A review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emission

Anthropogenic nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 313 observations across 109 studies evaluated the effect of N addition on the flux of three major GHGs: CO₂, CH₄ and N₂O. The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. Results indicate that N addition increased ecosystem carbon content of forests by 6%, marginally increased soil organic carbon of agricultural systems by 2%, but had no significant effect on net ecosystem CO₂ exchange for non-forest natural ecosystems. Across all ecosystems, N addition increased CH₄ emission by 97%, reduced CH₄ uptake by 38% and increased N₂O emission by 216%. The net effect of N on the global GHG budget is calculated and this topic is reviewed. Most often N addition is considered to increase forest C sequestration without consideration of N stimulation of GHG production in other ecosystems. However, our study indicated that although N addition increased the global terrestrial C sink, the CO₂ reduction could be largely offset (53–76%) by N stimulation of global CH₄ and N₂O emission from multiple ecosystems.     

Latitudinal differentiated water table control of carbon dioxide, methane and nitrous oxide fluxes from hydromorphic soils: feedbacks to climate change

The possibility of carbon (C) being locked away from the atmosphere for millennia is given in hydromorphic soils. However, the water-table-dependent feedback from soil organic matter (SOM) decomposition to the climate system is less clear. At least three greenhouse gases are produced: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). These gases show emission peaks at different water table positions and have different global warming potentials (GWP), for example a factor of 23 for CH₄ and 296 for N₂O as compared with the equivalent mass of CO₂ on a 100-year time horizon. This review of available annual data on all three gases revealed that the radiative forcing effect of SOM decomposition is principally dictated by CO₂ despite its low GWP. Anaerobic SOM decomposition generally has a lower potential feedback to the climatic system than aerobic SOM decomposition. Concrete values are constrained by a lack of data from tropical and subarctic regions. Furthermore, data on N₂O and on plant effects are generally rare. However, there is a clear latitudinal differentiation for the GWP of soils under anaerobic conditions compared with aerobic conditions when looking at CO₂ and CH₄: in the tropical and temperate regions, the anaerobic GWP showed a range of 25–60% of the aerobic value, but values varied between 80% and 110% in the boreal zone. Hence, particularly in the vulnerable boreal zone, the feedback from ecosystems to climate change will highly depend on plant responses to changing water tables at elevated temperatures.     

Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest

Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N₂O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N₂O, methane (CH₄) and carbon dioxide (CO₂) in field chambers monthly (bi-weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO₂ and CH₄ showed a strong seasonal pattern, with low rates in winter, but N₂O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N₂O flux and decreased CH₄ uptake in both treatment years, especially during winter. Annual N₂O fluxes in sugar maple treatment plots were 207 and 99 mg N m⁻² yr⁻¹ in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N₂O or CO₂ fluxes, but CH₄ uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N₂O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N₂O fluxes from northern hardwood forests.     

Activity and composition of ammonia oxidizing bacterial communities and emission dynamics of NH3 and N2O in a compost reactor treating organic household waste

Aims:  To monitor emissions of NH₃ and N₂O during composting and link these to ammonia oxidation rates and the community structure of ammonia oxidizing bacteria (AOB).
Methods and Results:  A laboratory-scale compost reactor treating organic household waste was run for 2 months. NH₃ emissions peaked when pH started to increase. Small amounts of N₂O and CH₄ were also produced. In total, 16% and less than 1% of the initial N was lost as NH₃-N and N₂O-N respectively. The potential ammonia oxidation rate, determined by a chlorate inhibition assay, increased fourfold during the first 9 days and then remained high. Initially, both Nitrosospira and Nitrosomonas populations were detected using DGGE analysis of AOB specific 16S rRNA fragments. Only Nitrosomonas europaea was detected under thermophilic conditions, but Nitrosospira populations re-established during the cooling phase.
Conclusions:  Thermophilic conditions favoured high potential ammonia oxidation rates, suggesting that ammonia oxidation contributed to reduced NH₃ emissions. Small but significant amounts of N₂O were emitted during the thermophilic phase. The significance of different AOBs detected in the compost for ammonia oxidation is not clear.
Significance and Impact of Study:  This study shows that ammonia oxidation occurs at high temperature composting and therefore most likely reduces NH₃ emissions.     

Spatial modelling of nitrous oxide emissions at the national scale using soil, climate and land use information

Nitrous oxide (N₂O) is a powerful greenhouse gas. The UK government is committed to reducing all greenhouse gas emissions and is required to make an inventory of the sources and emissions of these gases. Here, we extend work from a pilot study at the catchment scale reported in an earlier paper. This paper reports on the upscaling measurements of emissions to derive annual emission rates for specific combinations of soil type, land management and fertiliser practices to the national scale. Digital soil, climate and land use maps were combined within Geographic Information Systems (GIS) software. Upscaling of field emissions measurements involves adjusting measured annual N₂O emissions to fit combinations of crop growth cycles, soil wetness and the amount and timing of fertiliser applications. We have also taken account of the differences in emission rates from grazed pasture land due to differences in land management between land utilised for dairy production and land utilised for beef production. Calculated annual emission rates were then spatially scaled to derive national figures through the use of a GIS modelling framework, termed NitOx. The annual emission of N₂O from Scotland was determined as approximately 6 000 000 kg N yr⁻¹ (2.8 Mt carbon dioxide (CO₂) equivalents) and compares favourably with other national scale estimates such as the IPCC (1997) . The combination of animal grazing, high N inputs, climatic warmth and poorly drained soils means that the south west contributes significantly to the national total N₂O emissions. Localised areas of high emission can also be identified, but identification could be improved by applying this modelling approach at a larger scale. It would be beneficial to target these areas with mitigation strategies.     

Convergence in the relationship of CO2 and N2O exchanges between soil and atmosphere within terrestrial ecosystems

Ecosystem CO₂ and N₂O exchanges between soils and the atmosphere play an important role in climate warming and global carbon and nitrogen cycling; however, it is still not clear whether the fluxes of these two greenhouse gases are correlated at the ecosystem scale. We collected 143 pairs of ecosystem CO₂ and N₂O exchanges between soils and the atmosphere measured simultaneously in eight ecosystems around the world and developed relationships between soil CO₂ and N₂O fluxes. Significant linear regressions of soil CO₂ and N₂O fluxes were found for all eight ecosystems; the highest slope occurred in rice paddies and the lowest in temperate grasslands. We also found the dominant role of growing season on the relationship of annual CO₂ and N₂O fluxes. No significant relationship between soil CO₂ and N₂O fluxes was found across all eight ecosystem types. The estimated annual global N₂O emission based on our findings is 13.31 Tg N yr⁻¹ with a range of 8.19–18.43 Tg N yr⁻¹ for 1980–2000, of which cropland contributes nearly 30%. Our findings demonstrated that stoichiometric relationships may work on ecological functions at the ecosystem level. The relationship of soil N₂O and CO₂ fluxes developed here could be helpful in biogeochemical modeling and large-scale estimations of soil CO₂ and N₂O fluxes.     

Methane-dependent nitrate production by a microbial consortium enriched from a cultivated humisol

Abstract A consortium was enriched from a humisol incubated with 3.6 kPa CH₄ and NH₄⁺. This consortium oxidized NH₄⁺ to NO₂⁻ and NO₃⁻ (NO₃⁻/NO₂⁻ ratio about 20) with smaller amounts of N₂O. This oxidation stopped in the stationary phase after depletion of CH₄. CH₃OH or CO₂ did not support oxidation. Growth and resting cell experiments suggested that nitrification was associated with methanotrophic activity and that chemoautotrophic nitrifiers were absent.     

Development of a gas diffusion probe for the determination of methane concentrations and diffusion characteristics in flooded paddy soil

Abstract A probe for the measurement of dissolved CH₄ in anoxic methanogenic environments was developed. The probe was based on the diffusion of dissolved CH₄ through a silicone membrane into a gas space at the end of the probe. This gas space was flushed with N₂ and analyzed gas-chromatographically for CH₄. The probe had a spatial resolution of < 1.3 mm, the detection limit was about 20 μM CH₄, the precision of the measurement was 9%, and consecutive measurements could be made every 4 min. Memory effects after analysis of high CH₄ concentrations could be avoided by flushing the probe with N₂ between each measurement. The probe was sensitive for water movement and, therefore, was calibrated in an artificial sediment of glass beads (100 μm diam.) immersed by aqueous solutions of known CH₄ concentrations. Sensitivity of the probe for changes in the sediment's porosity could not presently be excluded. The probe was used to measure vertical profiles of dissolved CH₄ in microcosms of anoxic paddy soil. The vertical CH₄ profiles measured with the probe compared fairly well with those measured after an extraction procedure. The profiles clearly showed that CH₄ was produced in deeper layers and diffused upwards to be consumed in the oxic top 2 mm soil layers. The probe was also used to determine the diffusion coefficient of CH₄ in an inactivated paddy soil microcosm using a set-up which allowed modelling of a measured CH₄ concentration profile using Fick's 2nd law.     

Effects of temperature and fertilization on nitrogen cycling and community composition of an urban lawn

We examined the influence of temperature and management practices on the nitrogen (N) cycling of turfgrass, the largest irrigated crop in the United States. We measured nitrous oxide (N₂O) fluxes, and plant and soil N content and isotopic composition with a manipulative experiment of temperature and fertilizer application. Infrared lamps were used to increase surface temperature by 3.5±1.3 °C on average and control and heated plots were split into high and low fertilizer treatments. The N₂O fluxes increased following fertilizer application and were also directly related to soil moisture. There was a positive effect of warming on N₂O fluxes. Soils in the heated plots were enriched in nitrogen isotope ratio ( δ ¹⁵N) relative to control plots, consistent with greater gaseous losses of N. For all treatments, C₄ plant C/N ratio was negatively correlated with plant δ ¹⁵N, suggesting that low leaf N was associated with the use of isotopically depleted N sources such as mineralized organic matter. A significant and unexpected result was a large, rapid increase in the proportion of C₄ plants in the heated plots relative to control plots, as measured by the carbon isotope ratio ( δ ¹³C) of total harvested aboveground biomass. The C₄ plant biomass was dominated by crabgrass, a common weed in C₃ fescue lawns. Our results suggest that an increase in temperature caused by climate change as well as the urban heat island effect may result in increases in N₂O emissions from fertilized urban lawns. In addition, warming may exacerbate weed invasions, which may require more intensive management, e.g. herbicide application, to manage species composition.     

Soils,a sink for N2O? A review

Soils are the main sources of the greenhouse gas nitrous oxide (N₂O). The N₂O emission at the soil surface is the result of production and consumption processes. So far, research has concentrated on net N₂O production. However, in the literature, there are numerous reports of net negative fluxes of N₂O, (i.e. fluxes from the atmosphere to the soil). Such fluxes are frequent and substantial and cannot simply be dismissed as experimental noise.
Net N₂O consumption has been measured under various conditions from the tropics to temperate areas, in natural and agricultural systems. Low mineral N and large moisture contents have sometimes been found to favour N₂O consumption. This fits in with denitrification as the responsible process, reducing N₂O to N₂. However, it has also been reported that nitrifiers consume N₂O in nitrifier denitrification. A contribution of various processes could explain the wide range of conditions found to allow N₂O consumption, ranging from low to high temperatures, wet to dry soils, and fertilized to unfertilized plots. Generally, conditions interfering with N₂O diffusion in the soil seem to enhance N₂O consumption. However, the factors regulating N₂O consumption are not yet well understood and merit further study.
Frequent literature reports of net N₂O consumption suggest that a soil sink could help account for the current imbalance in estimated global budgets of N₂O. Therefore, a systematic investigation into N₂O consumption is necessary. This should concentrate on the organisms, reactions, and environmental factors involved.     

Isolation, characterization, and nitrification potential of a methylotroph and two heterotrophic bacteria from a consortium showing methane-dependent nitrification

Abstract A methanotrophic nitrifying consortium was previously obtained from a humisol which showed CH₄-dependent nitrification. Although the methanotroph could not be obtained in pure culture, three other members of the consortium have been isolated: An obligately methylotrophic Methylobacillus (Is-1) which grows only on CH₃OH and does not nitrify; a Pseudomonas (Is-2) which grows on Is-1 culture filtrate and produces NO₂⁻, NO₃⁻ and N₂O from NH₂OH, and NO₃⁻ from NO₂⁻; and a second Pseudomonas (Is-3) which produces NO₃⁻ from NH₄⁺ or NO₂⁻, and N₂O from NH₂OH. A model is proposed for the trophic relations and nitrogen transformations in the consortium which may apply to some natural systems.     

Measurement of denitrification in estuarine sediment using membrane inlet mass spectrometry

Abstract Denitrification was measured in intact sediment cores and in homogenised slurries using membrane inlet mass spectrometry. Dissolved concentrations of O₂, N₂, N₂O and CO₂ were simultaneously monitored. Using a 0.8 mm diameter needle probe, a comparison was made of the gas profiles of intact cores obtained under different conditions, i.e. with air or argon as the headspace gas and after the addition of nitrate and/or a carbon source to the sediment surface. O₂ was detectable to a depth of 1 cm under a headspace of air and the depth at which the maxima of denitrification products occurred was 1.5–2 cm. Denitrification products (N₂O, N₂) occurred in the surface layers where O₂ was above the minimum level of detectability (> 0.25 μM): diffusion of N₂ and N₂O upwards from the anoxic zone, local anaerobic microenvironments or aerobic denitrification are alternative explanations for this observation. The addition of nitrate and/or acetate increased the concentrations of N₂, N₂O and CO₂ in the sediment core. In sediment slurries, the pH, nitrate concentration, carbon source and the depth from which the sample was taken affected the rate of denitrification. Nitrogen was the sole detectable end product. Maximum denitrification occurred at pH 7.5 and at 20 mM nitrate. Denitrification was at a maximum in those slurries prepared from sections of core at 1–2 cm depth.     

Consumption of NO by methanotrophic bacteria in pure culture and in soil

Abstract The methanotrophs Methylomonas angile (type I) and Methylosinus trichosporium (type II) produced nitrite, nitrate and N₂O during growth on methane, apparently by heterotrophic nitrification of ammonium. The methanotrophs were also able to consume NO but did not produce it. After incubation of soil from a drained paddy field in the presence of CH₄ the numbers of methanotrophs increased from 10⁵ to 10⁷ per gram dry weigth. The thus enriched soil showed increased rates of NO consumption while rates of NO production did not change.     

Missing methane emissions from leaves of terrestrial plants

The controversial claim that attached leaves of terrestrial plants emit CH₄ aerobically remains to be corroborated. Here, we report CH₄ fluxes and CO₂ exchange rates for leaves of the C₄ species Zea mays using a high-accuracy traceable online analytical system. In contrast to earlier results for Z. mays , our measurements provide no evidence for substantial aerobic CH₄ emissions from photosynthesizing leaves illuminated with photosynthetically active radiation ( λ =400–700 nm), or from dark-respiring leaves. Preliminary measurements with the same system indicated a similar lack of aerobic CH₄ emissions in the light or dark from leaves of the C₃ species Nicotiana tabacum . These findings are supported by independent high-precision ¹³C-labeling studies that also failed to confirm substantial aerobic CH₄ emissions from plants. Nevertheless, we are not able to exclude the possibility that CH₄ emissions from plants may be linked to nonenzymatic processes with an action spectrum lying outside the wavelength range for photosynthesis.     

Production, oxidation and emission of methane in rice paddies

Abstract Production and emission of methane from submerged paddy soil was studied in laboratory rice cultures and in Italian paddy fields. Up to 80% of the CH₄ produced in the paddy soil did not reach the atmosphere but was apparently oxidized in the rhizosphere. CH₄ emission through the rice plants was inhibited by an atmosphere of pure O₂ but was stimulated by an atmosphere of pure N₂ or an atmosphere containing 5% acetylene. Gas bubbles taken from the submerged soil contained up to 60% CH₄, but only < 1% CH₄ after the bubbles had passed the soil-water interface or had entered the intercellular gas space system of the rice plants. CH₄ oxidation activities were detected in the oxic surface layer of the submerged paddy soil. Flooding the paddy soil with water containing > 0.15% sea salt (0.01% sulfate) resulted in a strong inhibition of the rates of methanogenesis and a decrease in the rates of CH₄ emission. This result explains the observation of relatively low CH₄ emission rates in rice paddy areas flooded with brackish water.     

Effects of oxygen, pH and nitrate concentration on denitrification by Pseudomonas species

Abstract The production of nitrogen-containing gases by denitrification in three organisms was examined using membrane inlet mass spectrometry. The effects of O₂ (during both growth and maintenance) and of pH, nitrate concentration and carbon source were tested in non-proliferating cell suspensions. Two strains of Pseudomonas aeruginosa were capable of co-respiration of NO₃⁻ and O₂ and, under controlled O₂ supply, gave oscillatory denitrification. Variations in culture and assay conditions affected both the rate of denitrification and the ratio of end products (N₂O:N₂). Higher rates were seen following anaerobic growth. Optimum values of pH and nitrate concentration for denitrification are given. Generally, the optimum pH was 7.0–7.5, approximately that of the growth medium. Optimum nitrate concentration was generally 20 mM.     

Hurricane-induced nitrous oxide fluxes from a wet tropical forest

Hurricane activity is predicted to increase over the mid-Atlantic as global temperatures rise. Nitrous oxide (N₂O), a greenhouse gas with a substantial source from tropical soils, may increase after hurricanes yet this effect has been insufficiently documented. On September 21, 1998, Hurricane Georges crossed Puerto Rico causing extensive defoliation. We used a before–after design to assess the effect of Georges on N₂O emissions, and factors likely influencing N₂O fluxes including soil inorganic nitrogen pools and soil water content in a humid tropical forest at El Verde, Puerto Rico. Emissions of N₂O up to 7 months post-Georges ranged from 5.92 to 4.26 ng cm⁻² h⁻¹ and averaged five times greater than fluxes previously measured at the site. N₂O emissions 27 months after the hurricane remained over two times greater than previously measured fluxes. Soil ammonium pools decreased after Georges and remained low. The first year after the hurricane, nitrate pools increased, but not significantly when compared against a single measurement made before the hurricane. Soil moisture and temperature did not differ significantly in the two sampling periods. These results suggest that hurricanes increase N₂O fluxes in these forests by altering soil N transformations and the relative availabilities of inorganic nitrogen.     

Field determination of denitrification in water-logged forest soils

Abstract Denitrification rates were measured as N₂O production in two water-logged forest soils at monthly intervals. The effect of acetylene inhibition and the addition of nitrate, glucose, acetate and celloboise in field incubations was examined.
N₂O release from the two soils was very low, 26 mg N₂m⁻²y⁻¹ in ash and 178 mg N₂ in alder. In acetylene inhibited incubations N₂O production was higher, 296 and 486 mg N₂m⁻² y⁻¹ in ash and alder respectively. After addition of nitrate and C-sources to a 10 mM concentration, denitrification rates increased to 5–15 times higher values.
The denitrification rates below 4°C were low and most N₂O was produced in late spring and summer.
The highest rate of denitrification during a 50 h incubation experiment occurred between 3 and 23 h.     

Reduction of nitrous oxide by a soil Cytophaga in the presence of acetylene and sulfide

Abstract A denitrifying Cytophaga was isolated from soil enriched by anaerobic incubation with glucose, sulfide (S²⁻), nitrous oxide (N₂O), and acetylene (C₂H₂). Such soil enrichments and pure cultures of the isolated Cytophaga reduced N₂O rapidly even in the presence of a normally inhibitory concentration of C₂H₂ (4 kPa) providing S²⁻ was present (8 μmol/g soil or 0.4 μmol/ml culture). Since C₂H₂ inhibition of the reduction of N₂O is used as a tool in the assay of denitrification, the presence in large numbers of such a Cytophaga may influence the effectiveness of this assay especially in sulfidic environments.     

Sensitivity of methanogenic bacteria from paddy soil to oxygen and desiccation

Abstract The effects of combinations of desiccation and exposure to O₂ were studied in pure cultures of Methanosarcina barkeri strain Fusaro and in a new Methanosarcina strain and a new Methanobacterium strain which were both isolated from dry oxic paddy soil. Incubation of bacterial suspensions under air for 200 min resulted in a decreased potential to produce CH₄, but not in a decreased viability. The inhibitory effect of O₂ slightly increased with increased salt concentration. Desiccation of bacterial suspensions under N₂ resulted in reduction of viability to 10% and of potential CH₄ production to 0.6%. Desiccation of bacterial suspensions under air resulted in a larger decrease of both viability (0.5%) and potential CH₄ production (0.03%). This decrease was smaller at rapid compared to slow desiccation. Survival and potential CH₄ production were further inhibited when the suspension was dried in the presence of sand grains or glass beads coated with FeS or FeNH₄PO₄. However, survival and potential CH₄ production increased dramatically in the presence of pyrite (FeS₂) grains. Then, as much as 10% of the initial methanogenic population survived oxic desiccation. This relatively good resistance is in agreement with observations that methanogens in rice fields survive the periods when the paddy soil is dry and oxic.