Sink For Atmospheric N2O Research Articles

Annual Greenhouse-Gas Emissions from Forest Soil of a Peri-Urban Conifer Forest in Greece under Different Thinning Intensities and Their Climate-Change Mitigation Potential

Abstract Knowledge of the response of thinning implementation on forest soil–atmospheric greenhouse gas (GHG) (CO2, CH4, N2O) fluxes exchange system in Mediterranean region is limited because of the high heterogeneity of both soil properties and forest biomass. The novelty of this study is grounded predominantly in evaluating for the first time the response of annual GHG fluxes to thinning in a coniferous peri-urban forest soil in Greece, thus contributing significantly to the enrichment of the GHG fluxes database from the Mediterranean forest ecosystem. Results suggest that CH4 uptake increased with increasing thinning intensity. The reduction in CO2 emissions in both thinning treatments was possibly related to an indirect effect of soil heterotrophic and autotrophic respiration. Coniferous peri-urban forests in Greece can act temporally as sinks of atmospheric N2O in the coldest months and a weak source of N2O fluxes in summer. The GHG variation depended largely on soil environmental factors with soil temperature representing the dominant factor for CO2 and CH4, whereas soil moisture correlated, albeit weakly, with N2O variability. Reduction in global warming potential was observed in both thinning treatments, markedly in selective treatment, giving an initial indication that high-intensity thinning in coniferous peri-urban forests in Greece presents a high potential for global change mitigation.

Nitrous oxide and methane seasonal variability in the epilimnion of a large tropical meromictic lake (Lake Kivu, East-Africa)

We report a data-set of monthly vertical profiles obtained from January 2012 to October 2013, from the surface to 70 m depth of nitrous oxide (N2O) and dissolved methane (CH4) in Lake Kivu, a large and deep meromictic tropical lake (East Africa). Vertical variations of N2O were modest, with ranges of 6–9 and 0–16 nmol L−1 in surface and bottom waters, respectively, and occasionally peaks of N2O (up to 58 nmol L−1) were observed at the oxic-anoxic interface. On the contrary, steep vertical gradients of CH4 were observed with values changing several orders of magnitude from surface (19–103 nmol L−1) to 70 m (~113,000–520,000 nmol L−1). Seasonal variations of CH4 were caused by annual cycles of mixing and stratification, during the dry and rainy seasons, respectively. This mixing allowed the establishment of a thick oxic layer (maximum 65 m deep), leading to decreased CH4 concentrations (minimum of 8 nmol L−1), presumably due to bacterial CH4 oxidation. During the stratification period, the oxic mixed layer was thinner (minimum 25 m deep), and an increase of CH4 concentrations in surface waters was observed (maximum of 103 nmol L−1), probably due to a lower integrated CH4 oxidation on the water column. Lake Kivu seasonally alternated between a source and a sink for atmospheric N2O, but on an annual scale was a small source of N2O to the atmosphere (on average 0.43 µmol m−2 day−1), while it was a small source of CH4 to the atmosphere throughout the year (on average 86 µmol m−2 day−1). Vertical and seasonal variations of N2O are discussed in terms of nitrification and denitrification, although from the present data-set it is not possible to unambiguously identify the main drivers of N2O production.

Pristine mangrove creek waters are a sink of nitrous oxide.

Nitrous oxide (N2O) is an important greenhouse gas, but large uncertainties remain in global budgets. Mangroves are thought to be a source of N2O to the atmosphere in spite of the limited available data. Here we report high resolution time series observations in pristine Australian mangroves along a broad latitudinal gradient to assess the potential role of mangroves in global N2O budgets. Surprisingly, five out of six creeks were under-saturated in dissolved N2O, demonstrating mangrove creek waters were a sink for atmospheric N2O. Air-water flux estimates showed an uptake of 1.52 ± 0.17 μmol m−2 d−1, while an independent mass balance revealed an average sink of 1.05 ± 0.59 μmol m−2 d−1. If these results can be upscaled to the global mangrove area, the N2O sink (~2.0 × 108 mol yr−1) would offset ~6% of the estimated global riverine N2O source. Our observations contrast previous estimates based on soil fluxes or mangrove waters influenced by upstream freshwater inputs. We suggest that the lack of available nitrogen in pristine mangroves favours N2O consumption. Widespread and growing coastal eutrophication may change mangrove waters from a sink to a source of N2O to the atmosphere, representing a positive feedback to climate change.

An unaccounted for N2O sink in the surface water of the eastern subtropical South Pacific: Physical versus biological mechanisms

Nitrous oxide (N2O) is a trace gas affecting atmospheric radiative forcing through its greenhouse effect in the troposphere and destroying the ozone in the stratosphere. The oceans account for one-third of the global atmospheric N2O emissions, in which they are primarily cycled by nitrification and denitrification, with high N2O production in the subsurface waters. The surface waters are generally reported to be in equilibrium or slightly supersaturated with respect to the atmosphere. However, surface N2O sub-saturations have been observed in several regions of the world’s oceans, such as off south-central Chile, which is bathed by the Sub-Antarctic Water Mass (SAAW), where N2O subsaturations as low as 35% have been registered during the austral spring and summer. An analysis of the mechanisms driving such surface N2O subsaturations (physical or biological) showed that physical mechanisms were not responsible for the high surface N2O deficit. In contrast, in situ potential experiments in surface waters with 15N2O addition showed an active biological N2O fixation (between 0.43 and 87.34nmol/L/d), with the highest N2O fixation rates associated with the SAAW (25.25–25.75kg/m3).Additionally, incubation experiments with 15N2O in surface water samples from one oceanic station showed high 15N-POM enrichment (0.44‰) and an inhibition of 15N-POM enrichment when an additional nitrogen source was added (NO2− and NH4+). These results suggest the existence of a mechanism able to use several nitrogen sources, including N2O. Molecular analyses (16S rRNA gene) from these experiments showed the presence of three major groups of bacteria: Gammaproteobacteria, Flavobacteria and Cyanobacteria, with Synechococcus sp. being the dominant group in the culture. However, the analysis of the nifH gene showed a taxonomic affiliation to the order Stigonematales associated with Mastigocladus sp. and Fischerella sp. and the order Oscillatoriales associated with Trichodesmium sp.Finally, the oceanic region exhibiting surface N2O subsaturations acts as a sink for atmospheric N2O, consuming ∼11.4 Gg of N2O in a six-month period. The N2O levels in the sink are 75% higher than those of the reported N2O source from the coastal band. The balance between the oceanic region and the coastal band results in a sink region of 4.94 Gg of N2O during this period.

First measurements of nitrous oxide in Arctic sea ice

Nitrous oxide (N2O) contributes ∼6% of the total radiative forcing from long‐lived greenhouse gases. While tropospheric concentrations have increased by 20% since the beginning of the industrial revolution, sources and sinks of N2O are still poorly quantified. In the Arctic, N2O atmospheric concentrations vary seasonally, due mainly to vertical mixing. The contributions of local natural sources to this cycle are still unknown. Here we report on N2O measurements conducted in the bottom 10 cm of the sea ice and in the underlying surface water (USW) from late March to early May 2008 in the southeastern Beaufort Sea and Amundsen Gulf. Bulk N2O concentrations in ice were low (∼6 nM) and were consistently undersaturated with respect to the USW (∼40% saturation) and the atmosphere (∼30% saturation). Loss of N2O via brine rejection during sea ice formation in fall and winter can explain these low N2O ice concentrations. An unknown fraction of this rejected N2O is likely ventilated to the atmosphere either directly from the ice or through leads during ice formation, while in spring and early summer, melting of the N2O‐depleted sea ice is expected to lower the partial pressure of N2O of newly open waters which could act as a sink for atmospheric N2O. These first measurements indicate that sea ice formation and melt has the potential to generate sea‐air or air‐sea fluxes of N2O, respectively.

Extending records of the isotopic composition of atmospheric N2O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core

A historical record of changes in the N2O isotope composition is important for a better understanding of the biogeochemical cycles involving atmospheric N2O. Here we combine measurements of trapped gases in the Greenland Ice Sheet Project II (GISP II) ice core and the interstitial air from the snowpack at the South Pole to reconstruct atmospheric N2O isotope records back to 1785 A.D. Results from six measurements of ice with gas ages between 1785 and 1819 A.D. indicate that the δ15N and δ18O of atmospheric N2O was 10.1 ± 0.6 and 47.4 ± 1.0‰, respectively. These values are 1.9 and 2.9‰ higher than present‐day values as measured in this study. A simple two‐box atmospheric model was constructed to simulate the various sources and sinks of atmospheric N2O over the last 250 years. Model results suggest that the total anthropogenic N2O emissions in 1995 were between 4.2 and 5.7 Tg N yr−1. The ice core and box model results were compared to δ15N and δ18O measurements of air trapped in the snowpack at South Pole spanning the 20th century. Our results suggest that δ15N and δ18O of atmospheric N2O have dropped by 1.7 and 0.9‰, respectively, during the last century. These data support previous predictions of decreasing atmospheric isotope ratios that are related to a ∼30% increase in total N2O emissions which are primarily related to agricultural activities.

Dynamics of dissolved O2, CO2, CH4, and N2O in a tropical coastal swamp in southern Thailand.

We studied the distribution of dissolved O2, CO2, CH4, and N2O in a coastal swamp system in Thailand with the goal to characterize the dynamics of these gases within the system. The gas concentrations varied spatially and seasonally in both surface and ground waters. The entire system was a strong sourcefor CO2 and CH4, and a possible sink for atmospheric N2O. Seasonal variation in precipitation primarily regulated the redox conditions in the system. However, distributions of CO2, CH4, and N2O in the river that received swamp waters were not always in agreement with redox conditions indicated by dissolvedO2 concentrations. Sulfate production through pyriteoxidation occurred in the swamp with thin peat layerunder aerobic conditions and was reflected by elevatedSO 4 2− /Cl− in the river water. When SO 4 2− /Cl− was high, CO2 and CH4 concentrations decreased, whereas the N2O concentration increased. The excess SO 4 2− in the river water was thus identified as a potential indicator for gas dynamics in this coastal swamp system.

Methods for measuring N2O flux from water surface and N2O dissolved in water from agricultural land

A new chamber method and a stripping method were developed for field measurements of the rate of N2O emission from the water surface and for determinations of dissolved N2O in water from agricultural land. These methods were used for the measurement of drainage canal water and flooded water of rice fields during the period of June 1982 to January 1983. The results demonstrate that aquatic systems of agricultural land may provide both source and sink for atmospheric N2O.

Denitrification loss from a grassland soil in the field receiving different rates of nitrogen as ammonium nitrate

SummaryDenitrification loss from a loam under a cut ryegrass sward receiving 0, 250 and 500 kg N ha−1 a−1 in four equal amounts was measured during 14 months using the acetylene‐inhibition technique. The rate of denitrification responded rapidly to changes in soil water content as affected by rain. Mean rates of denitrification exceeded 0.2 kg N ha−1 day−1 only when the soil water content was >20% (w/w) and nitrate was >5μ N g−1 in the upper 20 cm of the profile and when soil temperature at 2 cm was >5–8°C. When the soil dried to a water content <20%, denitrification decreased to <0.05 kg N ha−1 day−1. Highest rates (up to 2.0 kg N ha−1 day−1) were observed following application of fertilizer to soil at a water content of about 30% (w/w) in early spring. Denitrification in the control plot during this period was generally about a hundredth of that in plots treated with ammonium nitrate. High rates of N2O loss (up to 0.30 kg N ha−1 day‐1) were invariably associated with high rates of denitrification (> 0.2 kg N ha−1 day−1). However, within 2–3 weeks following application of fertilizer to the plot receiving 250 kg N ha−1 a−1 the soil acted as a sink for atmospheric N2O when its water content was >20% and its temperature >5–8°C. Annual N losses arising from denitrification were 1.6, 11.1 and 29.1 kg N ha−1 for the plots receiving 0, 250 and 500 kg N ha−1 a−1, respectively. More than 60% of the annual loss occurred during a period of 8 weeks when fertilizer was applied to soil with a water content >20%.

THE EFFECT OF FLOODING ON NITROUS OXIDE EMISSIONS FROM AN ORGANIC SOIL

To determine the effect of flooding of Pahokee muck (a drained, cultivated Histosol) on nitrous oxide emissions from the soil, we measured field N2O fluxes and N2O produced in soil samples incubated under controlled laboratory conditions. During the first 5 days of flooding of a field that had previously been cropped to sweet corn (Zea mays L. rugosa), the NO3--N levels declined from 68 to <0.1 microgram nitrogen per cubic centimeter. Similarly, the N2O flux decreased from 174 g N/ha/d prior to flooding to approximately 0 after flooding. Comparing the flux values and the N2O concentrations in the flood water suggested that the flooded field may have acted as a sink for atmospheric N2O. In laboratory-incubated samples of the preflood and flooded soils, the potential denitrification rates increased from 16.2 μg N/cm3/day prior to flooding to 31.2 μg N/cm3/d after flooding. Whereas N2O was the major product of denitrification in soil samples collected prior to flooding, very little N2O accumulated in samples collected from the flooded field. Nitrous oxide was readily reduced in the flooded soil in the presence of NO3--N and NO2--N concentrations of 100 and 3 mg/liter, respectively.

N2O exchange between a grassland soil and the atmosphere

The recognition that NO, derived from atmospheric N2O, is an important factor in the destruction of stratospheric ozone1, and that atmospheric N2O may reduce radiative heat loss from the Earth's surface2, has stimulated interest3–6 in the global sources and sinks for N2O. One possibility is that increasing use of industrially fixed N in agricultural production increases loss of N2O to the atmosphere. Until recently, however, field data for the evaluation of this concept were not available. Even now, only a few longer-term studies of N2O loss from representative agricultural land permit estimates of loss over a complete year or a cropping season. In the present study, field measurements of N2O exchange over 12 months between a grassland soil and the atmosphere indicated that the soil acts as both a source and a sink for atmospheric N2O, depending on soil conditions and the amount of nitrogenous fertilizer applied.

Diurnal variability in the concentration of nitrous oxide in surface air

Significant diurnal variations in the concentration of nitrous oxide (N2O) in surface air were detected by analysis of air samples collected at two sites in Iowa. At both sites, the highest concentrations of N2O were observed during early morning hours. The diurnal variations observed decreased with increasing height above ground level and indicated that soil in the areas studied was a source of atmospheric N2O. They differed markedly from the variations detected in recent work leading to postulation of the existence of a major ground surface sink for atmospheric N2O.

The cycle of atmospheric nitrous oxide

An attempt has been made to construct an N 2 O balance of the troposphere in order to demonstrate the state of knowledge of the cycle of atmospheric N 2 O. This balance has been constructed considering the parameters relevant for atmospheric N 2 O as far as known such as the horizontal and vertical distribution in the atmosphere and in the oceans, the variation of the atmospheric mixing ratio with time, the fluxes in the air-sea and in the air-soil systems, and man-made fluxes. The total atmospheric burden of N 2 O appears to be in the range 1600-2100 Mt, the tropospheric turn-over time between 4 and 12 years. Most of the atmospheric N 2 O seems to originate from microbial activities at the Earth’s surface with the oceans apparently being the major source with about 35 % of the total source as obtained from the tropospheric N 2 O mass and turn-over time (range of uncertainty 5-100 %) followed by soil and fresh water with about 25 % of the total source (range of uncertainty 4-100 %) including the contribution of industrial nitrogen fertilizers with about 10 % of the total source (range of uncertainty 2-24 %). Direct man-made N 2 O sources seem to be relatively small with about 2 % of the total source (range of uncertainty 0.5-5 %), but may grow considerably in the future. The N 2 O production by chemical reactions in the troposphere itself (e.g. electrical discharges) is not known; a figure of between 3 and 70 % of the total source has been inferred from laboratory experiments. The sinks of atmospheric N 2 O are poorly known. The only known major N 2 O destruction appears to occur in the stratosphere. The rate of transport into the stratosphere, however, limits this sink to about 10 % of the total amount of N 2 O to be destroyed under steady state conditions (range of uncertainty 3-22 %). The major problem of the cycle of atmospheric N 2 O is the unknown main sink. Since the stratosphere does not seem to come into question, the missing sink should be found somewhere in the troposphere, pedosphere, and/or hydrosphere.

Potential of soil as a sink for atmospheric nitrous oxide

Recent articles concerning the potential effect of nitrous oxide (N2O) on stratospheric ozone have emphasized the need for research to identify sources and sinks of atmospheric N2O. Work reported shows that soils have a significant capacity for uptake of N2O and may represent an important natural sink for atmospheric N2O. Uptake of N2O by soils is a microbial process involving reduction of N2O to N2. It is promoted by anaerobic conditions and by organic substances that promote growth of soil microorganisms, and it is retarded by nitrate. Studies with nine Iowa surface soils showed that their capacity for uptake of N2O under conditions favorable for denitrification of nitrate was much greater than their capacity for release of this gas.