scholarly journals Influence of mesoscale eddies on the distribution of nitrous oxide in the eastern tropical South Pacific

2016 ◽  
Vol 13 (4) ◽  
pp. 1105-1118 ◽  
Author(s):  
Damian L. Arévalo-Martínez ◽  
Annette Kock ◽  
Carolin R. Löscher ◽  
Ruth A. Schmitz ◽  
Lothar Stramma ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Coastal Upwelling ◽  
Mesoscale Eddies ◽  
South Pacific ◽  
N Cycling ◽  
Gene Marker ◽  
N Loss ◽  
Mode Water ◽  
N2o Production

Abstract. Recent observations in the eastern tropical South Pacific (ETSP) have shown the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production–consumption processes in the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source–sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked, and sampled during two surveys carried out in the ETSP in November–December 2012. A two-peak structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification, ammonium monooxygenase (amoA), and the gene marker for N2O production during denitrification, nitrite reductase (nirS). Conversely, we found strong N2O depletion in the core of the OMZ (O2 < 5 µmol L−1) to be consistent with nitrite (NO2−) accumulation and low levels of nitrate (NO3−), thus suggesting active denitrification. N2O depletion within the OMZ's core was substantially higher in the centre of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, “ageing” of mesoscale eddies tends to decrease N2O concentrations through the water column in response to the reduced supply of material to fuel N loss, although hydrographic variability might also significantly impact the pace of the production–consumption pathways for N2O. Our results evidence the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

scholarly journals Influence of mesoscale eddies on the distribution of nitrous oxide in the eastern tropical South Pacific

2015 ◽  
Vol 12 (12) ◽  
pp. 9243-9273 ◽  
Author(s):  
D. L. Arévalo-Martínez ◽  
A. Kock ◽  
C. R. Löscher ◽  
R. A. Schmitz ◽  
L. Stramma ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Coastal Upwelling ◽  
Mesoscale Eddies ◽  
South Pacific ◽  
N Cycling ◽  
Gene Marker ◽  
N Loss ◽  
Mode Water ◽  
N2o Production

Abstract. Recent observations in the eastern tropical South Pacific (ETSP) demonstrated the key role of meso- and submesoscale processes (e.g. eddies) in shaping its hydrographic and biogeochemical properties. Off Peru, elevated primary production from coastal upwelling in combination with sluggish ventilation of subsurface waters fuels a prominent oxygen minimum zone (OMZ). Given that nitrous oxide (N2O) production/consumption processes on the water column are sensitive to oxygen (O2) concentrations, the ETSP is a region of particular interest to investigate its source-sink dynamics. To date, no detailed surveys linking mesoscale processes and N2O distributions as well as their relevance to nitrogen (N) cycling are available. In this study, we present the first measurements of N2O across three mesoscale eddies (two mode water or anticyclonic and one cyclonic) which were identified, tracked and sampled during two surveys carried out in the ETSP in November-December 2012. A "two peak" structure was observed for N2O, wherein the two maxima coincide with the upper and lower boundaries of the OMZ, indicating active nitrification and partial denitrification. This was further supported by the abundances of the key gene for nitrification amoA and the gene marker for N2O production during denitrification, nirS. Conversely, we found strong N2O depletion in the core of the OMZ (O2 < 5 μmol L−1) to be consistent with nitrite (NO2−) accumulation and low levels of nitrate (NO3−), thus suggesting active denitrification. N2O depletion within the OMZ's core was substantially higher in the center of mode water eddies, supporting the view that eddy activity enhances N-loss processes off Peru, in particular near the shelf break where nutrient-rich, productive waters from upwelling are trapped before being transported offshore. Analysis of eddies during their propagation towards the open ocean showed that, in general, "aging" of mesoscale eddies tends to decrease N2O concentrations through the water column in response to reduced supply of material to fuel N-loss, although hydrographic variability might also significantly impact the pace of the production/consumption pathways for N2O. Our results demonstrate the relevance of mode water eddies for N2O distribution, thereby improving our understanding of the N-cycling processes, which are of crucial importance in times of climate change and ocean deoxygenation.

scholarly journals Coastal hypoxia/anoxia as a source of CH<sub>4</sub> and N<sub>2</sub>O

2009 ◽  
Vol 6 (5) ◽  
pp. 9455-9523 ◽  
Author(s):  
S. W. A. Naqvi ◽  
H. W. Bange ◽  
L. Farías ◽  
P. M. S. Monteiro ◽  
M. I. Scranton ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
Black Sea ◽  
Coastal Upwelling ◽  
Dead Zone ◽  
Nitrous Oxide Emissions ◽  
Quantitative Prediction ◽  
The Black Sea ◽  
N2o Production ◽  
Bottom Waters

Abstract. We review here available information on distributions of methane (CH4) and nitrous oxide (N2O) from major, mostly coastal, oxygen (O2)-deficient zones produced due to both natural processes and human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O2 levels. In the case of CH4, bottom-water O2 content probably affects emission from sediments, believed to be the main source of water-column CH4, as well as its oxidative loss in water itself. Highest CH4 accumulation (several μM) occurs in silled basins having anoxic deep waters such as the Black Sea and the Cariaco Basin. One to two orders of magnitude smaller, but still significant, accumulation also occurs in bottom waters of open margins experiencing anoxia and in silled basins containing suboxic/severely hypoxic waters. In highly eutrophic waters over open continental shelves (such as the upwelling zone off Namibia and the "dead zone" in the Gulf of Mexico) high CH4 concentrations (several hundred nM) may occur in non-sulphidic waters as well, but in these regions it is difficult to differentiate the hypoxia-induced enhancement from in situ production of CH4 in the water column and, sometimes, large inputs of CH4 associated with freshwater runoff or seepage from sediments. Despite the observed CH4 build-up in low-O2 bottom waters, methanotrophic activity severely restricts its emission from the ocean. As a result, an intensification or expansion of coastal hypoxic zones will probably not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH4 budget. The situation is different for N2O, the production of which is greatly enhanced in severely hypoxic waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of semi-enclosed/land-locked anoxic basins such as the Black Sea. Most O2-deficient systems serve as strong net sources of N2O to the atmosphere. This is especially true for regions of coastal upwelling with shallow oxygen minimum zones where a dramatic increase in N2O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N2O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N2O as well as insufficient data from some key coastal regions.

scholarly journals Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre

2007 ◽  
Vol 4 (5) ◽  
pp. 729-741 ◽  
Author(s):  
J. Charpentier ◽  
L. Farias ◽  
N. Yoshida ◽  
N. Boontanon ◽  
P. Raimbault
Keyword(s):  
Nitrous Oxide ◽  
Coastal Upwelling ◽  
South Pacific ◽  
Site Preference ◽  
Natural Environments ◽  
Intermediate Water ◽  
N2o Production ◽  
South Pacific Ocean ◽  
Nitrifier Denitrification ◽  
Organic Particles

Abstract. The mechanisms of microbial nitrous oxide (N2O) production in the ocean have been the subject of many discussions in recent years. New isotopomeric tools can further refine our knowledge of N2O sources in natural environments. This study compares hydrographic, N2O concentration, and N2O isotopic and isotopomeric data from three stations along a coast-perpendicular transect in the South Pacific Ocean, extending from the center (Sts. GYR and EGY) of the subtropical oligotrophic gyre (~26° S; 114° W) to the upwelling zone (St. UPX) off the central Chilean coast (~34° S). Although AOU/N2O and NO3− trends support the idea that most of the N2O (mainly from intermediate water (200–600 m)) comes from nitrification, N2O isotopomeric composition (intramolecular distribution of 15N isotopes) expressed as SP (site preference of 15N) shows low values (10 to 12\\permil) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N2O mediated by ammonium oxidizers). The coincidence of this SP signal with high – stability layer, where sinking organic particles can accumulate, suggests that N2O could be produced by nitrifier denitrification inside particles. It is postulated that deceleration of particles in the pycnocline can modify the advection - diffusion balance inside particles, allowing the accumulation of nitrite and O2 depletion suitable for nitrifier denitrication. As lateral advection seems to be relatively insignificant in the gyre, in situ nitrifier denitrification could account for 40–50% of the N2O produced in this layer. In contrast, coastal upwelling system is characterized by O2 deficient condition and some N deficit in a eutrophic system. Here, N2O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N2O production processes, which presumably reflect both the effect of nitrification and denitrification at low O2 levels on N2O production, but net N2O consumption by denitrification was not observed.

scholarly journals Investigating the effect of El Niño on nitrous oxide distribution in the eastern tropical South Pacific

2019 ◽  
Vol 16 (9) ◽  
pp. 2079-2093 ◽  
Author(s):  
Qixing Ji ◽  
Mark A. Altabet ◽  
Hermann W. Bange ◽  
Michelle I. Graco ◽  
Xiao Ma ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Water Column ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
South Pacific ◽  
Biological Properties ◽  
Nitrite Reduction ◽  
N2o Production ◽  
Near Surface

Abstract. The open ocean is a major source of nitrous oxide (N2O), an atmospheric trace gas attributable to global warming and ozone depletion. Intense sea-to-air N2O fluxes occur in major oceanic upwelling regions such as the eastern tropical South Pacific (ETSP). The ETSP is influenced by the El Niño–Southern Oscillation that leads to inter-annual variations in physical, chemical, and biological properties in the water column. In October 2015, a strong El Niño event was developing in the ETSP; we conduct field observations to investigate (1) the N2O production pathways and associated biogeochemical properties and (2) the effects of El Niño on water column N2O distributions and fluxes using data from previous non-El Niño years. Analysis of N2O natural abundance isotopomers suggested that nitrification and partial denitrification (nitrate and nitrite reduction to N2O) were occurring in the near-surface waters; indicating that both pathways contributed to N2O effluxes. Higher-than-normal sea surface temperatures were associated with a deepening of the oxycline and the oxygen minimum layer. Within the shelf region, surface N2O supersaturation was nearly an order of magnitude lower than that of non-El Niño years. Therefore, a significant reduction of N2O efflux (75 %–95 %) in the ETSP occurred during the 2015 El Niño. At both offshore and coastal stations, the N2O concentration profiles during El Niño showed moderate N2O concentration gradients, and the peak N2O concentrations occurred at deeper depths during El Niño years; this was likely the result of suppressed upwelling retaining N2O in subsurface waters. At multiple stations, water-column inventories of N2O within the top 1000 m were up to 160 % higher than those measured in non-El Niño years, indicating that subsurface N2O during El Niño could be a reservoir for intense N2O effluxes when normal upwelling is resumed after El Niño.

scholarly journals Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre

2007 ◽  
Vol 4 (3) ◽  
pp. 1673-1702 ◽  
Author(s):  
J. Charpentier ◽  
L. Farias ◽  
N. Yoshida ◽  
N. Boontanon ◽  
P. Raimbault
Keyword(s):  
Nitrous Oxide ◽  
Coastal Upwelling ◽  
South Pacific ◽  
Site Preference ◽  
Intermediate Water ◽  
Low Oxygen ◽  
N2o Production ◽  
Oxide Production ◽  
Nitrifier Denitrification ◽  
Nitrous Oxide Production

Abstract. The biogeochemical mechanism of bacterial N2O production in the ocean has been the subject of many discussions in recent years. New isotopomeric tools can help further knowledge on N2O sources in natural environments. This research shows and compares hydrographic, nitrous oxide concentration, and N2O isotopic and isotopomeric data from three stations across the South Pacific Ocean, from the center of the subtropical oligotrophic gyre (~26° S; 114° W) to the upwelling zone along the central Chilean coast (~34° S). Althought AOU/N2O and NO3− trends support the idea that most of N2O source (mainly from intermediate water (200–1000 m)) come from nitrification, N2O isotopomeric composition (intramolecular distribution of 15N isotopes in N2O) reveals an abrupt change in the mechanism of nitrous oxide production, always observed through lower SP (site preference of 15N), at a high – stability layer, where particles could act as microsites and N2O would be produced by nitrifier denitrification (reduction of nitrite to nitrous oxide mediated by primary nitrifiers). There, nitrifier denitrification can account for 40% and 50% (center and east border of the gyre, respectively) of the nitrous oxide produced in this specific layer. This process could be associated with the deceleration of sinking organic particles in highly stable layers of the water column. In constrast, coastal upwelling system is characterized by oxygen deficient condition and some N deficit in a eutrophic system. Here, nitrous oxide accumulates up to 480% saturation, and isotopic and isotopomer signal show highly complex nitrous oxide production processes, which presumably reflect both the effect of nitrification and denitrification at low oxygen levels on N2O production, but non N2O consumption by denitrification was observed.

scholarly journals Nitrogen and oxygen availabilities control water column nitrous oxide production during seasonal anoxia in the Chesapeake Bay

2018 ◽  
Vol 15 (20) ◽  
pp. 6127-6138 ◽  
Author(s):  
Qixing Ji ◽  
Claudia Frey ◽  
Xin Sun ◽  
Melanie Jackson ◽  
Yea-Shine Lee ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
Chesapeake Bay ◽  
Water Column ◽  
N2o Emissions ◽  
N2o Production ◽  
Isotope Tracer ◽  
Relative Contribution ◽  
Environmental Controls ◽  
Long Run ◽  
Nitrate And Nitrite

Abstract. Nitrous oxide (N2O) is a greenhouse gas and an ozone depletion agent. Estuaries that are subject to seasonal anoxia are generally regarded as N2O sources. However, insufficient understanding of the environmental controls on N2O production results in large uncertainty about the estuarine contribution to the global N2O budget. Incubation experiments with nitrogen stable isotope tracer were used to investigate the geochemical factors controlling N2O production from denitrification in the Chesapeake Bay, the largest estuary in North America. The highest potential rates of water column N2O production via denitrification (7.5±1.2 nmol-N L−1 h−1) were detected during summer anoxia, during which oxidized nitrogen species (nitrate and nitrite) were absent from the water column. At the top of the anoxic layer, N2O production from denitrification was stimulated by addition of nitrate and nitrite. The relative contribution of nitrate and nitrite to N2O production was positively correlated with the ratio of nitrate to nitrite concentrations. Increased oxygen availability, up to 7 µmol L−1 oxygen, inhibited both N2O production and the reduction of nitrate to nitrite. In spring, high oxygen and low abundance of denitrifying microbes resulted in undetectable N2O production from denitrification. Thus, decreasing the nitrogen input into the Chesapeake Bay has two potential impacts on the N2O production: a lower availability of nitrogen substrates may mitigate short-term N2O emissions during summer anoxia; and, in the long-run (timescale of years), eutrophication will be alleviated and subsequent reoxygenation of the bay will further inhibit N2O production.

scholarly journals Vertical segregation among pathways mediating nitrogen loss (N<sub>2</sub> and N<sub>2</sub>O production) across the oxygen gradient in a coastal upwelling ecosystem

2017 ◽  
Vol 14 (20) ◽  
pp. 4795-4813 ◽  
Author(s):  
Alexander Galán ◽  
Bo Thamdrup ◽  
Gonzalo S. Saldías ◽  
Laura Farías
Keyword(s):  
Coastal Upwelling ◽  
Nitrite Reduction ◽  
15N Tracer ◽  
Ammonium Oxidation ◽  
N Loss ◽  
N2o Production ◽  
Coastal System ◽  
Central Chile ◽  
N Removal ◽  
Bottom Waters

Abstract. The upwelling system off central Chile (36.5° S) is seasonally subjected to oxygen (O2)-deficient waters, with a strong vertical gradient in O2 (from oxic to anoxic conditions) that spans a few metres (30–50 m interval) over the shelf. This condition inhibits and/or stimulates processes involved in nitrogen (N) removal (e.g. anammox, denitrification, and nitrification). During austral spring (September 2013) and summer (January 2014), the main pathways involved in N loss and its speciation, in the form of N2 and/or N2O, were studied using 15N-tracer incubations, inhibitor assays, and the natural abundance of nitrate isotopes along with hydrographic information. Incubations were developed using water retrieved from the oxycline (25 m depth) and bottom waters (85 m depth) over the continental shelf off Concepción, Chile. Results of 15N-labelled incubations revealed higher N removal activity during the austral summer, with denitrification as the dominant N2-producing pathway, which occurred together with anammox at all times. Interestingly, in both spring and summer maximum potential N removal rates were observed in the oxycline, where a greater availability of oxygen was observed (maximum O2 fluctuation between 270 and 40 µmol L−1) relative to the hypoxic bottom waters ( <  20 µmol O2 L−1). Different pathways were responsible for N2O produced in the oxycline and bottom waters, with ammonium oxidation and dissimilatory nitrite reduction, respectively, as the main source processes. Ammonium produced by dissimilatory nitrite reduction to ammonium (DNiRA) could sustain both anammox and nitrification rates, including the ammonium utilized for N2O production. The temporal and vertical variability of δ15N-NO3− confirms that multiple N-cycling processes are modulating the isotopic nitrate composition over the shelf off central Chile during spring and summer. N removal processes in this coastal system appear to be related to the availability and distribution of oxygen and particles, which are a source of organic matter and the fuel for the production of other electron donors (i.e. ammonium) and acceptors (i.e. nitrate and nitrite) after its remineralization. These results highlight the links between several pathways involved in N loss. They also establish that different mechanisms supported by alternative N substrates are responsible for substantial accumulation of N2O, which are frequently observed as hotspots in the oxycline and bottom waters. Considering the extreme variation in oxygen observed in several coastal upwelling systems, these findings could help to understand the ecological and biogeochemical implications due to global warming where intensification and/or expansion of the oceanic OMZs is projected.

scholarly journals N<sub>2</sub> fixation in eddies of the eastern tropical South Pacific Ocean

2016 ◽  
Vol 13 (10) ◽  
pp. 2889-2899 ◽  
Author(s):  
Carolin R. Löscher ◽  
Annie Bourbonnais ◽  
Julien Dekaezemacker ◽  
Chawalit N. Charoenpong ◽  
Mark A. Altabet ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
N2 Fixation ◽  
South Pacific ◽  
Open Ocean ◽  
Cyclonic Eddy ◽  
Geochemical Data ◽  
Intermediate Water ◽  
N Loss ◽  
Mode Water ◽  
South Pacific Ocean

Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen-deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations into their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce.We examined an open-ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open-ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies.In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November 2012 and December 2012 also revealed a reduction in N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.

scholarly journals Investigating the effect of El Niño on nitrous oxide distribution in the Eastern Tropical South Pacific

2018 ◽  
Author(s):  
Qixing Ji ◽  
Mark A. Altabet ◽  
Hermann W. Bange ◽  
Michelle I. Graco ◽  
Xiao Ma ◽  
...  
Keyword(s):  
Nitrous Oxide ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
South Pacific ◽  
Biological Properties ◽  
Nitrite Reduction ◽  
Sea Surface ◽  
N2o Production ◽  
Annual Variations

Abstract. The open ocean is a major source of atmospheric warming and ozone depleting gas nitrous oxide (N2O). Intense sea-to-air fluxes of N2O occur in major oceanic upwelling regions such as the Eastern Tropical South Pacific Ocean (ETSP). The ETSP is influenced by the El Niño-Southern Oscillation that leads to inter-annual variations of physical, chemical and biological properties. A strong El Niño was developing in this region in October 2015, during which we investigated the N2O production pathways and, by comparing to previous non-El Niño years, the effects of El Niño on water column N2O distributions and fluxes. Analysis of N2O natural abundance isotopomers suggested that both nitrification and partial denitrification (nitrate and nitrite reduction to N2O) were important N2O production pathways. Higher than normal sea-surface temperatures were associated with a deepening of the oxycline, while the level of sea surface N2O supersaturation on the continental shelf was nearly an order of magnitude lower than those of non-El Niño years. Therefore, a significant reduction of N2O efflux in the ETSP occurred during the 2015 El Niño event. At both offshore and coastal stations, the N2O concentration profiles during El Niño showed moderate N2O concentration gradients, and peak N2O concentrations were deeper than during non-El Niño years; this was likely the result of suppressed upwelling retaining N2O in subsurface waters. The depth-integrated N2O concentrations during El Niño were nearly twice as high as those measured in non-El Niño years, indicating subsurface N2O during El Niño could be a reservoir for intense N2O effluxes when normal upwelling is resumed after El Niño.

scholarly journals Sources of nitrous oxide emitted from European forest soils

2005 ◽  
Vol 2 (5) ◽  
pp. 1353-1380 ◽  
Author(s):  
P. Ambus ◽  
S. Zechmeister-Boltenstern ◽  
K. Butterbach-Bahl
Keyword(s):  
Nitrous Oxide ◽  
Forest Soils ◽  
Forest Ecosystems ◽  
C:N Ratio ◽  
N Cycling ◽  
Radiative Properties ◽  
N2o Emissions ◽  
Gross Nitrification ◽  
N2o Production ◽  
Average Contribution

Abstract. Forest ecosystems may provide strong sources of nitrous oxide (N2O), which is important for atmospheric chemical and radiative properties. Nonetheless, our understanding of controls on forest N2O emissions is insufficient to narrow current flux estimates, which still are associated with great uncertainties. In this study, we have investigated the quantitative and qualitative relationships between N-cycling and N2O production in European forests in order to evaluate the importance of nitrification and denitrification for N2O production. Soil samples were collected in 11 different sites characterized by variable climatic regimes and forest types. Soil N-cycling and associated production of N2O was assessed following application of 15N-labeled nitrogen. The N2O emission varied significantly among the different forest soils, and was inversely correlated to the soil C:N ratio. The N2O emissions were significantly higher from the deciduous soils (13 ng N2O-N cm-3d-1) than from the coniferous soils (4 ng N2O-N cm-3d-1). Nitrate (NO3-) was the dominant substrate for N2O with an average contribution of 62% and exceeding 50% at least once for all sites. The average contribution of ammonium (NH4+) to N2O averaged 34%. The N2O emissions were correlated with gross nitrification activities, and as for N2O, gross nitrification was also higher in deciduous soils (3.4 μ g N cm-3d-1) than in coniferous soils (1.1 μ g N cm-3d-1). The ratio between N2O production and gross nitrification averaged 0.67% (deciduous) and 0.44% (coniferous). Our study suggests that changes in forest composition in response to land use activities and global change may have implications for regional budgets of greenhouse gases. From the study it also became clear that N2O emissions were driven by the nitrification activity, although the N2O was produced per se mainly from denitrification. Increased nitrification in response to accelerated N inputs predicted for forest ecosystems in Europe may thus lead to increased greenhouse gas emissions from forest ecosystems.

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