Μg N2O Research Articles

Effect of Scirpus mariqueter on nitrous oxide emissions from a subtropical monsoon estuarine wetland

The effects of the wetland plant Scirpus mariqueter on nitrous oxide (N2O) emissions in Yangtze estuary, China, were investigated using an in situ static chamber technique. Field measurements spanned the entire growing season (May to October) and encompassed a wide range of weather conditions typical of the subtropical monsoon climate of this region. Simultaneous measurement of carbon dioxide (CO2) and anatomical measurements were conducted to experimentally determine the gas transport mechanisms of S. mariqueter on N2O flux. S. mariqueter had a significant effect on N2O flux. Based on the comparison of light‐dark and clipped‐unclipped gas flux, N2O flux was negatively correlated with NEE (p < 0.0001) and NPP (p < 0.001) under light conditions when S. mariqueter was present but positively with temperatures in the dark condition or when S. mariqueter was clipped. Besides the plant uptake corresponding to the N2O negative flux in light chamber, it is reasonable to assume that because of the limitation of nitrate in sediment, coupled nitrification‐denitrification is the main process of N2O producing. O2 transported into the S. mariqueter rhizosphere during photosynthesis stimulated denitrifier also would consume the N2O and would be induced to the N2O diffusing from atmosphere into sediment. Although photosynthetic activity of S. mariqueter attenuated N2O flux significantly over the course of the entire study period, creating a net sink for atmospheric N2O under light condition, the marsh of Chongming Island Dongtan wetland was a net source of atmospheric N2O during the active S. mariqueter growth phase (averaged flux was 98.3 μg N2O m−2 h−1).

Effects of flooding-induced N2O production, consumption and emission dynamics on the annual N2O emission budget in wetland soil

Rapid flooding of wetland soil promotes subsurface N2O production in the soil and potential emission to the atmosphere in distinctive emission pulses. Changes in flooding frequency of wetland soil following future climate change will likely affect the timing and magnitude of nitrous oxide (N2O) emissions from the soil to the atmosphere. From October 2009 to October 2010, rapid flooding of a natural Danish wetland was observed twice in response to high precipitation events. A flooding-induced N2O emission pulse (delay ∼16 h; duration ∼12 h; total emission 1.83 mg N2O–N m−2, max. emissions ∼250 μg N2O–N m−2 h−1) was observed when the soil conditions in the top soil had been oxidized for more than 2–3 weeks prior to flooding. This flooding-induced N2O emission pulse constituted ∼2.5% of the annual net N2O emissions of 0.74 kg N2O–N ha−1 yr−1. A net uptake of atmospheric N2O was observed during mid-summer when the WL was at its seasonally lowest counterbalancing ∼6.4% of the total annual net N2O emission budget. Main surface emission periods of N2O were observed when the water level and associated peaks in subsurface N2O concentrations were gradually decreasing to soil depths down to 40 cm below the surface. These surface emission patterns are predominantly linked to variations in plant-mediated gas transport via Phalaris arundinacea and N2O producing/consuming processes in the root zone. Soil flooding experiments using high-resolution N2O microsensors demonstrate very large N2O production and consumption capacities where >500 nmol N2O cm−3 were sequentially produced and consumed in less than 24 h. It is concluded that a higher future frequency of flooding-induced N2O emissions will have a very limited effect on the net annual N2O emission budget as long as NO3− availability in the soil prior to rapid flooding is not dramatically increased.

Methane and nitrous oxide emissions from rice seedling nurseries under flooding and moist irrigation regimes in Southeast China

Measurements of methane (CH4) and nitrous oxide (N2O) fluxes have been extensively taken following rice seedlings transplanted into paddy fields, while little is known about CH4 and N2O fluxes from rice seedling nurseries. Fluxes of CH4 and N2O were simultaneously measured in rice seedling nurseries under the water regimes of continuous flooding and moist irrigation without waterlogging in Southeast China in 2010. Fluxes of CH4 and N2O from continuously flooded nurseries averaged 10.33–14.84mgm−2h−1 and 28.64–34.35μg N2O–Nm−2h−1 for the different fertilizer applied plots, respectively. Relative to continuous flooding, moist irrigation decreased total CH4 by 14–50% but increased N2O by 72–186%, dependent on the fertilizer types. Compared with inorganic N fertilizer, organic manure application increased CH4 by 44% and 148% in the continuously flooded and moist irrigation nurseries, respectively. Rice seedling growth parameters were the greatest in moist irrigation nurseries with inorganic N fertilizer application. Moist irrigation instead of continuous waterlogging and shifts from organic manure to combined organic/inorganic N fertilizer inputs have been increasingly experienced in Chinese rice seedling nurseries, which would benefit for mitigating the combined global warming potentials of CH4 and N2O from rice seedling nurseries in China.

Soil Greenhouse Gas Fluxes during Wetland Forest Retreat along the Lower Savannah River, Georgia (USA)

Tidal freshwater forested wetlands (tidal swamps) are periodically affected by salinity intrusion at seaward transitions with marsh, which, along with altered hydrology, may affect the balance of gaseous carbon (C) and nitrogen (N) losses from soils. We measured greenhouse gas emissions (CO2, CH4, N2O) from healthy, moderately degraded, and degraded tidal swamp soils undergoing sea-level-rise-induced retreat along the lower Savannah River, Georgia, USA. Soil CO2 flux ranged from 90.2 to 179.1 mg CO2 m−2 h−1 among study sites, and was the dominant greenhouse gas emitted. CO2 flux differed among sites in some months, while CH4 and N2O fluxes were 0.18 mg CH4 m−2 h−1 and 1.23 μg N2O m−2 h−1, respectively, with no differences among sites. Hydrology, soil temperature, and air temperature, but not salinity, controlled the annual balance of soil CO2 emissions from tidal swamp soils. No clear drivers were found for CH4 or N2O emissions. On occasion, large ebbing or very low tides were even found to draw CO2 fluxes into the soil (dark CO2 uptake), along with CH4 and N2O. Overall, we hypothesized a much greater role for salinity and site condition in controlling the suite of greenhouse gases emitted from tidal swamps than we discovered, and found that CO2 emissions—not CH4 or N2O–contributed most to the global warming potential from these tidal swamp soils.

Potential of denitrification and nitrous oxide production from agricultural soil profiles (Seine Basin, France)

The denitrification process and the associated nitrous oxide (N2O) production in soils have been poorly documented, especially in terms of soil profiles; most work on denitrification has concentrated on the upper layer (first 20 cm). The objectives of this study were to examine the origin of N2O emission and the effects of in situ controlling factors on soil denitrification and N2O production, also allowing the (N2O production)/(NO3−–N reduction) ratio to be determined through (1) the position on a slope reaching a river and (2) the depth (soil horizons: 10–30 and 90–110 cm). In 2009 and 2010, slurry batch experiments combined with molecular investigations of bacterial communities were conducted in a corn field and an adjacent riparian buffer strip. Denitrification rates, ranging from 0.30 μg NO3−–N g−1 dry soil h−1 to 1.44 μg NO3−–N g−1 dry soil h−1, showed no significant variation along the slope and depth. N2O production assessed simultaneously differed considerably over the depth and ranged from 0.4 ng N2O–N g−1 dry soil h−1 in subsoils (the 90–110-cm layer) to 155.1 ng N2O–N g−1 dry soil h−1 in the topsoils (the 10–30-cm layer). In the topsoils, N2O–N production accounted for 8.5–48.0% of the total denitrified NO3−–N, but for less than 1% in the subsoils. Similarly, N2O-consuming bacterial communities from the subsoils greatly differed from those of the topsoils, as revealed by their nosZ DGGE fingerprints. High N2O-SPPR (nitrous oxide semi potential production rates) in comparison to NO3-SPDR (nitrate semi potential reduction rates) for the topsoils indicated significant potential greenhouse N2O gas production, whereas lower horizons could play a role in fully removing nitrate into inert atmospheric N2. In terms of landscape management, these results call for caution in rehabilitating or constructing buffer zones for agricultural nitrate removal.

Field-Scale Transplantation Experiment To Investigate Structures of Soil Bacterial Communities at Pioneering Sites

Studies on the effect of environmental conditions on plants and microorganisms are a central issue in ecology, and they require an adequate experimental setup. A strategy often applied in geobotanical studies is based on the reciprocal transplantation of plant species at different sites. We adopted a similar approach as a field-based tool to investigate the relationships of soil bacterial communities with the environment. Soil samples from two different (calcareous and siliceous) unvegetated glacier forefields were reciprocally transplanted and incubated for 15 months between 2009 and 2010. Controls containing local soils were included. The sites were characterized over time in terms of geographical (bedrock, exposition, sunlight, temperature, and precipitation) and physicochemical (texture, water content, soluble and nutrients) features. The incubating local ("home") and transplanted ("away") soils were monitored for changes in extractable nutrients and in the bacterial community structure, defined through terminal restriction fragment length polymorphism (T-RFLP) of the 16S rRNA gene. Concentrations of soluble ions in most samples were more significantly affected by seasons than by the transplantation. For example, NO(3)(-) showed a seasonal pattern, increasing from 1 to 3 μg NO(3)(-) (g soil dry weight)(-1) after the melting of snow but decreasing to <1 μg NO(3)(-) (g soil dry weight)(-1) in autumn. Seasons, and in particular strong precipitation events occurring in the summer of 2010 (200 to 300 mm of rain monthly), were also related to changes of bacterial community structures. Our results show the suitability of this approach to compare responses of bacterial communities to different environmental conditions directly in the field.

Indirect N2O emissions from shallow groundwater in an agricultural catchment (Seine Basin, France)

Production and accumulation of nitrous oxide (N2O), a major greenhouse gas, in shallow groundwater might contribute to indirect N2O emissions to the atmosphere (e.g., when groundwater flows into a stream or a river). The Intergovernmental Panel on Climate Change (IPCC) has attributed an emission factor (EF5g) for N2O, associated with nitrate leaching in groundwater and drainage ditches—0.0025 (corresponding to 0.25% of N leached which is emitted as N2O)—although this is the subject of considerable uncertainty. We investigated and quantified the transport and fate of nitrate (NO3 −) and dissolved nitrous oxide from crop fields to groundwater and surface water over a 2-year period (monitoring from April 2008 to April 2010) in a transect from a plateau to the river with three piezometers. In groundwater, nitrate concentrations ranged from 1.0 to 22.7 mg NO3 −–N l−1 (from 2.8 to 37.5 mg NO3 −–N l−1 in the river) and dissolved N2O from 0.2 to 101.0 μg N2O–N l−1 (and from 0.2 to 2.9 μg N2O–N l−1 in the river). From these measurements, we estimated an emission factor of EF5g = 0.0026 (similar to the value currently used by the IPCC) and an annual indirect N2O flux from groundwater of 0.035 kg N2O–N ha−1 year−1, i.e., 1.8% of the previously measured direct N2O flux from agricultural soils.

Soil Degradation Due to Vicinal Intensive Hog Farming Operation Located in East Mediterranean

One of the main environmental impacts of concentrated animal feeding operations is the soil degradation in vicinity with the livestock breeding facilities due to substances such as ammonia emitted from the various stages of the process. Owing to the high temperatures of the Mediterranean ecosystems, the evolution of gasses is more extensive and the soil degradation is consequently more severe than those obtained in northern Europe. In this research, the soil degradation effects of a large meat-producing, processing, and packaging unit have been investigated. The investigated intensive hog farming operation (IHFO) is located at a limestone soil coastal area with sea to the north and hills to the south. Soil samples of the upper mineral soil were taken in various distances and directions from the IHFO boundaries. Thirteen experimental cycles were carried out in the duration of 1.5 years starting in March 2009 until October 2010. The soil samples were analyzed on pH and electrical conductivity (EC) values as well as NH4 + and NO3 − concentrations. Significantly higher concentrations of the two nitrogen forms were observed on samples at increasing proximity downwind from the farm (south). Southern soil average NH4 + and NO3 − concentrations ranged between 0.4–118 μg NH4 +-N g−1 soil and 6.1–88.4 μg NO3 −-N g−1 soil, respectively. The variation of emitted gasses depositions was clearly reflected in the average pH and EC values. Average pH and EC values downwind from IHFO boundaries varied between 7.1–8.2 and 140–268 μS/cm, respectively.

N2O and CH4 fluxes in undisturbed and burned holm oak, scots pine and pyrenean oak forests in central Spain

We investigated N2O and CH4 fluxes from soils of Quercus ilex, Quercus pyrenaica and Pinus sylvestris stands located in the surrounding area of Madrid (Spain). The fluxes were measured for 18 months from both mature stands and post fire stands using the static chamber technique. Simultaneously with gas fluxes, soil temperature, soil water content, soil C and soil N were measured in the stands. Nitrous oxide fluxes ranged from −11.43 to 8.34 μg N2O–N m−2 h−1 in Q.ilex, −7.74 to 13.52 μg N2O–N m−2 h−1 in Q. pyrenaica and −28.17 to 21.89 μg N2O–N m−2 h−1 in P. sylvestris. Fluxes of CH4 ranged from −8.12 to 4.11 μg CH4–C m−2 h−1 in Q.ilex, −7.74 to 3.0 μg CH4–C m−2 h−1 in Q. pyrenaica and −24.46 to 6.07 μg CH4–C m−2 h−1 in P. sylvestris. Seasonal differences were detected; N2O fluxes being higher in wet months whereas N2O fluxes declined in dry months. Net consumption of N2O was related to low N availability, high soil C contents, high soil temperatures and low moisture content. Fire decreased N2O fluxes in spring. N2O emissions were closely correlated with previous day’s rainfall and soil moisture. Our ecosystems generally were a sink for methane in the dry season and a source of CH4 during wet months. The available water in the soil influenced the observed seasonal trend. The burned sites showed higher CH4 oxidation rates in Q. ilex, and lower rates in P. sylvestris. Overall, the data suggest that fire alters both N2O and CH4 fluxes. However, the magnitude of such variation depends on the site, soil characteristics and seasonal climatic conditions.

N2O emission in relation to plant and soil properties and yield of rice varieties

Nitrous oxide (N2O) is a major greenhouse gas contributing to global warming. Rainfed rice fields are considered to be a notable source of atmospheric N2O emission. To investigate the dynamics of N2O emission and the relationship of plant and soil properties with emission of N2O in rice, a field experiment was conducted. The five popularly grown rice varieties Luit, Disang, Kapilli, Siana and Phorma were grown in the fall season under rainfed conditions. N2O emission was measured at seven-day intervals starting from the day of transplanting for the whole crop growing season. We also measured soil parameters, e.g. soil pH, soil temperature, soil organic carbon, soil NO 3 − -N, and field water level; and plant growth parameters: root-shoot dry weight, root length and leaf area. Our results show that N2O emission from the plant varieties ranged from 1.24 μg to 379.40 μg N2O−N m−2 h−1. Seasonal N2O emission from the rice varieties ranged from 77 to 150 mg N2O−N m−2. Root dry weight, shoot dry weight, soil NO 3 − -N, root length, leaf area and field water showed relationships with N2O emission. Root and shoot weight, soil NO 3 − -N and field water were found to be the main factors influencing N2O emission. The varieties Phorma and Siana, with lower grain productivity but profuse vegetative growth, showed higher seasonal N2O emission.

Nitrification and denitrification response to varying periods of desiccation and inundation in a western Kansas stream

Changing environmental conditions and increased water consumption have transformed many historically perennial stream systems into intermittent systems. Multiple drying and wetting events throughout the year might impact many stream processes including nitrification and denitrification, key components of the nitrogen (N) cycle. During summer 2007, an experimental stream was used to dry and then rewet stream sediments to determine the effects of desiccation and rewetting of stream sediment on nitrification and denitrification potentials. Mean (±SE) nitrification and denitrification rates in sediment not dried (controls) were 0.431 ± 0.017 μg NO3 −–N/cm2/h and 0.016 ± 0.002 μg N2O–N/cm2/h, respectively. As sediment samples dried, nitrification rates decreased. Rates in sediments dried less than 7 d recovered to levels equal or greater than those in the controls within 1 d of being rewetted. Denitrification rates were not affected by 1 d of drying, but samples dried greater than 1 d experienced reduced rates of denitrification. Denitrification in sediments dried 7 d or less recovered by day seven of being rewetted. Nitrification and denitrification processes failed to fully recover in sediments dried more than 7 d. These results demonstrate that alterations in stream’s hydrology can significantly affect N-cycle processes.

Responses of N2O fluxes to temperature, water table and N deposition in a northern boreal fen

Nitrous oxide (N2O) fluxes were measured fortnightly to monthly with manual chambers in 2004–2008 and hourly with automatic chambers during the snow‐free seasons of 2007 and 2008 in a sedge fen in northern Finland. The fluxes were generally low, varying from −45 to 37 µg N2O–N m−2 hour−1 (negative fluxes indicating uptake of N2O from the atmosphere into the soil) and showing large spatial and temporal variation. Slightly higher emissions were observed in winter than in summer. On an annual scale, the fen acted as a N2O source. The annual balances showed a clear decreasing trend from 1.1 kg N ha−1 year−1 (= 12.2 µg N m−2 hour−1) in 2004 to zero balances in 2007 and 2008. Two potential reasons for the decreasing mean flux were (i) a decreasing atmospheric N deposition during the snow‐free season, and (ii) a rising water‐table level (WTL), which restricts the availability of oxygen in the peat and therefore favours the formation of molecular nitrogen (N2) instead of N2O by the denitrifying microbes. The measurements conducted with the automatic chambers during the snow‐free season showed a positive exponential relationship between the N2O flux and the temperature in 2008, but not in 2007. Similarly, a unimodal relationship with the WTL was found in 2008, with maximum fluxes observed when the WTL was about 4 cm above the fen surface. No diurnal variation in N2O fluxes measured by automatic chambers was found. The fluxes measured by the manual or automatic chambers were similar in magnitude, but different in their temporal pattern. The daily N2O concentration at a depth of 0.15 m in the peat was always lower than the ambient atmospheric concentration, indicating that at this depth the atmospheric N2O was consumed. Together with the observed negative flux rates this suggests that microbial N2 production is a significant part of the N cycle in this fen.

Carbon and oxygen controls on N2O and N2 production during nitrate reduction

Abstract Here we provide evidence that the form of carbon compound and O2 concentration exert an inter-related regulation on the production and reduction of N2O in soil. 6.7 mM d -glucose, 6.7 mM D-mannitol, 8 mM L-glutamic acid or 10 mM butyrate (all equivalent to 0.48 g C l−1) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O2 concentrations were established at ∼2%, 10% and 21% O2 v/v for each C treatment, and 2 mM K15NO3 (25 atom % excess 15N) was applied, enabling quantification of 15N–N2 production, 15N–(N2O-to-N2) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O2 v/v) soils, where addition of butyrate and glutamic acid resulted in greater N2O production (0.61 and 0.3 μg N2O–N g−1 soil for butyrate and glutamic acid, respectively) than the addition of carbohydrates (glucose and mannitol). Although, there was no significant effect of C compound at low initial O2 concentrations (∼2% O2 v/v), production of 15N–N2 was greatest where headspace O2 concentrations were initially, or fallen to, ∼2% O2 v/v, with greatest reduction of N2O and lowering 15N–(N2O-to-N2) ratios (∼0–0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O2 concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of 15N in NH4+ from applied 15N–NO3−, indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O2 v/v concentrations. Our 15N approach has provided the first direct evidence for enhancement of N2O reduction in the presence of carbohydrates and the dual regulation of C compound and O2 concentration on N2O production and reduction, which has implications for management of N2O emissions through changing C inputs (exudates, rhizodeposition, residues) with plant species of differing C traits, or through plant breeding.

Carbon dioxide, methane, and nitrous oxide exchanges in an age‐sequence of temperate pine forests

AbstractWe investigated soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) exchanges in an age‐sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid‐April to mid‐December in 2006 and 2007. For both CH4 and N2O, we observed uptake and emission ranging from −160 to 245 μg CH4 m−2 h−1 and −52 to 21 μg N2O m−2 h−1, respectively (negative values indicate uptake). Mean fluxes from mid‐April to mid‐December across the 4, 17, 32, 67 years old stands were similar for CO2 fluxes (259, 246, 220, and 250 mg CO2 m−2 h−1, respectively), without pattern for N2O fluxes (−3.7, 1.5, −2.2, and −7.6 μg N2O m−2 h−1, respectively), whereas the uptake rates of CH4 increased with stand age (6.4, −7.9, −10.8, and −23.3 μg CH4 m−2 h−1, respectively). For the same period, the combined contribution of CH4 and N2O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO2 and aggregated soil exchanges of CH4 and N2O was on average 4%, &lt;1%, &lt;1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO2 fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH4 and N2O fluxes, but CH4 emission was observed following summer rainfall events. LFH layer removal reduced CO2 emissions by 43%, increased CH4 uptake during dry and warm soil conditions by more than twofold, but did not affect N2O flux. We suggest that significant alternating sink and source potentials for both CH4 and N2O may occur in N‐ and soil water‐limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.

Alternative Methods for Measuring Inorganic, Organic, and Total Dissolved Nitrogen in Soil

There are numerous methods for measuring inorganic, dissolved organic, and microbial N in soils, although many of these are complex or require expensive equipment. We have modified methods for the measurement of NH4+, NO3−, total dissolved N (TDN), and soil microbial biomass N (SMBN) in soils. The methods are based on a microtiter plate format and are rapid and simple to perform. Ammonium is quantified by a colorimetric method based on the Berthelot reaction. Total dissolved N and SMBN (by CH3Cl fumigation‐extraction) are quantified as NO3− after alkaline persulfate oxidation. Nitrate is estimated directly or after persulfate oxidation by reduction of NO3− to NO2− by VCl3 and subsequent colorimetric determination of NO2− by acidic Griess reaction. The new suite of methods was compared with conventional methods such as high‐performance anion‐exchange chromatography for NO3− and high‐temperature catalytic oxidation for TDN. Our methods produced comparable detection limits, linearities, and precisions compared with the conventional methods. Limits of quantification were 7 μg NH4+–N L−1, 55 μg NO3−–N L−1, and 0.275 mg TDN L−1 The accuracy of the proposed methods was excellent, with recoveries of added NH4+, NO3−, and glycine ranging between 96 and 99%. Linearities of the respective calibrations were high (R2 &gt; 0.99), and precisions for NH4+ (CV = 2.1%), NO3− (CV = 3.5%), and TDN (CV = 3.9%) were comparable to the reference methods. The simplicity, rapidity, and low cost of the proposed methods therefore allow an expansion of the scope and range of N cycle studies where sophisticated instrumentation is not available.

Sulfide Inhibition of Nitrate Removal in Coastal Sediments

Microbial nitrate (NO3−) removal via denitrification (DNF) at high sulfide (H2S) concentrations was compared in sediment from a coastal freshwater pond in a developed area that receives salt-water influx during storm events, and a saline pond proximal to an undeveloped estuary. Sediments were incubated with added SO42− (1,000 µg per gram dry weight basis (gdw)) to determine whether acid volatile sulfides (AVS) were formed. DNF in the sediments was measured with NO3–N (300 µg gdw−1) alone, and with NO3–N and H2S (1,000 µg S2− gdw−1). SO42− addition to the freshwater sediments resulted in AVS formation (970 ± 307 µg S gdw−1) similar to the wetland with no added SO42− (986 ± 156 µg S gdw−1). DNF rates measured with no added H2S were greater in the freshwater than the wetland site (10.6 ± 0.6 vs. 6.4 ± 0.1 µg N2O–N gdw−1 h−1, respectively). High H2S concentrations retained NH4–N in the undeveloped wetland and retained NO3–N in the developed freshwater site, suggesting that potential salt-water influx may reduce the ability of the freshwater sediments to remove NO3–N.

Potential Denitrification and Nitrous Oxide Production in the Sediments of the Seine River Drainage Network (France)

To investigate bottom sediment denitrification at the scale of the Seine drainage network, a semi-potential denitrification assay was used in which river sediments (and riparian soils) were incubated for a few hours under anaerobic conditions with non limiting nitrate concentrations. This method allowed the nitrous oxide (N(2)O) concentration in the headspace, as well as the nitrate, nitrite, and ammonium concentrations to be determined during incubation. The rates at which nitrate decreased and N(2)O increased were then used to assess the potential denitrification activity and associated N(2)O production in the Seine River Basin. We observed a longitudinal pattern characterized by a significant increase of the potential rate of denitrification from upstream sectors to large downstream rivers (orders 7-8), from approximately 3.3 to 9.1 microg NO(3)(-)-N g(-1) h(-1), respectively, while the N(2)O production rates was the highest both in headwaters and in large order rivers (0.14 and 0.09 N(2)O-N g(-1) h(-1), respectively) and significantly lower in the intermediate sectors (0.01 and 0.03 N(2)O-N g(-1) h(-1)). Consequently, the ratio N(2)O production:NO(3) reduction was found to reach 5% in headstreams, whereas it averaged 1.2% in the rest of the drainage network, an intermediate percentage being found for the riparian soils. Finally, the ignition loss of sediments, together with other redundant variables (particulate organic carbon content: g C 100 g(-1) dry weight [dw], moisture: g water 100 g(-1) dw, sediment size <50 mum: g material size <50 mum 100 g(-1) dw) were found to control these activities. However, the biodegradability of organic matter must be measured to better understand the factor controlling denitrification and its associated N(2)O production.

Estimation of Indirect Nitrous Oxide Emissions from a Shallow Aquifer in Northern Germany

Ground water is considered to be an important source for indirect N2O emissions. We investigated indirect N2O emissions from a shallow aquifer in Germany over a 1-yr period. Because N2O accumulated in considerable amounts in the surface ground water (mean, 52.86 microg N2O-N L(-1)) and corresponding fluxes were high (up to 34 microg N2O-N m(-2) h(-1)), it was hypothesized that significant indirect N2O emissions would occur via the vertical and the lateral emission pathway. Vertical N2O emissions were investigated by measuring N2O concentrations and calculating fluxes from the surface ground water to the unsaturated zone and at the soil surface. Lateral N2O fluxes were investigated by measuring ground water N2O and NO3- concentrations at five multilevel wells and at a waterworks well. Negligible amounts of N2O were emitted vertically into the unsaturated zone; most of it was convectively transported into the deeper autotrophic denitrification zone. Only a ground water level fall and rise triggered the emission of N2O (up to 3 microg N2O-N m(-2) h(-1)) into the unsaturated zone. Ground water-derived N2O was probably reduced during the upward diffusion, and soil surface emissions were governed by topsoil processes. Along the lateral pathway, N2O and NO3- concentrations decreased with increasing depth in the aquifer. Discharging ground water was almost free of N2O and NO3-, and indirect N2O emissions were small.

Soil processes and microbial community structures in 45- and 135-year-old lodgepole pine stands

As forests develop, changes in soil organic matter quantity and quality affect both nutrient dynamics and microbial community structure. Litter decomposition and nitrogen mineralization in association with soil microbial communities were compared between 45- and 135-year-old lodgepole pine ( Pinus contorta var. latifolia (Englem.)) stands in southeastern Wyoming, USA. Compared with the 45-year-old stand, the 135-year-old stand was found to have greater live-tree biomass, litter decomposition rates (264 versus 135 mg·(g litter)–1·year–1), soil nitrification rates (0.38 versus 0.19 µg NO3–·(g soil)–1 after 265 days of field incubation), and total phospholipid fatty acid (PLFA) concentrations (25 versus 9.2 nmol·(g soil)–1 at 0–5 cm depth). Canonical correspondence analysis indicated that variation of PLFA profiles within the 45-year-old stand was explained by soil pH and bulk density, whereas soil process rates explained the distributions of PLFA profiles within the 135-year-old stand. The results of these studies indicate that stand age influences live-tree biomass and soil properties that can lead to changes in litter decomposition rates and soil microbial communities in lodgepole pine forests.

Nitrous oxide emissions from an apple orchard soil in the semiarid Loess Plateau of China

Nitrous oxide (N2O) fluxes from an apple orchard soil in the semiarid Loess Plateau of China were measured using static chambers from September 2007 to September 2008. In this study, three sites were selected at distance of 2.5 m (D 2.5), 1.5 m (D 1.5), and 0.5 m (D 0.5) from the apple tree row. Nitrous oxide fluxes followed seasonal pattern, with high N2O emission rates occurring in the hot-humid summer and low rates in the cold-dry winter. Pulses of N2O emissions occurred after nitrogen fertilizer application, summer rainfall events, and during freeze-thaw cycles. Annual average N2O emission rates were the highest at D 0.5 site (48.2 ± 39.9 µg N2O m−2 h−1), the lowest at D 2.5 site (31.9 ± 18.2 µg N2O m−2 h−1), and intermediate at D1.5 site (36.8 ± 32.2 µg N2O m−2 h−1), suggesting that N2O emissions from the apple orchard soil increased when the chamber location was closer to the apple tree row. This may be due to the fertilization close to roots in hot and humid season. Over one third (37.1%) of the annual N2O emission occurred in the summer. Annual N2O emissions from the apple orchard soil averaged to 3.22 kg N2O ha−1 year−1. Annual emission factor of the apple orchard from the applied fertilizer (uncorrected for background emission) was 0.658%. This value was nearly a half (53%) of the default value provided by the Intergovernmental Panel on Climate Change for the application of synthetic fertilizers to cropland (1.25%). Therefore, the amount of N2O emissions from the semiarid apple orchard soil could be largely overestimated if no regional-specific factor is used.