Denitrification Enzyme Activity Research Articles

Preliminary results from monitoring of stream nitrogen concentrations, denitrification, and nitrification potentials in an urbanizing watershed in Xiamen, southeast China

As a part of the Long-term Urban Ecosystem Observation and Research Station in Xiamen (Xiamen LUEORS) with the Environmental Internet of Things (EIoT) as its technical support, southeast China by the Institute of Urban Environment, Chinese Academy of Sciences, long-term monitoring was established in an urbanizing watershed to understand the effects of urbanization on stream water quality. Six monitoring sites were established within forested headwater (Headwater), reservoir (Reservoir), agricultural (Agriculture, two locations), suburban residential (Residence), and estuarine (Estuary) locations within the Bantou reservoir watershed. Results from data collected in the whole year of 2012 show that in-stream total dissolved nitrogen (TDN) concentrations were significantly higher in the agricultural and residential sites and in the cool season. Although dissolved inorganic nitrogen (DIN) in stream water was dominated by -N rather than -N except in the headwater and estuary sites, -N concentrations were relatively greater in the residential and two agricultural sites than other sites. Dissolved organic nitrogen (DON) in stream water was high, comprising more than half of TDN in the forested headwater and estuary sites and nearly one third of TDN in other sites. Rain events (rainfall on or between the sampling dates) did not significantly influence nitrogen concentrations in stream water. Surface sediments from the different sites had similar denitrification enzyme activity (DEA), comparable to previous studies. Potential nitrification (potential ammonium oxidation, PAO) was significantly greater in the agricultural and residential sites (P < 0.05). These preliminary results suggest that land-use change has significant effects on watershed nitrogen dynamics in southeast China and that these effects differ somewhat from results observed in other parts of the world. The difference might be due to the ‘real’ urbanizing watershed in comparison with the stabilized watersheds with little development in the developed countries.

Denitrification in vadose zone material amended with dissolved organic matter from topsoil and subsoil

Limited information is available on the potential of subsoil and underlying alluvial gravels (vadose zone) to denitrify leached nitrate, though it is recognized that C availability is an important constraint. In this laboratory study, we used samples (<4 mm fraction) from subsoil (0.4–0.85 m depth) and alluvial gravel layers (1.2–4.8 m) derived from a site in Canterbury, New Zealand, to quantify denitrification potential and examine the denitrification response to inputs of dissolved organic C (DOC). As C sources, we used DOC extracted in either cold (20 °C) or hot (80 °C) water from topsoil and subsoil (assumed to represent C substrates likely to be transported into the subsoil in drainage). Measurements of potential denitrification enzyme activity (DEA, via samples amended with glucose and nitrate-N) showed the alluvial gravel samples had denitrification capability although rates were low (<5 μg N2O–N kg−1 h−1) compared with those reported for topsoils. The effect of the soil-derived DOC, at addition rates 16–215 mg kg−1, on denitrification was determined in a 7-d anaerobic incubation using acetylene to block reduction of N2O to N2. There was a lag of 24–36 h, during which N2O production was low, followed by a period of rapid increase (36–96 h), and a final phase (120–168 h) in which N2O concentrations remained relatively stable. During the rapid phase, the N2O production rate was very high (up to 440–500 μg N kg−1 h−1 at the highest DOC addition rate) and exceeded that measured in the 8-h DEA assay (a measure of indigenous denitrifier enzymes). The post-96 h decline in N2O production rate was attributed to depletion of available C. The temporal pattern of CO2–C production was similar to that of N2O, though the lag period was shorter (12–24 h). The ratio of N2O–N to CO2–C increased with time; the maximum was circa 0.3:1 at the highest addition rate of hot water extractable DOC. Our results suggest that, with input of DOC substrate, alluvial gravel materials could generate enough enzyme during a 1–2 day anaerobic period to denitrify significant quantities of nitrate. However, in situ denitrification in alluvial gravels may be low because of biophysical limitations such as low C inputs of bioavailable C and anaerobicity mostly confined to localized zones or lenses.

Spatial variability of soil microbial functioning in a tropical rainforest of French Guiana using nested sampling

Understanding the pattern in spatial distribution of soil microbial processes is critical to understand the environmental factors that regulate them as well as to scale up these processes to ecosystem. Soil samples from a 1ha tropical rainforest plot (Paracou, French Guiana) were analyzed according a nested sampling approach using different separation distances ranging from 0.4 to 40m. The variability of substrate induced respiration (SIR) and of denitrification enzyme activity (DEA) was characterized in relation to various soil properties (total C and N contents, NIRS related index of soil organic matter quality, SOMQ, and index of tree influence potential, IP). The variability of SIR and DEA was higher than that of environmental properties. The patterns of accumulated variance as a function of distance varied among the soil properties. The variability of SIR and DEA mainly occurred at small (1m) scale (and at the 10–40m-scales for SIR), probably reflecting the quality of litter input that results of the influence of local assemblage of different tree species, though changes in the soil N and C contents. Indeed, total soil C and N contents explained the microbial properties at every scale. Coefficients of codispersion showed that neither SOMQ nor IP did correlate with SIR and DEA, and confirmed that total C and N contents explained microbial properties in a scale dependent and complex manner. Such spatial dependency underlines the importance of soil heterogeneity in this tropical forest with implications for sampling strategies when studying the microbial processes and their response to disturbances.

Estimating Rates of Denitrification Enzyme Activity in Wetland Soils with Direct Simultaneous Quantification of Nitrogen and Nitrous Oxide by Membrane Inlet Mass Spectrometry

The microbial mediated process of denitrification is a major pathway for the removal of reactive nitrogen a pollutant in natural environments and waste treatment facilities. Denitrification potential, measured as denitrification enzyme activity (DEA), was quantified in novel short-term (4 h) anaerobic assays using a more sensitive and precise technique, membrane inlet mass spectrometry (MIMS) rather than the traditional headspace electron capture gas chromatography (GC-ECD) method. Using MIMS modifications made to the instrument and sample handling allowed for the simultaneous and direct measurement of reaction products nitrous oxide (N2O), a potent greenhouse gas, and the chemically unreactive dinitrogen (N2). Rate determinations were made from the slope of a linear curve generated by plotting increasing concentrations of the reaction products with time. Strong evidence for the validity of MIMS measured DEA rates was provided by showing consistent, linear accumulations of N2O or N2 and close agreement in rates from replicate reactions. Reactions were performed using wetland soils and cultures of Pseudomonas aeruginosa and P. chloroaphis that generated denitrification end products of N2 and N2O, respectively. Under acetylene inhibition P. aeruginosa produced the N2O end product at a rate equivalent to the rate obtained in the uninhibited reaction that produced N2. No significant (p>0.05) difference was observed between MIMS or headspace with GC-ECD, determined DEA in wetland soil reactions under acetylene inhibition. Because of anoxic conditions in the reaction vessels used with MIMS, detectable rates of N2O accumulation were only observed in acetylene blocked reactions or in cultures of P. chloroaphis. This method has potential applications ranging from near realtime wastewater treatment process measurements to field studies of nitrogen cycling. The continued development and application of these types of methods are needed to improve our understanding of the mechanisms regulating denitrification and its benign, N2, and harmful, N2O, end-products.

Abundance, diversity and spatio-temporal dynamics of nirS gene-harbouring denitrifiers in a potato field over the course of a growth season

The abundance and diversity of nirS-harbouring bacteria were evaluated in a potato field during a growth season using culture-independent techniques. A total of 182 operational taxonomical units were identified and most had low homology to known nirS sequences, which suggested the discovery of new denitrifiers. The diversity was significantly higher in the furrow, followed by the hill and the near-plant region and was inversely proportional to the denitrification enzyme activity. In contrast, the abundance was not altered by soil locations but was significantly lower at the end of the growth season.

Multiple small monthly doses of dicyandiamide (DCD) did not reduce denitrification in Waikato dairy pasture

The effectiveness of multiple small doses of the nitrification inhibitor dicyandiamide (DCD) to decrease denitrification under warm moist conditions was tested in a 1-year field trial on a grazed dairy pasture. DCD was applied approximately every 4 weeks as an aqueous spray onto ten replicate plots 3 days after rotational grazing by dairy cows. Each application was at the rate of 3 kg DCD ha−1, with a total annual application of 33 kg ha−1. Denitrification was assessed 5 days after each DCD application using the acetylene block method. At the end of the trial, the rate of degradation of DCD under summer conditions was measured. DCD significantly decreased the mean annual nitrate concentration by about 17%. Denitrification and denitrification enzyme activity were highly variable and no significant effect of DCD in decreasing denitrification was detected. In the summer month of December, DCD degraded rapidly with an estimated half-life of 5±3 days (mean and standard deviation).

Response of anaerobic carbon cycling to water table manipulation in an Alaskan rich fen

To test the effects of altered hydrology on organic soil decomposition, we investigated CO2 and CH4 production potential of rich-fen peat (mean surface pH = 6.3) collected from a field water table manipulation experiment including control, raised and lowered water table treatments. Mean anaerobic CO2 production potential at 10 cm depth (14.1 ± 0.9 μmol C g−1 d−1) was as high as aerobic CO2 production potential (10.6 ± 1.5 μmol C g−1 d−1), while CH4 production was low (mean of 7.8 ± 1.5 nmol C g−1 d−1). Denitrification enzyme activity indicated a very high denitrification potential (197 ± 23 μg N g−1 d−1), but net NO3− reduction suggested this was a relatively minor pathway for anaerobic CO2 production. Abundances of denitrifier genes (nirK and nosZ) did not change across water table treatments. SO42− reduction also did not appear to be an important pathway for anaerobic CO2 production. The net accumulation of acetate and formate as decomposition end products in the raised water table treatment suggested that fermentation was a significant pathway for carbon mineralization, even in the presence of NO3−. Dissolved organic carbon (DOC) concentrations were the strongest predictors of potential anaerobic and aerobic CO2 production. Across all water table treatments, the CO2:CH4 ratio increased with initial DOC leachate concentrations. While the field water table treatment did not have a significant effect on mean CO2 or CH4 production potential, the CO2:CH4 ratio was highest in shallow peat incubations from the drained treatment. These data suggest that with continued drying or with a more variable water table, anaerobic CO2 production may be favored over CH4 production in this rich fen. Future research examining the potential for dissolved organic substances to facilitate anaerobic respiration, or alternative redox processes that limit the effectiveness of organic acids as substrates in anaerobic metabolism, would help explain additional uncertainty concerning carbon mineralization in this system.

Soil properties are useful to examine denitrification function development in created mitigation wetlands

We investigated structural soil attributes, the development of denitrification potential (DP), and their relations in created and natural non-tidal freshwater wetlands in Virginia. Soil attributes included soil organic matter (SOM), total organic carbon (TOC), total nitrogen (TN), pH, gravimetric soil moisture (GSM), and bulk density (Db). A subset of soil attributes were analyzed across the sites, using Euclidean cluster analysis, resulting in three soil condition (SC) groups of increasing wetland soil development (i.e., SC1<SC2<SC3 less to more developed) as measured by accumulation of TOC and TN, the increase of GSM, and the decrease of Db. Denitrification enzyme activity (DEA) was measured for DP. DEA rates were somewhat different by wetland site, but with no age-based trajectory. No significant difference was seen in DEA rates by sampling period (p=0.06). However, DEA rates were clearly differentiated by SC groups, with the highest rates in SC3 followed by SC1 and lowest in SC2. The lowest DEA rates in SC2 seemed associated with a higher soil pH (∼6.6) in the group than that (∼5.3) of the other SC groups, but it needs a further investigation. The principal component analysis (PCA) of soil physicochemical properties and average DP showed the association between the development of denitrification function and the maturation of soil conditions in wetlands. The outcome of the study suggests that the use of a suite of simple soil properties may be useful to examine the development of denitrification function in created mitigation wetlands. The inclusion of soil properties in post-construction monitoring should be required to enhance our understanding and prediction of the functional development of created mitigation wetlands.

Denitrification Potential in Stormwater Control Structures and Natural Riparian Zones in an Urban Landscape

Humans have significantly altered urban landscapes, creating impervious surfaces, and changing drainage patterns that increase volume and velocity as well as frequency and timing of runoff following precipitation events. These changes in runoff have impaired streams and riparian areas that previously reduced watershed nitrogen (N) flux through uptake and denitrification. Stormwater control measures (SCM) are used most frequently to mitigate these hydrologic impacts. While SCM control runoff, their ability to remove N compared to natural riparian areas is not well-known. In this study we compared potential denitrification [as denitrification enzyme activity (DEA)] in five types of SCM (wet ponds, dry detention ponds, dry extended detention, infiltration basin, and filtering practices) and forested and herbaceous riparian areas in Baltimore, MD. DEA was higher in SCM (1.2 mg N kg(-1) hr(-1)) than in riparian areas (0.4 mg N kg(-1) hr(-1)). While DEA was highly correlated with soil moisture, organic matter, microbial biomass, and soil respiration areas across sites, it was always higher in SCM at equivalent levels of these variables. SCM appear to function as denitrification hotspots and, despite having similar microbial biomass, have higher potential denitrification than natural riparian areas.

Effects of salinity on the microbial removal of nitrate under varying nitrogen inputs within the marshland upwelling system

The marshland upwelling system (MUS) utilizes the natural properties of wetland soils to treat domestic wastewater injected into the marsh subsurface as the wastewater moves upwards and outwards from the injection site. The system is different from coarse media based wetland treatment systems common in Europe, though it relies on the same principles. A laboratory study was designed to simulate field conditions in order to investigate and quantify the removal of nitrogen from the wastewater by pumping wastewater into the bottom of cores and observing the changes as the wastewater moved upward to the surface. Two nitrogen treatments (100 mg NH4-N L−1 and 80 mg NH4-N L−1/20 mg NO3-N L−1) and two salinities (2 and 20‰) for each N treatment were studied. Dissolved organic carbon (DOC) demonstrated a removal efficiency of 90%, while NO3-N had a removal efficiency of > 99% throughout the 84 days of the study. Higher salinity had a temporary, significant lower removal of DOC, while nitrate removal was high and consistent over time. Microbial biomass C (MBC) and denitrification enzyme activity (DEA) were measured to determine the role of microbial processes within the MUS. Wastewater introduction increased microbial growth at the column surface, which led to increases in denitrification/nitrification coupling and net N loss, as estimated by DEA. Salinity and organic matter were found to have significant negative and positive impacts, respectively, on DEA rates and MBC. An understanding of the impacts of salinity on specific microbially-mediated N transformations is critical for improving the efficiency of the MUS in coastal environments to determine the long-term sustainability.

Responses of activities, abundances and community structures of soil denitrifiers to short-term mercury stress

The responses of activities, abundances and community structures of soil denitrifiers to mercury (Hg) stress were investigated through a short-term incubation experiment. Four soil treatments with different concentrations of Hg (CK, Hg25, Hg50, and Hg100, denoted as 0, 25,50, and 100 mg Hg/kg dry soil, respectively) were incubated for 28 days. Soil denitrification enzyme activity (DEA) was measured at day 3,7 and 28. The abundances and community structures of two denitrification concerning genes, nirS (cd1-nitrite reductase gene) and nosZ (nitrous oxide reductase gene), were analyzed using real-time PCR and denaturing gradient gel electrophoresis (DGGE). Results showed that soil DEA was significantly stimulated in the treatments of Hg25 and Hg50 compared with others at day 7. Meanwhile, no difference in the abundances of soil nirS and nosZ was found between Hg spiked treatments and CK, except the lower abundance of nirS (P < 0.05) in the Hg added treatments compared with that in the CK at day 28. The community structures of denitrifiers based on nirS gene presented obvious change at day 7 along with the Hg additions, however, no variation was found in all treatments based on the nosZ gene. The results indicated that Hg (Hg25 and Hg50) had a strongly short-term stimulation on soil DEA, and nirS gene is more sensitive than nosZ gene to Hg stress.

Temperature dependence of denitrification in phototrophic river biofilms

Denitrification is an ecosystem service of nitrogen load regulation along the terrestrial–freshwater–marine continuum. The present study documents the short-term temperature sensitivity of denitrification enzyme activity in phototrophic river biofilms as a typical microbial assemblage of this continuum. Denitrification measurements were performed using the acetylene inhibition method at four incubation temperatures: 1.1, 12.1, 21.2 and 30.9°C. For this range of temperature, N2O production could be fitted to an exponential function of incubation temperature, yielding mean (±standard error) activation energy of 1.42 (±0.24) eV and Q10 of 7.0 (±1.4). This first quantification of denitrification enzyme activity temperature dependence in phototrophic river biofilms compares with previous studies performed in soils and sediments. This demonstrates the high temperature dependence of denitrification as compared to other community-level metabolisms such as respiration or photosynthesis. This result suggests that global warming can unbalance natural community metabolisms in phototrophic river biofilms and affect their biogeochemical budget.

Denitrification Enzyme Activity in Swine Wastewater Effluent of a Nitrification/Denitrification Treatment System

Intensification of swine production in the U.S. and around the world requires advanced manure management. For swine manure management in the state of North Carolina, one system met all of the required advanced management criteria, and it was qualified as an environmentally superior technology. This investigation was part of the testing for this superior technology. The objectives of this investigation were to assess: (1) the denitrification enzyme activity (DEA) in the treatment systems homogenization tank and denitrification tank, and (2) the impact of the wastewater characteristics on this DEA. The DEA was measured by the acetylene inhibition method. Wastewater in the homogenization tank was fresh-flushed directly from the swine houses. Consequently, it was more concentrated than wastewaters in either the denitrification tank or typical swine wastewater lagoons; it had soluble biochemical oxygen demand (sBOD) of 676 mg L-1 and an electrical conductivity (EC) of 8.9 mS cm-1. However, the DEA in the homogenization tank was significantly limited by the low level of NO3-N, which was 0.1 mg L-1. Conversely, the DEA of the denitrification tank was limited by its lower level of carbon; it had only 53 mg L-1 sBOD. However, it had a NO3-N concentration of 150 mg L-1. When non-limiting glucose-C and NO3-N were added to the wastewaters of the homogenization and denitrification tanks, the homogenization tank had a significantly higher level of potential DEA: 17,943 vs. 10,055 mg N2O-N m-3 d-1, respectively. The DEA was generally well correlated by stepwise regression to the measured physiochemical characteristics. The findings of this investigation document that the DEA within this treated swine wastewater can be altered by manageable constituents of the processed swine wastewater, in particular soluble carbon and oxidized nitrogen.

Denitrification and Gas Emissions from Organic Soils under Different Water Table and Flooding Management

Draining the Florida Everglades for agricultural use has led to land subsidence and increased phosphorus (P) loads to the southern Everglades, environmental concerns that can be limited by controlling water table depth. The resulting anaerobic conditions in saturated soils may lead to increased denitrification and gaseous N2O release. Sugarcane was grown in lysimeters using four water table treatments, ranging from 40 to 16 cm in depth. Gas emissions were measured in April, July, and September using a photoacoustic gas analyzer. Denitrification enzyme activity (DEA) was performed on soil samples from the upper 15 cm, including non-limiting carbon and nitrogen conditions. No differences in N2O emissions or DEA were observed between water table treatments. However, emissions of N2O were higher in the spring, and CO2 emissions were greater in the late summer. Overall, incomplete DEA was very low in the soil samples, indicating that shallow water tables will not increase N2O emissions in these organic soils. Additions of C versus NO3 increased DEA, indicating that these organic soils are carbon limited. Neither the amoA nor nosZ genes were affected by water table management, but the abundance of the nosZ gene increased from April to September, while amoA gene abundance decreased.

Sedimentary Environment Influences the Effect of an Infaunal Suspension Feeding Bivalve on Estuarine Ecosystem Function

The suspension feeding bivalve Austrovenus stutchburyi is a key species on intertidal sandflats in New Zealand, affecting the appearance and functioning of these systems, but is susceptible to several environmental stressors including sedimentation. Previous studies into the effect of this species on ecosystem function have been restricted in space and time, limiting our ability to infer the effect of habitat change on functioning. We examined the effect of Austrovenus on benthic primary production and nutrient dynamics at two sites, one sandy, the other composed of muddy-sand to determine whether sedimentary environment alters this key species' role. At each site we established large (16 m2) plots of two types, Austrovenus addition and removal. In winter and summer we deployed light and dark benthic chambers to quantify oxygen and nutrient fluxes and measured sediment denitrification enzyme activity to assess denitrification potential. Rates of gross primary production (GPP) and ammonium uptake were significantly increased when Austrovenus was added, relative to removed, at the sandy site (GPP, 1.5 times greater in winter and summer; ammonium uptake, 8 times greater in summer; 3-factor analysis of variance (ANOVA), p<0.05). Denitrification potential was also elevated in Austrovenus addition plots at the sandy site in summer (by 1.6 times, p<0.1). In contrast, there was no effect of Austrovenus treatment on any of these variables at the muddy-sand site, and overall rates tended to be lower at the muddy-sand site, relative to the sandy site (e.g. GPP was 2.1 to 3.4 times lower in winter and summer, respectively, p<0.001). Our results suggest that the positive effects of Austrovenus on system productivity and denitrification potential is limited at a muddy-sand site compared to a sandy site, and reveal the importance of considering sedimentary environment when examining the effect of key species on ecosystem function.

Effects of soil aggregate size, moisture content and fertilizer management on nitrous oxide production in a volcanic ash soil

A laboratory incubation study was conducted to determine the effects of soil aggregate size, soil moisture content and manure application on nitrous oxide (N2O) production through nitrification and denitrification. In Southern Hokkaido, soil samples were taken from a mineral soil layer (2.5–10 cm) of a grassland to which chemical fertilizer and manure had been applied. The soil aggregates were air-dried and sieved with 4.5 mm and 2 mm sieves, and the soil moisture was adjusted to 60% and 80% of field water capacity (FWC). Immediately after moistening, incubation was initiated and lasted for 9 days at 20°C. Following the start of incubation, a flush of N2O, carbon dioxide (CO2) and nitric oxide (NO) was observed. Production of all gases was higher in larger aggregates from the manure-applied soil. Productions of CO2 and NO were not significantly influenced by soil moisture content, but N2O production was considerably higher in 80% FWC as compared with 60% FWC. Based on the results of the N2O–nitrogen (N)/NO–N ratio, the process of N2O production was mainly due to nitrification in 60% FWC and denitrification in 80% FWC. Soil chemical properties, especially ammonium–N –N), nitrate–N (NO3–N) and water extractable organic C (WEOC) and microbial biomass C (MBC) also changed immediately after moistening. These changes were higher in larger aggregates from the manure-applied soil. Potential denitrification enzyme activity (DEA) was significantly higher in larger aggregates from manure-applied soil with higher moisture content. The N2O production in both 60% and 80% FWC correlated significantly with MBC and DEA. Regardless of soil moisture conditions, MBC correlated significantly with DEA, WEOC consumption and apparent N mineralization. These facts suggest that larger soil aggregates could have quickly developed suitable internal conditions for microbial activity inside the aggregates and consequently stimulated N2O production through nitrification and denitrification.

Denitrification mitigates N flux through the stream–floodplain complex of a desert city

The Indian Bend Wash (IBW) flood-control project relies on a greenbelt to carry floods through Scottsdale, Arizona, USA. The greenbelt is characterized by a chain of shallow artificial lakes in a larger floodplain of irrigated turf, which has been protected from encroaching urban development. As such, this urban stream-floodplain complex can be divided into three subsystems: artificial lakes, channelized stream segments, and floodplain. We conducted experiments to evaluate which, if any, of these subsystems were important sites of denitrification, and to explore factors controlling denitrification rates. Denitrification enzyme activity (DEA) bioassays were conducted on sediments from eight lake and six stream segments as well as soil samples from eight floodplain transects. Mass-specific potential denitrification rates were significantly higher in lakes than in streams or floodplains. Nutrient limitation bioassays revealed that nitrate (NO3-) limited denitrification in lake sediments, a surprising finding given that NO3(-)-rich groundwater additions frequently raised lake NO3(-) concentration above 1 mg N/L. Experiments on intact lake cores suggested that denitrification was limited by the rate NO3(-) diffused into sediments, rather than its availability in overlying water. Floodplain denitrification was limited by water content, not NO3(-) or C, and irrigation of soils stimulated denitrification. We constructed a N budget for the IBW stream-floodplain complex based on our experimental results. We found that both lakes and floodplains removed large quantities of N, with denitrification removing 261 and 133 kg N ha(-1) yr(-1) from lake sediments and floodplain soils, respectively, indicating that lakes are hotspots for denitrification. Nevertheless, because floodplain area was >4.5 times that of lakes, floodplain soils removed nearly 2.5 times as much N as lake sediments. Given the desert's low annual precipitation, a finding that floodplain soils are active sites of denitrification might seem implausible; however, irrigation is common in urban landscapes, and it elevated annual denitrification in IBW. Based on our results, we conclude that construction of artificial lakes created hotspots while application of irrigation water created hot moments for denitrification in the stream-floodplain complex, demonstrating that management decisions can improve the ability of urban streams to provide critical ecosystem services like N retention.

Association of Earthworm-Denitrifier Interactions with Increased Emission of Nitrous Oxide from Soil Mesocosms Amended with Crop Residue

Earthworm activity is known to increase emissions of nitrous oxide (N(2)O) from arable soils. Earthworm gut, casts, and burrows have exhibited higher denitrification activities than the bulk soil, implicating priming of denitrifying organisms as a possible mechanism for this effect. Furthermore, the earthworm feeding strategy may drive N(2)O emissions, as it determines access to fresh organic matter for denitrification. Here, we determined whether interactions between earthworm feeding strategy and the soil denitrifier community can predict N(2)O emissions from the soil. We set up a 90-day mesocosm experiment in which (15)N-labeled maize (Zea mays L.) was either mixed in or applied on top of the soil in the presence or absence of the epigeic earthworm Lumbricus rubellus and/or the endogeic earthworm Aporrectodea caliginosa. We measured N(2)O fluxes and tested the bulk soil for denitrification enzyme activity and the abundance of 16S rRNA and denitrifier genes nirS and nosZ through real-time quantitative PCR. Compared to the control, L. rubellus increased denitrification enzyme activity and N(2)O emissions on days 21 and 90 (day 21, P = 0.034 and P = 0.002, respectively; day 90, P = 0.001 and P = 0.007, respectively), as well as cumulative N(2)O emissions (76%; P = 0.014). A. caliginosa activity led to a transient increase of N(2)O emissions on days 8 to 18 of the experiment. Abundance of nosZ was significantly increased (100%) on day 90 in the treatment mixture containing L. rubellus alone. We conclude that L. rubellus increased cumulative N(2)O emissions by affecting denitrifier community activity via incorporation of fresh residue into the soil and supplying a steady, labile carbon source.

Abundance, diversity and functional gene expression of denitrifier communities in adjacent riparian and agricultural zones

Lands under riparian and agricultural management differ in soil properties, water content, plant species and nutrient content and are therefore expected to influence denitrifier communities, denitrification and nitrous oxide (N(2) O) emissions. Denitrifier community abundance, denitrifier community structure, denitrification gene expression and activity were quantified on three dates in a maize field and adjacent riparian zone. N(2) O emissions were greater in the agricultural zone, whereas complete denitrification to N(2) was greater in the riparian zone. In general, the targeted denitrifier community abundance did not change between agricultural and riparian zones. However, nosZ gene expression was greater in the riparian zone than the agricultural zone. The community structure of nirS-gene-bearing denitrifiers differed in June only, whereas the nirK-gene-bearing community structure differed significantly between the riparian and the agricultural zones at all dates. The nirK-gene-bearing community structure was correlated with soil pH, while no significant correlations were found between nirS-gene-bearing community structure and soil environmental variables or N(2) O emissions, denitrification or denitrifier enzyme activity. The results suggested for the nirK and nirS-gene-bearing communities different factors control abundance vs. community structure. The nirK-gene-bearing community structure was also more responsive than the nirS-gene-bearing community structure to change between the two ecosystems.

Nitrification and Denitrification Gene Abundances in Swine Wastewater Anaerobic Lagoons

Although anaerobic lagoons are used globally for livestock waste treatment, their detailed microbial cycling ofN is only beginning to become understood. Within this cycling, nitrification can be performed by organisms that produce the enzyme ammonia monooxygenase. For denitrification, the reduction of nitrite to nitric oxide can be catalyzed by two forms of nitrite reductases, and N,O can be reduced by nitrous oxide reductase encoded by the gene nosZ The objectives of this investigation were to (i) quantify the abundance of the amoA, nirK, nirS, and nosZ genes; (ii) evaluate the influence of environmental conditions on their abundances; and (iii) evaluate their abundance relative to denitrification enzyme activity (DEA). Samples were analyzed via real-time quantitative polymerase chain reaction and collected from eight typical, commercial anaerobic, swine wastewater lagoons located in the Carolinas. The four genes assayed in this study were present in all eight lagoons. Their abundances relative to total bacterial populations were 0.04% (amoA), 1.33% (nirS), 5.29% (nirK), and 0.27% (nosZ). When compared with lagoon chemical characteristics, amoA and nirK correlated with several measured variables. Neither nirS nor nosZ correlated with any measured environmental variables. Although no gene measured in this study correlated with actual or potential DEA, nosZ copy numbers did correlate with the disparity between actual and potential DEA. Phylogenetic analysis ofnosZdid not reveal any correlations to DEA rates. As with other investigations, analyses of these genes provide useful insight while revealing the underlying greater complexity of N cycling within swine waste lagoons.