal 16S rRNA genes, complemented by chemical analyses of petroleum hydrocarbons, methane, oxygen and alternative terminal electron acceptors. Multivariate statistics revealed concentrations of contaminants and the position of the water table as significant factors shaping the microbial community composition. Micro-organisms with highest T-RFLP abundances were related to sulphate reducers belonging to the genus Desulfosporosinus, fermenting bacteria of the genera Sedimentibacter and Smithella, and aerobic hydrocarbon degraders of the genus Acidovorax. Furthermore, the acetoclastic methanogens Methanosaeta, and hydrogenotrophic methanogens Methanocella and Methanoregula were detected. Whereas sulphate and sulphate reducers prevail at the contamination source, the detection of methane, fermenting bacteria and methanogenic archaea further downstream points towards syntrophic hydrocarbon degradation.

Oxygen availability and distance to surface environments determine community composition and abundance of ammonia-oxidizing prokaroytes in two superimposed pristine limestone aquifers in the Hainich region, Germany

We followed the abundance and compared the diversity of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in the groundwater of two superimposed pristine limestone aquifers located in the Hainich region (Thuringia, Germany) over 22 months. Groundwater obtained from the upper aquifer (12 m depth) was characterized by low oxygen saturation (0-20%) and low nitrate concentrations (0-20 μM), contrasting with 50-80% oxygen saturation and 40-200 μM nitrate in the lower aquifer (48 m and 88 m depth). Quantitative PCR targeting bacterial and archaeal amoA and 16S rRNA genes suggested a much higher ammonia oxidizer fraction in the lower aquifer (0.4-7.8%) compared with the upper aquifer (0.01-0.29%). In both aquifers, AOB communities were dominated by one phylotype related to Nitrosomonas ureae, while AOA communities were more diverse. Multivariate analysis of amoA DGGE profiles revealed a stronger temporal variation of AOA and AOB community composition in the upper aquifer, pointing to a stronger influence of surface environments. Parallel fluctuations of AOA, AOB, and total microbial abundance suggested that hydrological factors (heavy rain falls, snow melt) rather than specific physicochemical parameters were responsible for the observed community dynamics.

Gas entrapment and microbial N2O reduction reduce N2O emissions from a biochar-amended sandy clay loam soil

Nitrous oxide (N2O) is a potent greenhouse gas that is produced during microbial nitrogen transformation processes such as nitrification and denitrification. Soils represent the largest sources of N2O emissions with nitrogen fertilizer application being the main driver of rising atmospheric N2O concentrations. Soil biochar amendment has been proposed as a promising tool to mitigate N2O emissions from soils. However, the underlying processes that cause N2O emission suppression in biochar-amended soils are still poorly understood. We set up microcosm experiments with fertilized, wet soil in which we used 15N tracing techniques and quantitative polymerase chain reaction (qPCR) to investigate the impact of biochar on mineral and gaseous nitrogen dynamics and denitrification-specific functional marker gene abundance and expression. In accordance with previous studies our results showed that biochar addition can lead to a significant decrease in N2O emissions. Furthermore, we determined significantly higher quantities of soil-entrapped N2O and N2 in biochar microcosms and a biochar-induced increase in typical and atypical nosZ transcript copy numbers. Our findings suggest that biochar-induced N2O emission mitigation is based on the entrapment of N2O in water-saturated pores of the soil matrix and concurrent stimulation of microbial N2O reduction resulting in an overall decrease of the N2O/(N2O + N2) ratio.

Release of nitrous oxide and dinitrogen from a transition bog under drained and rewetted conditions due to denitrification: results from a [15N]nitrate-bromide double-tracer study.

Denitrification is well known being the most important nitrate-consuming process in water-logged peat soils, whereby the intermediate compound nitrous oxide (N2O) and the end product dinitrogen (N2) are ultimately released. The present study was aimed at evaluating the release of these gases (due to denitrification) from a nutrient-poor transition bog ecosystem under drained and three differently rewetted conditions at the field scale using a 15N-tracer approach ([15N]nitrate application, 30 kg N ha−1) and a common closed-chamber technique. The drained site is characterized by a constant water table (WT) of –30 cm (here referred to as D30), while rewetted sites represent a constant WT of –15 cm, a constant WT of 0 cm (i.e. waterlogged), and an initial WT of 0 cm (which decreased slightly during the experiment), respectively, (here referred to as R15, R0, and R0d, respectively). The highest N2O emissions were observed at D30 (291 µg N2O–N m−2 h−1) as well as at R0d (665 µg N2O–N m−2 h−1). At the rewetted peat sites with a constant WT (i.e. R15 and R0), considerably lower N2O emissions were observed (maximal 37 µg N2O–N m−2 h−1). Concerning N2 only at the initially water-logged peat site R0d considerable release rates (up to 3110 µg N2–N m−2 h−1) were observed, while under drained conditions (D30) no N2 emission and under rewetted conditions with a constant WT (R15 and R0) significantly lower N2 release rates (maximal 668 µg N2–N m–2 h−1) could be detected. In addition, it has been found that natural WT fluctuations at rewetted peat sites, in particular a rapid drop down of the WT, can induce high emission rates for both N2O and N2.

Effects of grass species and grass growth on atmospheric nitrogen deposition to a bog ecosystem surrounded by intensive agricultural land use

We applied a 15N dilution technique called “Integrated Total Nitrogen Input” (ITNI) to quantify annual atmospheric N input into a peatland surrounded by intensive agricultural practices over a 2-year period. Grass species and grass growth effects on atmospheric N deposition were investigated using Lolium multiflorum and Eriophorum vaginatum and different levels of added N resulting in increased biomass production. Plant biomass production was positively correlated with atmospheric N uptake (up to 102.7 mg N pot−1) when using Lolium multiflorum. In contrast, atmospheric N deposition to Eriophorum vaginatum did not show a clear dependency to produced biomass and ranged from 81.9 to 138.2 mg N pot−1. Both species revealed a relationship between atmospheric N input and total biomass N contents. Airborne N deposition varied from about 24 to 55 kg N ha−1 yr−1. Partitioning of airborne N within the monitor system differed such that most of the deposited N was found in roots of Eriophorum vaginatum while the highest share was allocated in aboveground biomass of Lolium multiflorum. Compared to other approaches determining atmospheric N deposition, ITNI showed highest airborne N input and an up to fivefold exceedance of the ecosystem-specific critical load of 5–10 kg N ha−1 yr−1.

Nitrate turnover in a peat soil under drained and rewetted conditions: results from a [15N]nitrate–bromide double-tracer study

Under natural conditions, peatlands are generally nitrate-limited. However, recent concerns about an additional N input into peatlands by atmospheric N deposition have highlighted the risk of an increased denitrification activity and hence the likelihood of a rise of emissions of the greenhouse gas nitrous oxide. Therefore, the aim of the present study was to investigate the turnover of added nitrate in a drained and a rewetted peatland using a [15N]nitrate–bromide double-tracer method. The double-tracer method allows a separation between physical effects (dilution, dispersion and dislocation) and microbial and chemical nitrate transformation by comparing with the conservative Br− tracer. In the drained peat site, low NO3− consumption rates have been observed. In contrast, NO3− consumption at the rewetted peat site rises rapidly to about 100% within 4 days after tracer application. Concomitantly, the 15N abundances of nitrite and ammonium in soil water increased and lead to the conclusion that, besides commonly known NO3− reduction to nitrite (i.e. denitrification), a dissimilatory nitrate reduction to ammonium has simultaneously taken place. The present study reveals that increasing NO3− inputs into rewetted peatlands via atmospheric deposition results in a rapid NO3− consumption, which could lead to an increase in N2O emissions into the atmosphere.

Analysis of the coexisting pathways for NO and N2O formation in Chernozem using the 15N-tracer SimKIM-Advanced model

The nitrogen (N) cycle consists of a variety of microbial processes. These processes often occur simultaneously in soils, but respond differently to local environmental conditions due to process-specific biochemical restrictions (e.g. oxygen levels). Hence, soil nitrogen cycling (e.g. soil N gas production through nitrification and denitrification) is individually affected through these processes, resulting in the complex and highly dynamic behaviour of total soil N turnover. The development and application of methods that facilitate the quantification of individual contributions of coexisting processes is a fundamental prerequisite for (i) understanding the dynamics of soil N turnover and (ii) implementing these processes in ecosystem models. To explain the unexpected results of the triplet tracer experiment (TTE) of Russow et al. (Role of nitrite and nitric oxide in the processes of nitrification and denitrification in soil: results from 15N tracer experiments. Soil Biol Biochem. 2009;41:785–795) the existing SimKIM model was extended to the SimKIM-Advanced model through the addition of three separate nitrite subpools associated with ammonia oxidation, oxidation of organic nitrogen (Norg), and denitrification, respectively. For the TTE, individual treatments with 15N ammonium, 15N nitrate, and 15N nitrite were conducted under oxic, hypoxic, and anoxic conditions, respectively, to clarify the role of nitric oxide as a denitrification intermediate during N2O formation. Using a split nitrite pool, this analysis model explains the observed differences in the 15N enrichments in nitric oxide (NO) and nitrous oxide (N2O) which occurred in dependence on different oxygen concentrations. The change from oxic over hypoxic to anoxic conditions only marginally increased the NO and N2O release rates (1.3-fold). The analysis using the model revealed that, under oxic and hypoxic conditions, Norg-based N2O production was the dominant pathway, contributing to 90 and 50 % of the total soil N2O release. Under anoxic conditions, denitrification was the dominant process for soil N2O release. The relative contribution of Norg to the total soil NO release was small. Ammonia oxidation served as the major pathway of soil NO release under oxic and hypoxic conditions, while denitrification was dominant under anoxic conditions. The model parameters for soil with moderate soil organic matter (SOM) content were not scalable to an additional data set for soil with higher SOM content, indicating a strong influence of SOM content on microbial N turnover. Thus, parameter estimation had to be re-calculated for these conditions, highlighting the necessity of individual soil-dependent parameter estimations.

Impact of potassium on plant uptake of non-exchangeable NH4 +-N

Aims and backgroundRelease of ‘non-exchangeable’ NH4+-N from interlayers of 2:1 clay minerals is postulated to depend not only on soil solution NH4+-N concentration but also on the concentration of K+ and Ca2+. Concentrations of all three cations are altered in rhizosphere compared to soil solution at larger distance from the root surface.MethodsNon-exchangeable NH4+-N pool was labelled with 15 N. Treatments including application of K+, Ca2+ and K+ + Ca2+ were established. In a compartment system approach we analysed changes in soil solution concentrations of 15NH4+-N, 15NO3−-N, K+ and Ca2+in situ at different distances from the root surface over time and related them to the release of non-exchangeable 15NH4+-N and uptake of 15 N by plants.Results and conclusionsThe 15 N enrichment in plant tissue was significantly lower in treatments with K+ application compared to those without. This was in line with smaller depletion of non-exchangeable 15NH4+-N in the rhizosphere for these treatments and also with lower 15 N abundance in soil solution NO3−-N fraction. Hence, K+ application hampered the release of NH4+ from the interlayers. A promoting effect of increasing Ca2+ concentrations on release of non-exchangeable NH4+-N could not be evaluated since the Ca2+ concentration in soil solution was largely controlled by small amounts of carbonate contained in the substrate and thus the addition of Ca2+ did not result in a relevant increase of soil solution Ca2+ concentration as originally intended.The use of 15 N to follow the fate of non-exchangeable NH4+-N proved very useful as it provides a higher sensitivity for all measured fractions compared to total N. However, as soil N fractions equilibrate with each other labelling one fraction exclusively is not possible.