Estimates Of Denitrification Research Articles

Soil Denitrification, the Missing Piece in the Puzzle of Nitrogen Budget in Lowland Agricultural Basins

Denitrification is a key process buffering the environmental impacts of agricultural nitrate loads but, at present, remains the least understood and poorly quantified sink in nitrogen budgets at the watershed scale. The present work deals with a comprehensive and detailed analysis of nitrogen sources and sinks in the Burana–Volano–Navigabile basin, the southernmost portion of the Po River valley (Northern Italy), an intensively cultivated (> 85% of basin surface) low-lying landscape. Agricultural census data, extensive monitoring of surface–groundwater interactions, and laboratory experiments targeting N fluxes and pools were combined to provide reliable estimates of soil denitrification at the basin scale. In the agricultural soils of the basin, nitrogen inputs exceeded outputs by nearly 40% (~ 80 kg N ha−1 year−1), but this condition of potential N excess did not translate into widespread nitrate pollution. The general scarcity of inorganic nitrogen species in groundwater and soils indicated limited leakage and storage. Multiple pieces of evidence supported that soil denitrification was the process that needed to be introduced in the budget to explain the fate of the missing nitrogen. Denitrification was likely boosted in the soils of the studied basin, prone to waterlogged conditions and consequently oxygen-limited, owing to peculiar features such as fine texture, low hydraulic conductivity, and shallow water table. The present study highlighted the substantial contribution of soil denitrification to balancing nitrogen inputs and outputs in agricultural lowland basins, a paramount ecosystem function preventing eutrophication phenomena.

Optimality-based non-Redfield plankton–ecosystem model (OPEM v1.1) in UVic-ESCM 2.9 – Part 2: Sensitivity analysis and model calibration

Abstract. We analyse 400 perturbed-parameter simulations for two configurations of an optimality-based plankton–ecosystem model (OPEM), implemented in the University of Victoria Earth System Climate Model (UVic-ESCM), using a Latin hypercube sampling method for setting up the parameter ensemble. A likelihood-based metric is introduced for model assessment and selection of the model solutions closest to observed distributions of NO3-, PO43-, O2, and surface chlorophyll a concentrations. The simulations closest to the data with respect to our metric exhibit very low rates of global N2 fixation and denitrification, indicating that in order to achieve rates consistent with independent estimates, additional constraints have to be applied in the calibration process. For identifying the reference parameter sets, we therefore also consider the model's ability to represent current estimates of water-column denitrification. We employ our ensemble of model solutions in a sensitivity analysis to gain insights into the importance and role of individual model parameters as well as correlations between various biogeochemical processes and tracers, such as POC export and the NO3- inventory. Global O2 varies by a factor of 2 and NO3- by more than a factor of 6 among all simulations. Remineralisation rate is the most important parameter for O2, which is also affected by the subsistence N quota of ordinary phytoplankton (Q0,phyN) and zooplankton maximum specific ingestion rate. Q0,phyN is revealed as a major determinant of the oceanic NO3- pool. This indicates that unravelling the driving forces of variations in phytoplankton physiology and elemental stoichiometry, which are tightly linked via Q0,phyN, is a prerequisite for understanding the marine nitrogen inventory.

Mobile continuous-flow isotope-ratio mass spectrometer system for automated measurements of N2 and N2O fluxes in fertilized cropping systems

The use of synthetic N fertilizers has grown exponentially over the last century, with severe environmental consequences. Most of the reactive N will ultimately be removed by denitrification, but estimates of denitrification are highly uncertain due to methodical constraints of existing methods. Here we present a novel, mobile isotope ratio mass spectrometer system (Field-IRMS) for in-situ quantification of N2 and N2O fluxes from fertilized cropping systems. The system was tested in a sugarcane field continuously monitoring N2 and N2O fluxes for 7 days following fertilization using a fully automated measuring cycle. The detection limit of the Field-IRMS proved to be highly sensitive for N2 (54 g ha−1 day−1) and N2O (0.25 g ha−1 day−1) emissions. The main product of denitrification was N2 with total denitrification losses of up to 1.3 kg N ha−1 day−1. These losses demonstrate sugarcane systems in Australia are a hotspot for denitrification where high emissions of N2O and N2 can be expected. The new Field-IRMS allows for the direct and highly sensitive detection of N2 and N2O fluxes in real time at a high temporal resolution, which will help to improve our quantitative understanding of denitrification in fertilized cropping systems.

Linking denitrification with ecosystem respiration in mountain streams

AbstractRivers denitrify a portion of their nitrate () load, but estimates are difficult using microcosm or reach‐scale measurements that require specific biogeochemical and hydrologic conditions. Measuring reach‐scale oxygen (O2) respiration fluxes is easier than nitrogen (N2) fluxes, thus we paired microcosm estimates of denitrification by N2 production with estimates of aerobic respiration. The median molar ratio of ΔN2:−ΔO2 from 13 streams was 0.011 (95% credible interval 0.0002–0.027 mol:mol). We then measured diel O2 concentrations from 11 streams and converted to ecosystem respiration (ER) using a multiday oxygen model. Given reach‐scale ER of −160 mmol O2 m−2 d−1, the estimated median denitrification was 1.5 mmol N2 m−2 d−1 (credible interval (CI): 0.18–4.21) across our streams. Our estimates of denitrification constituted 19% of gross uptake (CI: 0–51%). In streams, ΔN2:−ΔO2 was lower than in estuarine and marine ecosystems. Despite multiple sources of error, this approach estimates reach‐scale denitrification and variation with concentrations.

Stratification of reactivity determines nitrate removal in groundwater

Biogeochemical reactions occur unevenly in space and time, but this heterogeneity is often simplified as a linear average due to sparse data, especially in subsurface environments where access is limited. For example, little is known about the spatial variability of groundwater denitrification, an important process in removing nitrate originating from agriculture and land use conversion. Information about the rate, arrangement, and extent of denitrification is needed to determine sustainable limits of human activity and to predict recovery time frames. Here, we developed and validated a method for inferring the spatial organization of sequential biogeochemical reactions in an aquifer in France. We applied it to five other aquifers in different geological settings located in the United States and compared results among 44 locations across the six aquifers to assess the generality of reactivity trends. Of the sampling locations, 79% showed pronounced increases of reactivity with depth. This suggests that previous estimates of denitrification have underestimated the capacity of deep aquifers to remove nitrate, while overestimating nitrate removal in shallow flow paths. Oxygen and nitrate reduction likely increases with depth because there is relatively little organic carbon in agricultural soils and because excess nitrate input has depleted solid phase electron donors near the surface. Our findings explain the long-standing conundrum of why apparent reaction rates of oxygen in aquifers are typically smaller than those of nitrate, which is energetically less favorable. This stratified reactivity framework is promising for mapping vertical reactivity trends in aquifers, generating new understanding of subsurface ecosystems and their capacity to remove contaminants.

Measuring diel and spatial variation in biogenic N2 delivery, production, and loss with natural tracers: Application to watershed‐scale estimation of denitrification

AbstractComplete understanding of the nitrogen cycle is hindered by the challenges of measuring denitrification at watershed scales. Here, we refine the open‐channel approach, a fundamental field method in stream biogeochemistry that can be used to quantify denitrification in streams based on N2 and N2O production. We explicitly consider biogenic, groundwater‐derived N2 inputs to a stream, investigate patterns of diel and spatial variability in stream N2 fluxes, and explore the use of two potential natural tracers of gas exchange, argon and radon, in place of the artificial tracers often employed in open‐channel studies. We conducted two open‐channel studies, 12 h and 24 h in duration, in a channelized stream on the eastern shore of Maryland. Twenty‐two to forty‐three percent of total N2 inputs from groundwater and stream sediments to the stream were biogenic, with the remainder coming from atmospheric N2 in groundwater recharge. Of biogenic N2 (bN2) inputs, 37–100% came from groundwater and the remaining 0–63% were from in‐stream production. Radon can be measured continuously in the field and, unlike other potential natural tracers of gas exchange (Ar and O2), has a negligible atmospheric concentration. This provides more consistent estimates of the evasion coefficient K (min−1) and lower uncertainty than argon. Furthermore, the relative constancy of N2 fluxes during nighttime hours suggested the possibility of a simplified radon‐based sampling procedure requiring ∼ 6–8 h per site. This approach enables quantification of bN2 at many points within a stream system and estimation of watershed‐scale bN2 fluxes from stream beds and inflowing groundwater.

Ecosystem scale nitrification and watershed support of tidal creek productivity revealed using whole system isotope tracer labeling

AbstractTidal creeks are common coastal features of low‐slope continental margins that connect nitrogen (N)‐replete watersheds to N‐limited coastal oceans. We conducted a 3‐week whole tidal creek 15 tracer labeling experiment to quantify creek‐scale inputs and transformations and to evaluate ecosystem dependence on watershed delivery. Isotopic 15N enrichments of creek were hundreds to thousands of per mil depending on tidal stage and watershed discharge. We detected tracer movement into microalgae, macrophytes, zooplankton, epifauna, water column ammonium, and bulk sediments throughout the tidal prism. Tracer enrichment of dissolved N2 was detected along the study reach and permitted estimation of denitrification at the whole‐system scale. All data were used to build a isotope budget to quantify whole system nitrification and to calibrate a first order biota tracer model that yielded species‐specific turnover times and estimates of how much N demand was ultimately derived from the labeled creek 15 . Combining results from the isotope budget and the biota tracer model supported the following conclusions: (1) Under base flow conditions, the watershed flux of was small compared to the amount of produced within the creek via nitrification; (2) Approximately half of all inputs to the creek were attenuated within the creek principally by biological assimilation prior to downstream transport; (3) Biological uptake reduced watershed derived loads by ∼ 50%, but the majority of their N requirement was generated within the creek.

Nitrogen Balance in a Poorly Draining Intensively Cultivated Soil

The increasing use of nitrogen in fertilizers has not only increased the productivity of agricultural crops but also the concern for the effects of high N inputs on the natural environment. Economic, agronomic and environmental concerns have led to greater efforts towards more effective utilization of nitrogenous fertilizers, which can be achieved through a better quantitative understanding of the fate of nitrogen when applied to the soil. So Nitrogen balance was investigated in a poorly draining intensively cultivated soil. Nitrogen inputs and outputs, such as additions by precipitation (Natm), irrigation water (Nirr), commercial fertilizers (Nfert), mineralization (Nmin), denitrification (Ndenitr), volatilization (Ngas), capillary rise (Ncap), uptake (Nupt), residual (Nres) and leaching (Nleac) were studied and quantified during a period of 72 weeks for two growing crops (maize and wheat). Nitrogen excess or deficit at time ‘t’ (Nt) was calculated from the following balance equation: Nt = Nres + Natm + Nirr + Nmin + Nfert + Ncap − Nupt − Ndenitr – Ngas − Nleac. Existing models were used after proper adjustments to calculate these fluxes. Only a single model, based on soil temperature at 60 cm and NO3--N concentration in ground water was developed for the estimation of denitrification at the ground water boundary. The calculated values satisfactorily matched the respective experimental data. The results shown that the greater part of the applied nitrogen fertilizer was lost to ground water during the winter causing considerable environmental concern. For sustainable land use, inputs and outputs of nutrients must be balanced in order to avoid negative impacts on the environment, especially in intensively exploited agricultural ecosystems.

Denitrification in upland of China: Magnitude and influencing factors

AbstractA better understanding of influencing factors and accurate estimate of soil denitrification is a global concern. Here we present a synthesis of 300 observations of denitrification in Chinese upland soils from 39 field and laboratory studies using the acetylene inhibition technique. The results of a linear mixed model analysis showed that the rates of soil denitrification were significantly affected by crop type, soil organic carbon, soil pH, the measurement period, and the rate of N application. The emission factor (EF) and N2O/(N2O + N2) ratio for soil denitrification were on average 2.11 ± 0.17% and 0.508 ± 0.020, respectively. Our meta‐analysis indicated that N fertilization increased soil denitrification by 311% (95% CI: 279–346%) and 112% (95% CI: 66–171%) in the field and laboratory studies, respectively. Substantial interactive effects between soil properties and N fertilization on soil denitrification were found. Although the highest values of both the rate of denitrification and the EF were found in vegetable fields, the size of the stimulating effect of N fertilization on soil denitrification was lower in vegetable fields than in maize and wheat fields. These results suggest that the crop‐specific effect is important and that vegetable fields are potential hot spots of denitrification in Chinese uplands. Based on either the EF or the N2O/(N2O + N2) ratio obtained, the estimated amount of total denitrification from the upland soils was an order of magnitude lower than that from budget calculations, suggesting that the acetylene inhibition technique may significantly underestimate denitrification in Chinese upland soils.

Sediment, water column, and open‐channel denitrification in rivers measured using membrane‐inlet mass spectrometry

AbstractRiverine biogeochemical processes are understudied relative to headwaters, and reach‐scale processes in rivers reflect both the water column and sediment. Denitrification in streams is difficult to measure, and is often assumed to occur only in sediment, but the water column is potentially important in rivers. Dissolved nitrogen (N) gas flux (as dinitrogen (N2)) and open‐channel N2 exchange methods avoid many of the artificial conditions and expenses of common denitrification methods like acetylene block and 15N‐tracer techniques. We used membrane‐inlet mass spectrometry and microcosm incubations to quantify net N2 and oxygen flux from the sediment and water column of five Midwestern rivers spanning a land use gradient. Sediment and water column denitrification ranged from below detection to 1.8 mg N m−2 h−1 and from below detection to 4.9 mg N m−2 h−1, respectively. Water column activity was variable across rivers, accounting for 0–85% of combined microcosm denitrification and 39–85% of combined microcosm respiration. Finally, we estimated reach‐scale denitrification at one Midwestern river using a diel, open‐channel N2 exchange approach based on reach‐scale metabolism methods, providing an integrative estimate of riverine denitrification. Reach‐scale denitrification was 8.8 mg N m−2 h−1 (95% credible interval: 7.8–9.7 mg N m−2 h−1), higher than combined sediment and water column microcosm estimates from the same river (4.3 mg N m−2 h−1) and other estimates of reach‐scale denitrification from streams. Our denitrification estimates, which span habitats and spatial scales, suggest that rivers can remove N via denitrification at equivalent or higher rates than headwater streams.

Predicting groundwater redox status on a regional scale using linear discriminant analysis

Reducing conditions are necessary for denitrification, thus the groundwater redox status can be used to identify subsurface zones where potentially significant nitrate reduction can occur. Groundwater chemistry in two contrasting regions of New Zealand was classified with respect to redox status and related to mappable factors, such as geology, topography and soil characteristics using discriminant analysis. Redox assignment was carried out for water sampled from 568 and 2223 wells in the Waikato and Canterbury regions, respectively. For the Waikato region 64% of wells sampled indicated oxic conditions in the water; 18% indicated reduced conditions and 18% had attributes indicating both reducing and oxic conditions termed “mixed”. In Canterbury 84% of wells indicated oxic conditions; 10% were mixed; and only 5% indicated reduced conditions. The analysis was performed over three different well depths, <25m, 25 to 100 and >100m. For both regions, the percentage of oxidised groundwater decreased with increasing well depth. Linear discriminant analysis was used to develop models to differentiate between the three redox states. Models were derived for each depth and region using 67% of the data, and then subsequently validated on the remaining 33%. The average agreement between predicted and measured redox status was 63% and 70% for the Waikato and Canterbury regions, respectively. The models were incorporated into GIS and the prediction of redox status was extended over the whole region, excluding mountainous land. This knowledge improves spatial prediction of reduced groundwater zones, and therefore, when combined with groundwater flow paths, improves estimates of denitrification.

Effects of wetland plants on denitrification rates: a meta-analysis.

Human activity is accelerating changes in biotic communities worldwide. Predicting impacts of these changes on ecosystem services such as denitrification, a process that mitigates the consequences of nitrogen pollution, remains one of the most important challenges facing ecologists. Wetlands especially are valued as important sites of denitrification, and wetland plants are expected to have differing effects on denitrification. We present the results of a meta-analysis, conducted on 419 published estimates of denitrification in wetlands dominated by different plant species. Plants increased denitrification rates by 55% on average. This effect varied significantly among communities as defined by the dominant plant species, but surprisingly did not differ substantially among methods for measuring denitrification or among types of wetlands. We conclude that mechanistically linking functional plant traits to denitrification will be key to predicting the role of wetlands in nitrogen mitigation in a changing world.

Higher diversity and abundance of denitrifying microorganisms in environments than considered previously.

Denitrification is an important process in the global nitrogen cycle. The genes encoding NirK and NirS (nirK and nirS), which catalyze the reduction of nitrite to nitric oxide, have been used as marker genes to study the ecological behavior of denitrifiers in environments. However, conventional polymerase chain reaction (PCR) primers can only detect a limited range of the phylogenetically diverse nirK and nirS. Thus, we developed new PCR primers covering the diverse nirK and nirS. Clone library and qPCR analysis using the primers showed that nirK and nirS in terrestrial environments are more phylogenetically diverse and 2-6 times more abundant than those revealed with the conventional primers. RNA- and culture-based analyses using a cropland soil also suggested that microorganisms with previously unconsidered nirK or nirS are responsible for denitrification in the soil. PCR techniques still have a greater capacity for the deep analysis of target genes than PCR-independent methods including metagenome analysis, although efforts are needed to minimize the PCR biases. The methodology and the insights obtained here should allow us to achieve a more precise understanding of the ecological behavior of denitrifiers and facilitate more precise estimate of denitrification in environments.

Impacts of low phytoplankton NO3−:PO43− utilization ratios over the Chukchi Shelf, Arctic Ocean

The impact of Arctic denitrification is seen in the extremely low values for the geochemical tracer of microbial nitrogen (N) cycle source/sink processes N⁎⁎ (Mordy et al. 2010). The utility of N⁎⁎ as an oceanic tracer of microbial N cycle processes, however, relies on the assumption that phytoplankton utilize dissolved N and P in Redfield proportions, and thus changes in N⁎⁎ are due to either N2-fixation or denitrification. We present results from two cruises to the Chukchi Sea that quantify nutrient drawdown, nutrient deficits, and particulate nutrient concentrations to estimate production over the Chukchi Shelf and document lower than Redfield N:P utilization ratios by phytoplankton. These low ratios are used to calculate N⁎⁎ (assuming a Redfield NO3−:PO43− utilization ratio) and NNR⁎⁎ (using the measured particulate N:P ratios) and, combined with current flow speed and direction measurements, to diagnose denitrification rates on the Chukchi Shelf. Our estimates of denitrification rates are up to 40% higher when Redfield proportions are used. However, the denitrification rates we calculate using NNR⁎⁎ are still higher than previous estimates (up to 8 fold) of denitrification on the Chukchi shelf. These estimates suggest that Arctic shelves may be a greater sink of oceanic N than previously thought.

Modeling denitrification in an agricultural catchment in Central New York

Abstract Denitrification, the microbially mediated reduction of NO3− to N2, NO, or N2O gas, provides an important ecosystem service by reducing N loads to downstream waters. To incorporate denitrification services into management planning, it is important to quantify the benefit; however, it is difficult to quantify denitrification rates and even harder to extrapolate them spatially and temporally to generate landscape-scale estimates of denitrification. We developed a coupled hydrologic-denitrification model that predicts daily denitrification rates across an agricultural watershed and calibrated it using in situ denitrification measurements and two types of hydrologic observations (streamflow and upland soil moisture). The model fits well with the observed denitrification (R2 = 0.77, RMSE = 272 kg N ha−1 yr−1, NRMSE = 0.77), stream discharge (NSE = 0.66) and soil moisture (NSE = 0.77), and quantifies the denitrification ecosystem service provided by the watershed as a whole, as well as by the various land classes. Over the seven year model run, mean annual denitrification rates were 21 kg N ha−1 yr−1 watershed-wide, 47 kg N ha−1 yr−1 in the wetland, 52 kg N ha−1 yr−1 in cropped areas, and 4 kg N ha−1 yr−1 in pastures and forests. Quantification of ecosystem services is an essential prerequisite to accounting for those benefits in management decisions. This study is one step towards addressing the lack of field validated ecosystem service quantification studies. In the future, the model will allow us to further examine the spatial and temporal patterns of denitrification and to explore the implications of changes in management practices or climate.

Microbial nitrogen transformation in constructed wetlands treating contaminated groundwater

Pathways of ammonium (NH4 (+)) removal were investigated using the stable isotope approach in constructed wetlands (CWs). We investigated and compared several types of CWs: planted horizontal subsurface flow (HSSF), unplanted HSSF, and floating plant root mat (FPRM), including spatial and seasonal variations. Plant presence was the key factor influencing efficiency of NH4 (+) removal in all CWs, what was illustrated by lower NH4 (+)-N removal by the unplanted HSSF CW in comparison with planted CWs. No statistically significant differences in NH4 (+) removal efficiencies between seasons were detected. Even though plant uptake accounted for 32-100% of NH4 (+) removal during spring and summer in planted CWs, throughout the year, most of NH4 (+) was removed via simultaneous nitrification-denitrification, what was clearly shown by linear increase of δ(15)N-NH4 (+) with decrease of loads along the flow path and absence of nitrate (NO3 (-)) accumulation. Average yearly enrichment factor for nitrification was -7.9 ‰ for planted HSSF CW and -5.8 ‰ for FPRM. Lack of enrichment for δ(15)N-NO3 (-) implied that other processes, such as nitrification and mineralization were superimposed on denitrification and makes the stable isotope approach unsuitable for the estimation of denitrification in the systems obtaining NH4 (+) rich inflow water.

Application of chemical tracers to an estimate of benthic denitrification in the Okhotsk Sea

To estimate benthic denitrification in a marginal sea, we assessed the usefulness of \({\text{N}}_{2}^{*}\), a new tracer to measure the excess nitrogen gas (N2) using dissolved N2 and argon (Ar) with N* in the intermediate layer (26.6–27.4σ θ ) of the Okhotsk Sea. The examined parameters capable of affecting \({\text{N}}_{2}^{*}\) are denitrification, air injection and rapid cooling. We investigated the relative proportions of these effects on \({\text{N}}_{2}^{*}\) using multiple linear regression analysis. The best model included two examined parameters of denitrification and air injection based on the Akaike information criterion as a measure of the model fit to data. More than 80 % of \({\text{N}}_{2}^{*}\) was derived from the denitrification, followed by air injection. Denitrification over the Okhotsk Sea shelf region was estimated to be 5.6 ± 2.4 μmol kg−1. The distribution of \({\text{N}}_{2}^{*}\) was correlated with potential temperature (θ) between 26.6 and 27.4σ θ (r = −0.55). Therefore, we concluded that \({\text{N}}_{2}^{*}\) and N* can act complementarily as a quasi-conservative tracer of benthic denitrification in the Okhotsk Sea. Our findings suggest that \({\text{N}}_{2}^{*}\) in combination with N* is a useful chemical tracer to estimate benthic denitrification in a marginal sea.

Multi-decade Responses of a Tidal Creek System to Nutrient Load Reductions: Mattawoman Creek, Maryland USA

We developed a synthesis using diverse monitoring and modeling data for Mattawoman Creek, Maryland, USA to examine responses of this tidal freshwater tributary of the Potomac River estuary to a sharp reduction in point-source nutrient loading rate. Oligotrophication of these systems is not well understood; questions concerning recovery pathways, threshold responses, and lag times remain to be clarified and eventually generalized for application to other systems. Prior to load reductions Mattawoman Creek was eutrophic with poor water clarity (Secchi depth <0.5 m), no submerged aquatic vegetation (SAV), and large algal stocks (50–100 μg L−1 chlorophyll-a). A substantial modification to a wastewater treatment plant reduced annual average nitrogen (N) loads from 30 to 12 g N m−2 year−1 and phosphorus (P) loads from 3.7 to 1.6 g P m−2 year−1. Load reductions for both N and P were initiated in 1991 and completed by 1995. There was no trend in diffuse N and P loads between 1985 and 2010. Following nutrient load reduction, NO2 + NO3 and chlorophyll-a decreased and Secchi depth and SAV coverage and density increased with initial response lag times of one, four, three, one, and one year, respectively. A preliminary N budget was developed and indicated the following: diffuse sources currently dominate N inputs, estimates of long-term burial and denitrification were not large enough to balance the budget, sediment recycling of NH4 was the single largest term in the budget, SAV uptake of N from sediments and water provided a modest seasonal-scale N sink, and the creek system acted as an N sink for imported Potomac River nitrogen. Finally, using a comparative approach utilizing data from other shallow, low-salinity Chesapeake Bay ecosystems, strong relationships were found between N loading and algal biomass and between algal biomass and water clarity, two key water quality variables used as indices of restoration in Chesapeake Bay.

The role of benthic foraminifera in the benthic nitrogen cycle of the Peruvian oxygen minimum zone

Abstract. The discovery that foraminifera are able to use nitrate instead of oxygen as an electron acceptor for respiration has challenged our understanding of nitrogen cycling in the ocean. It was thought before that only prokaryotes and some fungi are able to denitrify. Rate estimates of foraminiferal denitrification have been very sparse and limited to specific regions in the oceans, not comparing stations along a transect of a certain region. Here, we present estimates of benthic foraminiferal denitrification rates from six stations at intermediate water depths in and below the Peruvian oxygen minimum zone (OMZ). Foraminiferal denitrification rates were calculated from abundance and assemblage composition of the total living fauna in both surface and subsurface sediments, as well as from individual species specific denitrification rates. A comparison with total benthic denitrification rates as inferred by biogeochemical models revealed that benthic foraminifera probably account for the total denitrification in shelf sediments between 80 and 250 m water depth. The estimations also imply that foraminifera are still important denitrifiers in the centre of the OMZ around 320 m (29–50% of the benthic denitrification), but play only a minor role at the lower OMZ boundary and below the OMZ between 465 and 700 m (2–6% of total benthic denitrification). Furthermore, foraminiferal denitrification has been compared to the total benthic nitrate loss measured during benthic chamber experiments. The estimated foraminiferal denitrification rates contribute 2 to 46% to the total nitrate loss across a depth transect from 80 to 700 m, respectively. Flux rate estimates range from 0.01 to 1.3 mmol m−2 d−1. Furthermore we show that the amount of nitrate stored in living benthic foraminifera (3 to 3955 μmol L−1) can be higher by three orders of magnitude as compared to the ambient pore waters in near-surface sediments sustaining an important nitrate reservoir in Peruvian OMZ sediments. The substantial contribution of foraminiferal nitrate respiration to total benthic nitrate loss at the Peruvian margin, which is one of the main nitrate sink regions in the world ocean, underpins the importance of the previously underestimated role of benthic foraminifera in global biogeochemical cycles.

Direct flux and 15N tracer methods for measuring denitrification in forest soils

Estimates of denitrification are one of the key uncertainties in the terrestrial nitrogen (N) cycle, primarily because reliable measurements of this highly variable process—especially the production of its terminal product (N2)—are difficult to obtain. We evaluated the ability of gas-flow soil core and 15N tracer methods to provide reliable estimates of denitrification in forest soils. Our objectives were to: (1) describe and present typical results from new gas-flow soil core and in situ 15N tracer methods for measuring denitrification, (2) discuss factors that affect the relevance of these methods to actual in situ denitrification, and (3) compare denitrification estimates produced by the two methods for a series of sites in a northern hardwood forest ecosystem. Both methods were able to measure accumulations of N2 over relatively short (2–5 h) incubations of either unamended or tracer-amended intact soils. Denitrification rates measured by the direct flux soil core method were very sensitive to incubation oxygen (O2) concentration and decreased with increased O2 levels. Denitrification rates measured by the in situ 15N tracer method were very sensitive to the 15N content of the nitrate (NO3 −) pool undergoing denitrification, which limits the applicability of this method for quantifying denitrification in N-poor ecosystems. While its ability to provide accurate estimates of denitrification was limited, the 15N tracer method provided estimates of the short-term abiotic and biotic transformations of atmospheric N deposition to gas. Furthermore, results suggest that denitrification is higher and that N2O:N2 ratios are lower (<0.02) than previously thought in the northern hardwood forest and that short-term abiotic and biotic transformations of atmospheric N deposition to gas are significant in this ecosystem.