Methane and nitrous oxide emissions from irrigated lowland rice paddies after wheat straw application and midseason aeration

Straw application and midseason drainage play role in controlling methane (CH₄) and nitrous oxide (N₂O) emissions from rice paddy fields, but little information is available on their integrative effect on CH₄ and N₂O emissions. A two-year field experiment was conducted to study the combined effect of timing and duration of midseason aeration and wheat straw incorporation on mitigation of global warming potential (GWP) of CH₄ and N₂O emissions from irrigated lowland rice paddy fields. Results showed that incorporation of wheat straw increased CH₄ by a factor of 5–9 under various water regimes, but simultaneously decreased N₂O emission by 19–42 % during the rice growing season. Without straw incorporation, prolonged aeration significantly reduced the net 100-year GWP of CH₄ and N₂O emissions by 6 %, but also decreased rice production when compared with normal aeration. With straw incorporation, the lowest GWP was found by early aeration, which reduced GWP by 7 and 20 % in 2007 and 2008, respectively. Estimation of net GWPs of CH₄ and N₂O emissions indicated that early midseason drainage with straw incorporation offered the potential to mitigate CH₄ and N₂O emissions from irrigated lowland rice paddies in China.     

Mitigation of N<Subscript>2</Subscript>O and CH<Subscript>4</Subscript> Emission from Rice and Wheat Cropping Systems Using Dicyandiamide and Hydroquinone

Agriculture contributes considerably to the emission of greenhouse gases, such as N₂O and CH₄. Here we summarize results from previous pot experiments assessing the effectiveness of urease and nitrification inhibitors reducing both N₂O and CH₄ emissions from wheat and rice cropping systems fertilized with urea (U). For the wheat cropping system, using a cambisol, we observed that the application of U with hydroquinone (HQ, a urease inhibitor), U with dicyandiamide (DCD, a nitrification inhibitor) and U with HQ plus DCD decreased the N₂O emissions by 11.4, 22.3 and 25.1%, respectively. For the rice copping system, using a luvisol, we found that the application of U with HQ, U with DCD and U with HQ plus DCD decreased N₂O emissions by 10.6, 47.0 and 62.3%, respectively, and CH₄ emissions by 30.1, 53.1 and 58.3%, respectively. In terms of total global warming potential (GWP) a reduction of 61.2% could be realized via the combined addition of HQ and DCD. The addition of wheat straw reduced the activity of HQ and DCD in the rice cropping experiments. In terms of total GWP only a reduction of 30.7% could be achieved. In general, both in upland and flooded conditions, the application of HQ and DCD alone was less effective than HQ in combination with DCD, but not significantly for U plus DCD treatment. Our observations may be further constrained, however, by practical, economic or social problems and should therefore be tested at the scale of a region (e.g. a watershed) and related to an integrated abatement of agricultural N losses.     

Emissions of N<Subscript>2</Subscript>O from fertilized and grazed grassland on organic soil in relation to groundwater level

Intensively managed grasslands on organic soils are a major source of nitrous oxide (N₂O) emissions. The Intergovernmental Panel on Climate Change (IPCC) therefore has set the default emission factor at 8 kg N–N₂O ha⁻¹ year⁻¹ for cultivation and management of organic soils. Also, the Dutch national reporting methodology for greenhouse gases uses a relatively high calculated emission factor of 4.7 kg N–N₂O ha⁻¹ year⁻¹. In addition to cultivation, the IPCC methodology and the Dutch national methodology account for N₂O emissions from N inputs through fertilizer applications and animal urine and faeces deposition to estimate annual N₂O emissions from cultivated and managed organic soils. However, neither approach accounts for other soil parameters that might control N₂O emissions such as groundwater level. In this paper we report on the relations between N₂O emissions, N inputs and groundwater level dynamics for a fertilized and grazed grassland on drained peat soil. We measured N₂O emissions from fields with different target groundwater levels of 40 cm (‘wet’) and 55 cm (‘dry’) below soil surface in the years 1992, 1993, 2002, 2006 and 2007. Average emissions equalled 29.5 kg N₂O–N ha⁻¹ year⁻¹ and 11.6 kg N–N₂O ha⁻¹ year⁻¹ for the dry and wet conditions, respectively. Especially under dry conditions, measured N₂O emissions exceeded current official estimates using the IPCC methodology and the Dutch national reporting methodology. The N₂O–N emissions equalled 8.2 and 3.2% of the total N inputs through fertilizers, manure and cattle droppings for the dry and wet field, respectively and were strongly related to average groundwater level (R ² = 0.74). We argue that this relation should be explored for other sites and could be used to derive accurate emission data for fertilized and grazed grasslands on organic soils.     

Methane and nitrous oxide emissions from rice paddy soil as influenced by timing of application of hydroquinone and dicyandiamide

A pot trial and a field experiment were conducted to study the effect of timing of application of nitrification inhibitor dicyandiamide (DCD) on N₂O and CH₄ emissions from rice paddy soil. Four treatments including Treatment CK₁, DCD-1 (application of DCD with basal fertilizer), DCD-2 (DCD with tillering fertilizer) and DCD-3 (DCD with panicle initiation fertilizer), were designed and implemented in pot experiment. Total N₂O and CH₄ emissions from DCD-treated soils were decreased profoundly when compared with that from urea alone (P < 0.05). Application of DCD together with basal fertilizer, tillering fertilizer and panicle initiation fertilizer reduced N₂O emission by 8, 30 and 2%, respectively, while those for CH₄ were 21, 8 and 1%. The field experiment with four treatments was carried out subsequently, and a kind of urease inhibitor hydroquinone (HQ) was also incorporated with DCD simultaneously. The combined use of HQ and DCD with basal fertilizer, tillering fertilizer and panicle initiation fertilizer decreased N₂O emissions by 24, 56 and 17%, respectively, while those for CH₄ were 35, 19 and 12%. N₂O emission was effectively reduced by the inhibitor(s) applied with tillering fertilizer before midseason aeration, while CH₄ emission was effectively decreased by the combined use of inhibitor(s) with basal fertilizer before rice transplanting. Furthermore, an increase in rice yield and a reduction of total global warming potential (GWP) of CH₄ and N₂O could be achieved by using inhibitor(s) in rice paddy field.     

Effect of drainage in the fallow season on reduction of CH<Subscript>4</Subscript> production and emission from permanently flooded rice fields

Field and incubation experiments were conducted during 2007–2009 to study the effect of drainage in the fallow season on CH₄ production and emission from permanently flooded rice fields. It was found that drainage in the fallow season significantly affected the temporal variations of CH₄ production and emission from permanently flooded rice fields. CH₄ production and emission from permanently flooded rice fields (Treatment FF) mainly occurred during the rice season, where they were found to be much lower in the late fallow season. No CH₄ flux was detected from drained fields (Treatment DF) in the fallow season. Compared with Treatment FF, Treatment DF was delayed not only its onset of CH₄ production and emission, but also appearance of the highest peak of CH₄ production during the rice season. A significant positive relationship was observed between CH₄ production rates of paddy soil and corresponding CH₄ fluxes (P < 0.01). CH₄ production in rice roots was the highest in rate at the rice booting stage, but was obviously lower at the rice tillering, grain filling and ripening stages, and the highest value reached at the same time as the peak of CH₄ production occurred in the paddy soil. Drainage in the fallow season significantly decreased CH₄ production and emission from Treatment FF. Compared with Treatment FF, Treatment DF was about 42–61% lower in mean CH₄ production rate in the paddy soil during the rice season, and was reduced by approximately 56% in mean CH₄ production rate in rice roots. Accordingly, Treatment DF was 20.6–30.2 g CH₄ m⁻², 39–52% lower than Treatment FF in total CH₄ emission during the rice season, and 44–57% lower in annual total CH₄ emission. Rice yield in Treatment DF tended to be 4–7% lower than that in Treatment FF.     

Effect of increased N use and dry periods on N<Subscript>2</Subscript>O emission from a fertilized grassland

To better understand the effects of increased N input and dry periods on soil nitrous oxide (N₂O) emission, we examined a unique data-set of weather, soil microclimate, N input, and N₂O emissions (using the eddy covariance method), measured at a fertilized grassland over the period 2003–2008. We found that the N₂O emission (11.5 kg N ha⁻¹ year⁻¹), the ratio of N₂O emission to N input (3.4), and the duration of elevated N₂O flux (57 days) in 2003 were about two times greater than those of the following years. 2003 had the highest annual N input (343 kg N ha⁻¹ year⁻¹) which exceeded the agronomical requirements for Irish grasslands (up to 306 kg ha⁻¹ year⁻¹). In the summer of 2003, the site had a significantly higher soil temperature, lower WFPS and lowest rainfall of all years. Large N₂O emission events followed rainfall after a long dry period in the summer of 2003, attributed to dominant nitrification processes. Furthermore, in the non summer periods, when temperature was lower and WFPS was higher and when there were prior N applications, lower N₂O emissions occurred and were attributed to dominant denitrification processes. Throughout the study period, the N input and soil dryness related factors (duration of WFPS under 50%, summer average WFPS, and low rainfall) showed exponential relationships with N₂O emission and the ratio of N₂O emission to N input. Based on these findings, we infer that the observed anomalously high N₂O emission in 2003 may have been caused by the combined effects of excess N input above the plant uptake rate, elevated soil temperature, and N₂O flux bursts that followed the rewetting of dry soil after an unusually long dry summer period. These results suggest that high N input above plant uptake rate and extended dry periods may cause abnormal increases in N₂O emissions.     

Effects of rice straw returning methods on N<Subscript>2</Subscript>O emission during wheat-growing season

A field experiment was conducted in Jurong of Nanjing, Jiangsu Province, China from 2006 to 2008 to investigate N₂O emission during the wheat-growing season as affected by various rice straw returning methods prior to wheat cultivation. The study was designed to have four treatments: no rice straw applied (CK), rice straw burnt in situ (RB), rice straw evenly incorporated into the topsoil (RI), rice straw evenly spread over the field as mulch (RM). Results showed that N₂O emission was decreased by 24–29% in Treatment RB and by 3–18% in Treatment RI, but increased by 15–39% in Treatment RM, compared with that in Treatment CK. The contents of soil total C and N at wheat harvest were significantly increased by 7–13% and by 8–12% in Treatment RI, respectively, compared with that in Treatment CK. The wheat grain yield in Treatment RI was 1.0–1.2 times that in the Treatment CK. Based on these results, the best management practice of returning rice straw to the soil prior to wheat cultivation is evenly incorporating rice straw into the topsoil, as the method tended to reduce N₂O emission during the wheat-growing season and increase wheat yield and soil fertility.     

Measurement of Net Global Warming Potential in Three Agroecosystems

When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the ‘greenhouse’ effect, the entire suite of biogenic greenhouse gases – carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) – needs to be considered. Storage of atmospheric CO₂ into stable organic carbon pools in the soil can sequester CO₂ while common crop production practices can produce CO₂, generate N₂O, and decrease the soil sink for atmospheric CH₄. The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO₂, i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO₂ equivalents ha⁻¹ y⁻¹. Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N₂O emission rates decline or increase with time after conversion to NT.     

Sorption studies of CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript>, N<Subscript>2</Subscript>, CO,O<Subscript>2</Subscript> and Ar on nanoporous aluminum terephthalate [MIL-53(Al)]

Aluminum terephthalate, MIL-53(Al), metal–organic framework synthesized hydrothermally and purified by solvent extraction method was used as an adsorbent for gas adsorption studies. The synthesized MIL-53(Al) was characterized by powder X-Ray diffraction analysis, surface area measurement using N₂ adsorption–desorption at 77 K, FTIR spectroscopy and thermo gravimetric analysis. Adsorption isotherms of CO₂, CH₄, CO, N₂, O₂ and Ar were measured at 288 and 303 K. The absolute adsorption capacity was found in the order CO₂>CH₄>CO>N₂>Ar>O₂. Henry’s constants, heat of adsorption in the low pressure region and adsorption selectivities for the adsorbate gases were calculated from their adsorption isotherms. The high selectivity and low heat of adsorption for CO₂ suggests that MIL-53(Al) is a potential adsorbent material for the separation of CO₂ from gas mixtures. The high selectivity for CH₄ over O₂ and its low heat of adsorption suggests that MIL-53(Al) could also be a compatible adsorbent for the separation of methane from methane–oxygen gas mixtures.     

The Effect of Future Climate Perturbations on N<Subscript>2</Subscript>O Emissions from a Fertilized Humid Grassland

N₂O emissions from a fertilized humid grassland near Cork, Ireland were continuously measured during 2003 using an eddy covariance system. For most of the year emissions were close to zero and 60% of the emissions occurred in eight major events of 2–20 days’ duration. Two hundred and seven kg ha⁻¹ of synthetic N and 130 kg ha⁻¹ organic N were applied over the year and the total measured annual N₂O emission was 11.6 kg N ha⁻¹. The flux data were used to test the prediction of N₂O emissions by the DNDC (DeNitrification – DeComposition) model. The model predicted total emissions of 15.4 kg N ha⁻¹, 32 % more than the observed emissions. On this basis the model was further used to simulate (a) background (non-anthropogenic) N₂O emissions and (b) the effect on N₂O emissions of future climate perturbations based on the Hadley Center model output of the IS92a scenario for Ireland. DNDC predicts 1.7 kg N ha⁻¹ year⁻¹ of background N₂O emissions, accounting for 15% of the observed emissions. Climate shifts will increase total annual modeled N₂O emissions from 15.4 kg N ha⁻¹ to 22.4 kg N ha⁻¹ if current levels of N applications are maintained, or to 21.2 kg N ha⁻¹ if synthetic N applications are reduced to 170 kg N ha⁻¹ to comply with recent EU water quality legislation. Thus the projected increase in N₂O emissions due to climate change is far larger than the decrease expected from reduced fertilizer applications.     

Separation characteristics of tetrapropylammoniumbromide templating silica/alumina composite membrane in CO<Subscript>2</Subscript>/N<Subscript>2</Subscript>, CO<Subscript>2</Subscript>/H<Subscript>2</Subscript> and CH<Subscript>4</Subscript>/H<Subscript>2</Subscript> systems

Nanoporous silica membrane without any pinholes and cracks was synthesized by organic templating method. The tetrapropylammoniumbromide (TPABr)-templating silica sols were coated on tubular alumina composite support ( γ-Al₂O₃/ α-Al₂O₃ composite) by dip coating and then heat-treated at 550 °C. By using the prepared TPABr templating silica/alumina composite membrane, adsorption and membrane transport experiments were performed on the CO₂/N₂, CO₂/H₂ and CH₄/H₂ systems. Adsorption and permeation by using single gas and binary mixtures were measured in order to examine the transport mechanism in the membrane. In the single gas systems, adsorption characteristics on the α-Al₂O₃ support and nanoporous unsupport (TPABr templating SiO₂/ γ-Al₂O₃ composite layer without α-Al₂O₃ support) were investigated at 20–40 °C conditions and 0.0–1.0 atm pressure range. The experimental adsorption equilibrium was well fitted with Langmuir or/and Langmuir-Freundlich isotherm models. The α-Al₂O₃ support had a little adsorption capacity compared to the unsupport which had relatively larger adsorption capacity for CO₂ and CH₄. While the adsorption rates in the unsupport showed in the order of H₂> CO₂> N₂> CH₄ at low pressure range, the permeate flux in the membrane was in the order of H₂≫N₂> CH₄> CO₂. Separation properties of the unsupport could be confirmed by the separation experiments of adsorbable/non-adsorbable mixed gases, such as CO₂/H₂ and CH₄/H₂ systems. Although light and non-adsorbable molecules, such as H₂, showed the highest permeation in the single gas permeate experiments, heavier and strongly adsorbable molecules, such as CO₂ and CH₄, showed a higher separation factor (CO₂/H₂=5-7, CH₄/H₂=4-9). These results might be caused by the surface diffusion or/and blocking effects of adsorbed molecules in the unsupport. And these results could be explained by surface diffusion. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Characterization of Methane Emissions from Rice Fields in Asia. III. Mitigation Options and Future Research Needs

Methane (CH₄) emissions from rice fields were determined using automated measurement systems in China, India, Indonesia, Thailand, and the Philippines. Mitigation options were assessed separately for different baseline practices of irrigated rice, rainfed, and deepwater rice. Irrigated rice is the largest source of CH₄ and also offers the most options to modify crop management for reducing these emissions. Optimizing irrigation patterns by additional drainage periods in the field or an early timing of midseason drainage accounted for 7–80% of CH₄ emissions of the respective baseline practice. In baseline practices with high organic amendments, use of compost (58–63%), biogas residues (10–16%), and direct wet seeding (16–22%) should be considered mitigation options. In baseline practices using prilled urea as sole N source, use of ammonium sulfate could reduce CH₄ emission by 10–67%. In all rice ecosystems, CH₄ emissions can be reduced by fallow incorporation (11%) and mulching (11%) of rice straw as well as addition of phosphogypsum (9–73%). However, in rainfed and deepwater rice, mitigation options are very limited in both number and potential gains. The assessment of these crop management options includes their total factor productivity and possible adverse effects. Due to higher nitrous oxide (N₂O) emissions, changes in water regime are only recommended for rice systems with high baseline emissions of CH₄. Key objectives of future research are identifying and characterizing high-emitting rice systems, developing site-specific technology packages, ascertaining synergies with productivity, and accounting for N₂O emissions.     

Field Validation of DNDC Model for Methane and Nitrous Oxide Emissions from Rice-based Production Systems of India

The DNDC (DeNitrification and DeComposition) model was tested against experimental data on CH₄ and N₂O emissions from rice fields at different geographical locations in India. There was a good agreement between the simulated and observed values of CH₄ and N₂O emissions. The difference between observed and simulated CH₄ emissions in all sites ranged from −11.6 to 62.5 kg C ha⁻¹ season⁻¹. Most discrepancies between simulated and observed seasonal fluxes were less than 20% of the field estimate of the seasonal flux. The relative deviation between observed and simulated cumulative N₂O emissions ranged from −237.8 to 28.6%. However, some discrepancies existed between observed and simulated seasonal patterns of CH₄ and N₂O emissions. The model simulated zero N₂O emissions from continuously flooded rice fields and poorly simulated CH₄ emissions from Allahabad site. For all other simulated cases, the model satisfactorily simulated the seasonal variations in greenhouse gas emission from paddy fields with different land management. The model also simulated the C and N balances in all the sites, including other gas fluxes, viz. CO₂, NO, NO₂, N₂ and NH₃ emissions. Sensitivity tests for CH₄ indicate that soil texture and pH significantly influenced the CH₄ emission. Changes in organic C content had a moderate influence on CH₄ emission on these sites. Introducing the mid-season drainage reduced CH₄ emissions significantly. Process-based biogeochemical modeling, as with DNDC, can help in identifying strategies for optimizing resource use, increasing productivity, closing yield gaps and reducing adverse environmental impacts.     

Toward Improved Coefficients for Predicting Direct N<Subscript>2</Subscript>O Emissions from Soil in Canadian Agroecosystems

Agricultural soils emit nitrous oxide (N₂O), a potent greenhouse gas. Predicting and mitigating N₂O emissions is not easy. To derive national coefficients for N₂O emissions from soil, we collated over 400 treatment evaluations (measurements) of N₂O fluxes from farming systems in various ecoregions across Canada. A simple linear coefficient for fertilizer-induced emission of N₂O in non-manured soils (1.18% of N applied) was comparable to that used by the Intergovernmental Panel on Climate Change (IPCC) (1.25% of N applied). Emissions were correlated to soil and crop management practices (manure addition, N fertilizer addition and inclusion of legumes in the rotation) as well as to annual precipitation. The effect of tillage on emissions was inconsistent, varying among experiments and even within experiments from year to year. In humid regions (e.g., Eastern Canada) no-tillage tended to enhance N₂O emissions; in arid regions (e.g., Western Prairies) no-tillage sometimes reduced emissions. The variability of N₂O fluxes shows that we cannot yet always distinguish between potential mitigation practices with small (e.g., <10%) differences in emission. Our analysis also emphasizes the need for developing consistent experimental approaches (e.g., ‘control’ treatments) and methodologies (i.e. measurement period lengths) for estimating N₂O emissions.     

BF<Subscript>3</Subscript>OEt<Subscript>2</Subscript>-coinitiated cationic polymerization of cyclopentadiene in the presence of water at room temperature

The cationic polymerization of cyclopentadiene (CPD) with 1-(4-methoxyphenyl)ethanol (1)/BF₃OEt₂ initiating system in CH₂Cl₂:CH₃CN 4:1 (v/v) mixture at room temperature and in the presence of water ([H₂O]/[BF₃OEt₂] up to 8) is reported. The number-average molecular weights of obtained polymers increased in direct proportion to monomer conversion or initial monomer concentration (M _n ≤ 4,000 g mol⁻¹) in agreement with calculated values, and were inversely proportional to initiator concentration. Polymer MWDs were relatively narrow (M _w/M _n = 1.4–1.7) up to 60% of monomer conversion. It was also shown that regioselectivity of CPD polymerization with 1/BF₃OEt₂ initiating system did not depend significantly on water, monomer, or initiator concentration (1,4-structures content was nearly 60% in all cases).     

Soil CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript>, and N<Subscript>2</Subscript>O fluxes over and between tile drains on corn,soybean, and forage fields under tile drainage management

Controlled tile drainage (CTD) can benefit the environment and crop production. However, CTD has the potential to increase soil greenhouse gas (GHG: CO₂, CH₄, N₂O) emissions by increasing soil water contents and elevating field water levels. A paired-field (CTD and uncontrolled tile drainage (UTD)) approach was used to compare soil GHG emissions for silt loam corn, soybean, and forage fields under CTD and UTD management in eastern Ontario, Canada during a drier and a wetter growing season. A total of five field pairs were examined. Soil GHG emissions directly over tile drains (OT) and between tile drains (BT) in the CTD fields were also assessed. Average soil GHG emissions did not significantly differ (p > 0.05) for CTD and UTD field pairs, except for CO₂ emissions (greater emissions from UTD fields) among two field pairs studied (forage in the drier growing season and soybean in the wetter growing season), and N₂O emissions from a soybean field pair in the wet growing season (greater emissions from CTD field). Significantly higher soil water contents in the UTD forage field may have augmented CO₂ fluxes there. There were some significantly higher N₂O (in the wetter growing season) and CO₂ emissions (in both growing seasons) BT relative to OT locations in some fields; but these differences were not translated significantly to other BT and OT site comparisons. The wetter growing season examined resulted in greater average daily soil CO₂ fluxes overall, but similar CH₄ and N₂O fluxes for soybean fields compared to soybean fields in the drier growing season. Overall, there were no spatially or temporally systematic differences in GHG emissions among CTD and UTD field pairs, or among BT and OT locations in CTD fields.     

Simultaneous Records of Methane and Nitrous Oxide Emissions in Rice-Based Cropping Systems under Rainfed Conditions

Rainfed rice (Oryza sativa L.)-based cropping systems are characterized by alternate wetting and drying cycles as monsoonal rains come and go. The potential for accumulation and denitrification of NO₃ ^– is high in these systems as is the production and emission of CH₄ during the monsoon rice season. Simultaneous measurements of CH₄ and N₂O emissions using automated closed chamber methods have been reported in irrigated rice fields but not in rainfed rice systems. In this field study at the International Rice Research Institute, Philippines, simultaneous and continuous measurements of CH₄ and N₂O were made from the 1994 wet season to the 1996 dry season. During the rice-growing seasons, CH₄ fluxes were observed, with the highest emissions being in organic residue-amended plots. Nitrous oxide fluxes, on the other hand, were generally nonexistent, except after fertilization events where low N₂O fluxes were observed. Slow-release N fertilizer further reduced the already low N₂O emissions compared with prilled urea in the first rice season. During the dry seasons, when the field was planted to the upland crops cowpea [Vigna unguiculata (L.) Walp] and wheat (Triticum aestivum L.), positive CH₄ fluxes were low and insignificant except after the imposition of a permanent flood where high CH₄ fluxes appeared. Evidences of CH₄ uptake were apparent in the first dry season, especially in cowpea plots, indicating that rainfed lowland rice soils can act as sink for CH₄ during the upland crop cycle. Large N₂O fluxes were observed shortly after rainfall events due to denitrification of accumulated NO₃ ^–. Cumulative CH₄ and N₂O fluxes observed during this study in rainfed conditions were lower compared with previous studies on irrigated rice fields.     

Subsoil <Superscript>15</Superscript>N-N<Subscript>2</Subscript>O Concentrations in a Sandy Soil Profile After Application of <Superscript>15</Superscript>N-fertilizer

Studies on emissions of nitrous oxide (N₂O) from agricultural soils mostly focus on fluxes between the soil and the atmosphere or are limited to the atmosphere in the topsoil. However, in soils with shallow water tables, significant N₂O formation may occur closer to the groundwater. The aims of this study were (i) to determine the importance of subsoil N₂O formation in a sandy soil; and (ii) to obtain a quantitative insight in the contribution of subsoil N₂O to the overall losses of N₂O to the environment. We applied ¹⁵N labeled fertilizer at a rate of 5.22 kg ¹⁵N ha⁻¹; 50% as Ca(NO₃)₂ and 50% as NH₄Cl, on a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.), and traced soil N₂O concentrations and fluxes over a one-year period. Throughout the year, total N₂O and the amount of ¹⁵N recovered in soil N₂O were highest in the subsoil, with a maximum concentration at 48 cm depth in mid-February of 19900 μl m⁻³ and 24 μg ¹⁵N m⁻³, respectively. The maximum concentration coincided with the highest water-filled pore space of 71%. The cumulative flux of N₂O was 446 g N₂O-N ha⁻¹, the recovery of ¹⁵N in this flux was 0.06%. During the summer, maximum fluxes followed high soil N₂O concentrations. During winter, no such relation was found. We concluded that the formation of N₂O was the highest in the subsoil, largely controlled by water-filled pore space rather than NO₃⁻ concentration or temperature. Although high subsoil N₂O concentrations did not lead to high surface fluxes of N₂O in the winter, artificial draining may lead to high indirect N₂O emissions through supersaturated drainage water.     

Promoting Effect of Triglyme on Lean NO<Subscript>x</Subscript> Reduction Over Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Abstract  

The highly oxygenated hydrocarbon triethylene glycol dimethyl ether or triglyme (CH₃O–(C₂H₄O–)₃CH₃) was found to efficiently reduce NO_x under lean conditions over Ag/Al₂O₃, but gave a low NO_x conversion over Cu-ZSM-5. Furthermore, triglyme showed an extraordinary promoting effect when added together with propene as reducing agent for NO_x over Ag/Al₂O₃ at low temperature. This is most likely due to that triglyme promotes the activation of propene.     

Electrochemical promotion of CO<Subscript>2</Subscript> hydrogenation on Rh/YSZ electrodes

The electrochemical promotion of the CO₂ hydrogenation reaction on porous Rh catalyst–electrodes deposited on Y₂O₃-stabilized-ZrO₂ (or YSZ), an O²⁻ conductor, was investigated under atmospheric total pressure and at temperatures 346–477 °C, combined with kinetic measurements in the temperature range 328–391 °C. Under these conditions CO₂ was transformed to CH₄ and CO. The CH₄ formation rate increased by up to 2.7 times with increasing Rh catalyst potential (electrophobic behavior) while the CO formation rate was increased by up to 1.7 times with decreasing catalyst potential (electrophilic behavior). The observed rate changes were non-faradaic, exceeding the corresponding pumping rate of oxygen ions by up to approximately 210 and 125 times for the CH₄ and CO formation reactions, respectively. The observed electrochemical promotion behavior is attributed to the induced, with increasing catalyst potential, preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH₄ and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.