Plant–microbe interactions and nitrogen dynamics during wetland establishment in a desert stream

In late-successional steady state ecosystems, plants and microbes compete for nutrients and nutrient retention efficiency is expected to decline when inputs exceed biotic demand. In carbon (C)-poor environments typical of early primary succession, nitrogen (N) uptake by C-limited microbes may be limited by inputs of detritus and exudates derived from contemporaneous plant production. If plants are N-limited in these environments, then this differential limitation may lead to positive relationships between N inputs and N retention efficiency. Further, the mechanisms of N removal may vary as a function of inputs if plant-derived C promotes denitrification. These hypotheses were tested using field surveys and greenhouse microcosms simulating the colonization of desert stream channel sediments by herbaceous vegetation. In field surveys of wetland (ciénega) and gravelbed habitat, plant biomass was positively correlated with nitrate (NO₃ ^?) concentration. Manipulation of NO₃ ^? in flow-through microcosms produced positive relationships among NO₃ ^? supply, plant production, and tissue N content, and a negative relationship with root:shoot ratio. These results are consistent with N limitation of herbaceous vegetation in Sycamore Creek and suggest that N availability may influence transitions between and resilience of wetland and gravelbed stable states in desert streams. Increased biomass in high N treatments resulted in elevated rates of denitrification and shifts from co-limitation by C and NO₃ ^? to limitation by NO₃ ^? alone. Overall NO₃ ^? retention efficiency and the relative importance of denitrification increased with increasing N inputs. Thus the coupling of plant growth and microbial processes in low C environments alters the relationship between N inputs and exports due to increased N removal under high input regimes that exceed assimilative demand.     

Denitrification rates and controlling factors in two agriculturally influenced temporary Mediterranean saline streams

In this study, we tested the hypothesis that agriculture, through its influence on water NO₃ ^?-N availability, would control denitrification in agriculturally influenced temporary saline streams, and that water salinity would not affect this process. We also tested the effect of summer drought on the denitrification process. We approached these objectives by estimating sediment denitrification (using the acetylene inhibition technique) in two temporary Mediterranean streams following an increased natural water salinity and agricultural gradient under pre- and post-drought conditions. During the pre-drought conditions, the water NO₃ ^?-N concentration was the main predictor of denitrification rates. Together with the water NO₃ ^?-N concentration, sediment redox conditions and water salinity appeared to be significant predictors, the latter showing a negative effect. During the post-drought, denitrification rates dropped significantly in both streams and no abiotic factors seemed to significantly influence this process. Our results suggest that high water salinity and drought affected negatively the stream-denitrifying capacity. This study highlights that stressors such as water salinity and hydrological intermittency should be considered in future stream management plans in order to preserve the role of streams on controlling the NO₃ ^?-N export, especially in the context of a warmer and drier climate.     

Influence of bioturbation on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in freshwater sediments

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO₃ ^?) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O₂, NO₃ ^?, NH₄ ⁺, CO₂, N₂O, and CH₄) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO₃ ^? concentrations (using N¹⁵ isotope tracing) interact to affect N cycling processes. At the lowest NO₃ ^? concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO₃ ^? concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O₂, NO₃ ^?, NH₄ ⁺ fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO₃ ^? concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.     

Enhanced Vadose Zone Nitrogen Removal by Poplar During Dormancy

A pilot-scale, engineered poplar tree vadose zone system was utilized to determine effluent nitrate (NO₃^?) and ammonium concentrations resulting from intermittent dosing of a synthetic wastewater onto sandy soils at 4.5°C. The synthetic wastewater replicated that of an industrial food processor that irrigates onto sandy soils even during dormancy which can leave groundwater vulnerable to NO₃^? contamination. Data from a 21-day experiment was used to assess various Hydrus model parameterizations that simulated the impact of dormant roots. Bromide tracer data indicated that roots impacted the hydraulic properties of the packed sand by increasing effective dispersion, water content and residence time. The simulated effluent NO₃^? concentration on day 21 was 1.2 mg-N L^?1 in the rooted treatments compared to a measured value of 1.0 ± 0.72 mg-N L^?1. For the non-rooted treatment, the simulated NO₃^? concentration was 4.7 mg-N L^?1 compared to 5.1 ± 3.5 mg-N L^?1 measured on day 21. The model predicted a substantial “root benefit” toward protecting groundwater through increased denitrification in rooted treatments during a 21-day simulation with 8% of dosed nitrogen converted to N₂ compared to 3.3% converted in the non-rooted test cells. Simulations at the 90-day timescale provided similar results, indicating increased denitrification in rooted treatments.     

Characteristics of Nitrate Reduction Using Fe (II) as Electron Donor in Activated Sludge

Static experiments were conducted to investigate the effects of environmental factors on nitrate (NO₃^?-N)-removal efficiency, such as NO₃^?-N loading, pH value, C/N ratio and temperature in activated sludge using Fe (II) as electron donor. The results demonstrated that the average denitrification rate increased from 1.25 to 2.23 mg NO₃^?-N/(L·h) with NO₃^?-N loading increased from 30 to 60 mg/L. When pH increased from 7 to 8, the concentration of NO₃^?-N and nitrite (NO₂^?-N) in effluent were all maintained at quite low levels. C/N ratio had little impact on denitrification process, i.e., inorganic carbon (C) source could still be enough for denitrification process with C/N ratio as low as 5. Temperature had a significant effect on the denitrification efficiency, and NO₃^?-N removal efficiency of 92.03%, 96.77%, 97.67% and 98.23% could be obtained with temperature of 25°C, 30°C, 35°C and 40°C, respectively. SEM, XRD and XRF analysis was used to investigate microscopic surface morphology and chemical composition of the denitrifying activated sludge, and mechanism of the nitrate-dependent anaerobic ferrous oxidation (NAFO) bacterias could be explored with this research.     

Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA

Denitrification is an important net sink for NO₃ ^? in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO₃ ^? concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO₃ ^? concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U _denit, in μmol N m^?2 h^?1), results of different methods for a given set of conditions commonly were in agreement within a factor of 2–3. At approximately constant temperature (~20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U _denit (~0–4,000 μmol N m^?2 h^?1) and stream NO₃ ^? concentration (~20–1,100 μmol L^?1) representing seasonal variation from spring high flow (high NO₃ ^?) to late summer low flow (low NO₃ ^?). The temporal dependence of U _denit on NO₃ ^? was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, U _denit ≈26 * [NO₃ ^?]^0.44 or U _denit ≈640 * [NO₃ ^?]/[180 + NO₃ ^?]; for a partially weighted dataset, U _denit ≈14 * [NO₃ ^?]^0.54 or U _denit ≈700 * [NO₃ ^?]/[320 + NO₃ ^?]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO₃ ^? concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO₃ ^? concentration. Hypothetical models based on our results illustrate: (1) U _denit was inversely related to denitrification rate constant (k1_denit, in day^?1) and vertical transfer velocity (v _f,denit, in m day^?1) at seasonal and possibly event time scales; (2) although k1_denit was relatively large at low flow (low NO₃ ^?), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U _denit; and (3) although NO₃ ^? assimilation and denitrification were linked through production of organic reactants, rates of NO₃ ^? loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1_denit and v _f,denit are linked implicitly with stream depth, NO₃ ^? concentration, and(or) NO₃ ^? load, estimates of U _denit may be related more directly to field factors (including NO₃ ^? concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U _denit and stream NO₃ ^? concentration and accounting for temporal variation.     

Denitrification and nitrous oxide effluxes in boreal, eutrophic river sediments under increasing nitrate load: a laboratory microcosm study

Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N₂) and nitrous oxide (N₂O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 μM of ¹⁵N enriched nitrate (NO₃ ^?, ~98 at. %). Effluxes of both N₂ and N₂O from sediment to the overlying water increased with increasing NO₃ ^? load. Although the ratio of N₂O to N₂ increased with increasing NO₃ ^? load, it remained below 0.04, N₂ always being the main product. At the NO₃ ^? concentrations most frequently found in the studied river water (10–100 μM), up to 8% of the NO₃ ^? was removed in denitrification, whereas with the highest concentration (300 μM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO₃ ^? was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO₃ ^? load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO₃ ^? was assimilated by microbes during their growth. The ¹⁵N content in ammonium (NH₄ ⁺) in the efflux was low, suggesting that reduction of NO₃ ^? to NH₄ ⁺ was not the reason for the NO₃ ^? removal. This study provides the first published information on denitrification and N₂O fluxes and their regulation by NO₃ ^? load in eutrophic high latitude rivers.     

Nitrate removal processes in a constructed wetland treating drainage from dairy pasture

¹⁵N-labelled NO₃^? was used in a surface-flow constructed wetland in spring to examine the relative importance of competing NO₃^? removal processes. In situ mesocosms (0.25 m²) were dosed with 2 l of ¹⁵NO₃^? (NaNO₃, 300 mg N l^?1, 99 atom% ¹⁵N) and bromide (Br^?) solution (LiBr, 4.3 g l^?1, as a conservative tracer). Concentrations of NO₃^?, Br^?, dissolved oxygen and ¹⁵N₂ were monitored periodically and replicate mesocosms were destructively sampled prior to and 6 days after ¹⁵N addition. Denitrification, immobilisation, plant uptake and dissimilatory NO₃^? reduction to NH₄⁺ (DNRA) accounted for 77, 11, 9 and 2% of ¹⁵NO₃^? transformed during the experiment. Only 6% of denitrification gases were directly measured as atmospheric or dissolved ¹⁵N₂; the remainder (71%) was determined via ¹⁵N mass balance. This indicated that a large proportion of the denitrification gases were entrapped within the soil matrix and/or plant aerenchyma. The floating plant Lemna minor exhibited a significantly higher NO₃^? uptake rate (221 mg kg^?1 d^?1) than Typha orientalis (10 mg kg^?1 d^?1), but periodic harvest of plants would remove <3% of annual NO₃^? inputs. Our results suggest that this 6-year-old constructed wetland functions effectively as a sink for NO₃^? during the growing season with less than one-quarter of the NO₃^? processed sequestered into wetland plant, algal and microbial N pools and the balance permanently removed by denitrification.     

Evaluation of denitrification in an urban stream receiving wastewater effluent

Urban streams often contain elevated concentrations of nitrogen (N) which can be amplified in systems receiving effluent from wastewater treatment plants (WWTP). In this study, we evaluated the importance of denitrification in a stream draining urban Greensboro, NC, USA, using two approaches: (1) natural abundance of ¹⁵N–NO₃⁻ in conjunction with background NO₃⁻–N concentrations along a 7 km transect downstream of a WWTP; and (2) C₂H₂ block experiments at three sites and at three habitat types within each site. Overall lack of a longitudinal pattern of δ¹⁵N–NO₃⁻ and NO₃⁻–N, combined with high concentrations of NO₃⁻–N suggested that other factors were controlling NO₃⁻–N flux in the study transect. However, denitrification did appear to be significant along one portion of the transect. C₂H₂ block experiments showed that denitrification rates were much higher downstream of the WWTP compared to upstream, and showed that denitrification rates were highest in erosional and depositional areas downstream of the WWTP and in erosional areas upstream of the plant. Thus, the combination of the two methods for evaluating denitrification provided more insight into the spatial dynamics of denitrification activity than either approach alone. Denitrification appeared to be a significant sink for NO₃⁻–N upstream of the WWTP, but not downstream. Approximately 46% of the total NO₃⁻–N load was removed via denitrification in the upstream, urban section of the stream, while only 2.3% of NO₃⁻–N was lost downstream of the plant. This result suggests that controlling NO₃⁻–N loading from the plant could result in considerable improvement of downstream water quality.     

Effects of urban stream burial on organic matter dynamics and reach scale nitrate retention

Nitrogen (N) retention in streams is an important ecosystem service that may be affected by the widespread burial of streams in stormwater pipes in urban watersheds. We predicted that stream burial suppresses the capacity of streams to retain nitrate (NO₃ ^?) by eliminating primary production, reducing respiration rates and organic matter availability, and increasing specific discharge. We tested these predictions by measuring whole-stream NO₃ ^? removal rates using ¹⁵NO₃ ^? isotope tracer releases in paired buried and open reaches in three streams in Cincinnati, Ohio (USA) during four seasons. Nitrate uptake lengths were 29 times greater in buried than open reaches, indicating that buried reaches were less effective at retaining NO₃ ^? than open reaches. Burial suppressed NO₃ ^? retention through a combination of hydrological and biological processes. The channel shape of two of the buried reaches increased specific discharge which enhanced NO₃ ^? transport from the channel, highlighting the relationship between urban infrastructure and ecosystem function. Uptake lengths in the buried reaches were further lengthened by low stream biological NO₃ ^? demand, as indicated by NO₃ ^? uptake velocities 17-fold lower than that of the open reaches. We also observed differences in the periphyton enzyme activity between reaches, indicating that the effects of burial cascade from the microbial to the ecosystem scale. Our results suggest that stream restoration practices involving “daylighting” buried streams have the potential to increase N retention. Further work is needed to elucidate the impacts of stream burial on ecosystem functions at the larger stream network scale.     

Efficient removal of NH4+ accumulated during anaerobic treatment of distillery wastewater from Shochu making

Propionate and NH₄⁺ were accumulated in the effluent during anaerobic treatment of five-fold diluted distillery wastewater from shochu making. Propionate could be removed efficiently during biological denitrification by the addition of NO₃⁻ (4.2 g/l) to the anaerobically treated wastewater. At a hydraulic retention time of more than 2 h, a TOC removal efficiency of 90% could be achieved. The wastewater was then treated aerobically by biological nitrification. With a hydraulic retention time of more than 14 h the efficiency of reduction of NH₄⁺ could be maintained above 97%. In order to reduce the amount of NO₃⁻ addition necessary for the removal of propionate, simultaneous removal of propionate and NH₄⁺ was studied by recirculating the effluent from a nitrification process to a denitrification process using denitrification and nitrification reactors connected in series. At a recirculation ratio of 2, the amount of NO₃⁻ that had to be added was reduced to 0.3 g/l of anaerobically treated wastewater, which corresponds to 6.9% of the theoretical value. Under the same conditions except for the addition of NO₃⁻ at 1.0 g/l, TOC and BOD in the effluent from the nitrification were 23 and 5 mg/l respectively, which are sufficiently low to allow discharge into river water. Moreover, the NO₃⁻ concentration in the effluent decreased with increases in the recirculation ratio.     

The saturation of N cycling in Central Plains streams: <Superscript>15</Superscript>N experiments across a broad gradient of nitrate concentrations

We conducted ¹⁵NO₃⁻ stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed to measure nitrate (NO₃⁻) cycling dynamics. Mean ambient NO₃⁻ concentrations of the streams ranged from 0.9 to 21,000 μg l⁻¹ NO₃⁻–N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which stream biota utilized available NO₃⁻-decreased along the gradient of increasing NO₃⁻. Observed functional relationships of biological N transformations (uptake and nitrification) with NO₃⁻ concentration did not support a 1st order model and did not show signs of Michaelis–Menten type saturation. The empirical relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient of NO₃⁻ concentrations, but only accounted for ∼1% of total NO₃⁻ uptake. On average, 20% of stream water NO₃⁻ was lost to denitrification per km, but the percentage removed in most streams was <5% km⁻¹. Although the rate of cycling was greater in streams with larger NO₃⁻ concentrations, the relative proportion of NO₃⁻ retained per unit length of stream decreased as NO₃⁻ concentration increased. Due to the rapid rate of NO₃⁻ turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove N through denitrification is highly variable.     

“Hotspots” and “Hot Moments” of Denitrification in Urban Brownfield Wetlands

The influence of hydrology and soil properties on disproportionately high (“hot”) rates of nitrate (NO₃ ^?) removal via denitrification has been relatively well established. It is poorly understood, however, how the unique soil characteristics of brownfield wetlands contribute to or hinder denitrification. In this study, we examined drivers of “hot” denitrification rates over time (“hot moments”) and space (“hotspots”) in a watershed located on an unrestored brownfield in New Jersey, USA. We carried out measurements of denitrification over 9-day sequences during three seasons in sites with the same vegetation (Phragmites australis) but different soils (fill material, remnant marsh soils, flooded organic-rich soils). Denitrification rates above the 3rd quartile value of the data distribution were defined as “hot” and the most important drivers of these rates were determined using mixed models. Porosity and NO₃ ^? availability were the strongest spatial and temporal predictors, respectively, of high denitrification rates, with coarse-textured, unflooded fill materials unexpectedly supporting the highest rates. These results suggest that pore-scale hydrology is a more complex controller of wetland denitrification than previously thought. Course-textured, unflooded soils have high fractions of air-filled pores relative to flooded soils, leading to more endogenous NO₃ ^? production, and less diffusion constraints than fine-textured soils, leading to higher NO₃ ^? availability to denitrifiers in suboxic pores. Laboratory studies confirmed denitrifiers were limited by NO₃ ^? availability. However, denitrification rates in all soils matched or exceeded atmospheric NO₃ ^? deposition and stormwater NO₃ ^? loading at the site, suggesting that brownfields may play an important role in NO₃ ^? removal from urban stormwater.     

Effects of nitrate concentration on the denitrification potential of a calcic cambisol and its fractions of N2, N2O and NO

Background and aims

The direct measurement of denitrification dynamics and its product fractions is important for parameterizing process-oriented model(s) for nitrogen cycling in various soils. The aims of this study are to a) directly measure the denitrification potential and the fractions of nitrogenous gases as products of the process in laboratory, b) investigate the effects of the nitrate (NO ₃ ^? ) concentration on emissions of denitrification gases, and c) test the hypothesis that denitrification can be a major pathway of nitrous oxide (N₂O) and nitric oxide (NO) production in calcic cambisols under conditions of simultaneously sufficient supplies of carbon and nitrogen substrates and anaerobiosis as to be found to occur commonly in agricultural lands.

Methods

Using the helium atmosphere (with or without oxygen) gas-flow-soil-core technique in laboratory, we directly measured the denitrification potential of a silt clay calcic cambisol and the production of nitrogen gas (N₂), N₂O and NO during denitrification under the conditions of seven levels of NO ₃ ^? concentrations (ranging from 10 to 250 mg N kg^?1 dry soil) and an almost constant initial dissolved organic carbon concentration (300 mg C kg^?1 dry soil).

Results

Almost all the soil NO ₃ ^? was consumed during anaerobic incubation, with 80–88 % of the consumed NO ₃ ^? recovered by measuring nitrogenous gases. The results showed that the increases in initial NO ₃ ^? concentrations significantly enhanced the denitrification potential and the emissions of N₂ and N₂O as products of this process. Despite the wide range of initial NO ₃ ^? concentrations, the ratios of N₂, N₂O and NO products to denitrification potential showed much narrower ranges of 51–78 % for N₂, 14–36 % for N₂O and 5–22 % for NO.

Conclusions

These results well support the above hypothesis and provide some parameters for simulating effects of variable soil NO ₃ ^? concentrations on denitrification process as needed for biogeochemical models.     

Heterotrophic nitrification and aerobic denitrification by a novel Halomonas campisalis

A novel halophilic strain that could carry out heterotrophic nitrification and aerobic denitrification was isolated and named as Halomonas campisalis ha3. It removed inorganic nitrogen compounds (e.g. NO₃ ^?, NO₂ ^? and NH₄ ⁺) simultaneously, and grew well in the medium containing up to 20 % (w/v) NaCl. PCR revealed four genes in the genome of ha3 related to aerobic denitrification: napA, nirS, norB and nosZ. The optimal conditions for aerobic denitrification were pH 9.0, at 37 °C, with 4 % (w/v) NaCl and sodium succinate as carbon source. The nitrogen removal rate was 87.5 mg NO₃ ^?–N l^?1 h^?1. Therefore, this strain is a potential aerobic denitrifier for the treatment of saline wastewater.     

Denitrification in soil amended with thermophile-fermented compost suppresses nitrate accumulation in plants

NO ₃ ^? is a major nitrogen source for plant nutrition, and plant cells store NO ₃ ^? in their vacuoles. Here, we report that a unique compost made from marine animal resources by thermophiles represses NO ₃ ^? accumulation in plants. A decrease in the leaf NO ₃ ^? content occurred in parallel with a decrease in the soil NO ₃ ^? level, and the degree of the soil NO ₃ ^? decrease was proportional to the compost concentration in the soil. The compost-induced reduction of the soil NO ₃ ^? level was blocked by incubation with chloramphenicol, indicating that the soil NO ₃ ^? was reduced by chloramphenicol-sensitive microbes. The compost-induced denitrification activity was assessed by the acetylene block method. To eliminate denitrification by the soil bacterial habitants, soil was sterilized with γ irradiation and then compost was amended. After the 24-h incubation, the N₂O level in the compost soil with presence of acetylene was approximately fourfold higher than that in the compost soil with absence of acetylene. These results indicate that the low NO ₃ ^? levels that are often found in the leaves of organic vegetables can be explained by compost-mediated denitrification in the soil.     

An in-depth look into a tropical lowland forest soil: nitrogen-addition effects on the contents of N2O, CO2 and CH4 and N2O isotopic signatures down to 2-m depth

Atmospheric nitrogen (N) deposition is rapidly increasing in tropical regions. We investigated how a decade of experimental N addition (125 kg N ha^?1 year^?1) to a seasonal lowland forest affected depth distribution and contents of soil nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), as well as natural abundance isotopic signatures of N₂O, nitrate (NO₃ ^?) and ammonium (NH₄ ⁺). In the control plots during dry season, we deduced limited N₂O production by denitrification in the topsoil (0.05–0.40 m) as indicated by: ambient N₂O concentrations and ambient ¹⁵N-N₂O signatures, low water-filled pore space (35–60%), and similar ¹⁵N signatures of N₂O and NO₃ ^?. In the subsoil (0.40–2.00 m), we detected evidence of N₂O reduction to N₂ during upward diffusion, indicating denitrification activity. During wet season, we found that N₂O at 0.05–2.00 m was mainly produced by denitrification with substantial further reduction to N₂, as indicated by: lighter ¹⁵N-N₂O than ¹⁵N-NO₃ ^? throughout the profile, and increasing N₂O concentrations with simultaneously decreasing ¹⁵N-N₂O enrichment with depth. These interpretations were supported by an isotopomer map and by a positive correlation between ¹⁸O-N₂O and ¹⁵N-N₂O site preferences. Long-term N addition did not affect dry-season soil N₂O-N contents, doubled wet-season soil N₂O-N contents, did not affect ¹⁵N signatures of NO₃ ^?, and reduced wet-season ¹⁵N signatures of N₂O compared to the control plots. These suggest that the increased NO₃ ^? concentrations have stimulated N₂O production and decreased N₂O-to-N₂ reduction. Soil CO₂-C contents did not differ between treatments, implying that N addition essentially did not influence soil C cycling. The pronounced seasonality in soil respiration was largely attributable to enhanced topsoil respiration as indicated by a wet-season increase in the topsoil CO₂-C contents. The N-addition plots showed reduced dry-season soil CH₄-C contents and threshold CH₄ concentrations were reached at a shallower depth compared to the control plots, revealing an N-induced stimulation of methanotrophic activity. However, the net soil CH₄ uptake rates remained similar between treatments possibly because diffusive CH₄ supply from the atmosphere largely limited CH₄ oxidation.     

The importance of denitrification for N2O emissions from an N-saturated forest in SW China: results from in situ 15N labeling experiments

Long-term elevated atmogenic deposition (~5 g m^?2 year^?1) of reactive nitrogen (N) causes N saturation in forests of subtropical China which may lead to high nitrous oxide (N₂O) emissions. Recently, we found high N₂O emission rates (up to 1,730 μg N₂O–N m^?2 h^?1) during summer on well-drained acidic acrisols (pH = 4.0) along a hill slope in the forested Tieshanping catchment, Chongqing, southwest China. Here, we present results from an in situ ¹⁵N–NO₃ ^? labeling experiment to assess the contribution of nitrification and denitrification to N₂O emissions in these soils. Two loads of 99 at.% K¹⁵NO₃ (equivalent to 0.2 and 1.0 g N m^?2) were applied as a single dose to replicated plots at two positions along the hill slope (at top and bottom, respectively) during monsoonal summer. During a 6-day period after label application, we found that 71–100 % of the emitted N₂O was derived from the labeled NO₃ ^? pool irrespective of slope position. Based on this, we assume that denitrification is the dominant process of N₂O formation in these forest soils. Within 6 days after label addition, the fraction of the added ¹⁵N–NO₃ ^? emitted as ¹⁵N–N₂O was highest at the low-N addition plots (0.2 g N m^?2), amounting to 1.3 % at the top position of the hill slope and to 3.2 % at the bottom position, respectively. Our data illustrate the large potential of acid forest soils in subtropical China to form N₂O from excess NO₃ ^? most likely through denitrification.     

Denitrification kinetics and denitrifier abundances in sediments of lakes receiving atmospheric nitrogen deposition (Colorado, USA)

The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5–8?kg?N?ha^?1?y^?1) or low (<2?kg?N?ha^?1?y^?1) inputs of atmospheric N deposition. The nitrate (NO₃ ^?) concentration was significantly greater in high-deposition lakes (11.3?μmol?l^?1) compared to low-deposition lakes (3.3?μmol?l^?1). Background denitrification was positively related to NO₃ ^? concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose–response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis–Menten kinetics, the maximum denitrification rate and half-saturation NO₃ ^? concentration did not differ between deposition regions and were 765?μmol?N?m^?2?h^?1 and 293?μmol?l^?1?NO₃ ^?, respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification.     

Factors Controlling Sediment Denitrification in Midwestern Streams of Varying Land Use

We investigated controls on stream sediment denitrification in nine headwater streams in the Kalamazoo River Watershed, Michigan, USA. Factors influencing denitrification were determined by using experimental assays based on the chloramphenicol-amended acetylene inhibition technique. Using a coring technique, we found that sediment denitrification was highest in the top 5 cm of the benthos and was positively related to sediment organic content. To determine the effect of overlying water quality on sediment denitrification, first-order stream sediments were assayed with water from second- and third-order downstream reaches, and often showed higher denitrification rates relative to assays using site-specific water from the first-order stream reach. Denitrification was positively related to nitrate (NO₃ ⁻) concentration, suggesting that sediments may have been nutrient-limited. Using stream-incubated inorganic substrata of varying size classes, we found that finer-grained sand showed higher rates of denitrification compared to large pebbles, likely due to increased surface area per volume of substratum. Denitrification was measurable on both inorganic substrata and fine particulate organic matter loosely associated with inorganic particles, and denitrification rates were related to organic content. Using nutrient-amended denitrification assays, we found that sediment denitrification was limited by NO₃ ⁻ or dissolved organic carbon (DOC, as dextrose) variably throughout the year. The frequency and type of limitation differed with land use in the watershed: forested streams were NO₃ ⁻-limited or co-limited by both NO₃ ⁻ and DOC 92% of the time, urban streams were more often NO₃ ⁻-limited than DOC-limited, whereas agricultural stream sediments were DOC-limited or co-limited but not frequently limited by NO₃ ⁻ alone.