Agricultural fields are an important anthropogenic source of atmospheric nitrous oxide (N 2 O) and nitric oxide (NO). Although many field studies have tested the effectiveness of possible mitigation options on N 2 O and NO emissions, the effectiveness of each option varies across sites due to environmental factors and field management. To combine these results and evaluate the overall effectiveness of enhanced-efficiency fertilizers [i.e., nitrification inhibitors (NIs), polymer-coated fertilizers (PCFs), and urease inhibitors (UIs)] on N 2 O and NO emissions, we performed a meta-analysis using field experiment data (113 datasets from 35 studies) published in peer-reviewed journals through 2008. The results indicated that NIs significantly reduced N 2 O emissions (mean: À38%, 95% confidential interval: À44% to À31%) compared with those of conventional fertilizers. PCFs also significantly reduced N 2 O emissions (À35%, À58% to À14%), whereas UIs were not effective in reducing N 2 O. NIs and PCFs also significantly reduced NO (À46%, À65% to À35%; À40%, À76% to À10%, respectively). The effectiveness of NIs was relatively consistent across the various types of inhibitors and land uses. However, the effect of PCFs showed contrasting results across soil and land-use type: they were significantly effective for imperfectly drained Gleysol grassland (À77%, À88% to À58%), but were ineffective for well-drained Andosol upland fields. Because available data for PCFs were dominated by certain regions and soil types, additional data are needed to evaluate their effectiveness more reliably. NIs were effective in reducing N 2 O emission from both chemical and organic fertilizers. Moreover, the consistent effect of NIs indicates that they are potent mitigation options for N 2 O and NO emissions.

[1] The Intergovernmental Panel on Climate Change (IPCC) regularly publishes guidelines for national greenhouse gas inventories and methane emission (CH 4 ) from rice paddies has been an important component of these guidelines. While there have been many estimates of global CH 4 emissions from rice fields, none of them have been obtained using the IPCC guidelines. Therefore, we used the Tier 1 method described in the 2006 IPCC guidelines to estimate the global CH 4 emissions from rice fields. To accomplish this, we used country-specific statistical data regarding rice harvest areas and expert estimates of relevant agricultural activities. The estimated global emission for 2000 was 25.6 Tg a À1 , which is at the lower end of earlier estimates and close to the total emission summarized by individual national communications. Monte Carlo simulation revealed a 95% uncertainty range of 14.8-41.7 Tg a À1; however, the estimation uncertainty was found to depend on the reliability of the information available regarding the amount of organic amendments and the area of rice fields that were under continuous flooding. We estimated that if all of the continuously flooded rice fields were drained at least once during the growing season, the CH 4 emissions would be reduced by 4.1 Tg a À1. Furthermore, we estimated that applying rice straw off season wherever and whenever possible would result in a further reduction in emissions of 4.1 Tg a À1 globally. Finally, if both of these mitigation options were adopted, the global CH 4 emission from rice paddies could be reduced by 7.6 Tg a À1

Rice cultivation is an important anthropogenic source of atmospheric methane (CH 4 ), the emission of which is affected by management practices. Many field measurements have been conducted in major rice-producing countries in Asia. We compiled a database of CH 4 emissions from rice fields in Asia from peer-reviewed journals. We developed a statistical model to relate CH 4 flux in the rice-growing season to soil properties, water regime in the rice-growing season, water status in the previous season, organic amendment and climate. The statistical results showed that all these variables significantly affected CH 4 flux, and explained 68% of the variability. Organic amendment and water regime in the rice-growing season were the top two controlling variables; climate was the least critical variable. The average CH 4 fluxes from rice fields with single and multiple drainages were 60% and 52% of that from continuously flooded rice fields. The flux from fields that were flooded in the previous season was 2.8 times that from fields previously drained for a long season and 1.9 times that from fields previously drained for a short season. In contrast to the previously reported optimum soil pH of around neutrality, soils with pH of 5.0-5.5 gave the maximum CH 4 emission. The model results demonstrate that application of rice straw at 6 t ha À1 before rice transplanting can increase CH 4 emission by 2.1 times; when applied in the previous season, however, it increases CH 4 emission by only 0.8 times. Default emission factors and scaling factors for different water regimes and organic amendments derived from this work can be used to develop national or regional emission inventories.

[1] Isotopomer ratios of N 2 O (bulk nitrogen and oxygen isotope ratios, d 15 N bulk and d 18 O, and intramolecular 15 N site preference, SP) are useful parameters that characterize sources of this greenhouse gas and also provide insight into production and consumption mechanisms. We measured isotopomer ratios of N 2 O emitted from typical Japanese agricultural soils (Fluvisols and Andisols) planted with rice, wheat, soybean, and vegetables, and treated with synthetic (urea or ammonium) and organic (poultry manure) fertilizers. The results were analyzed using a previously reported isotopomeric N 2 O signature produced by nitrifying/denitrifying bacteria and a characteristic relationship between d 15 N bulk and SP during N 2 O reduction by denitrifying bacteria. Relative contributions from nitrification (hydroxylamine oxidation) and denitrification (nitrite reduction) to gross N 2 O production deduced from the analysis depended on soil type and fertilizer. The contribution from nitrification was relatively high (40%-70%) in Andisols amended with synthetic ammonium fertilizer, while denitrification was dominant (50%-90%) in the same soils amended with poultry manure during the period when N 2 O production occurred in the surface layer. This information on production processes is in accordance with that obtained from flux/concentration analysis of N 2 O and soil inorganic nitrogen. However, isotopomer analysis further revealed that partial reduction of N 2 O was pronounced in high-bulk density, alluvial soil (Fluvisol) compared to low-bulk density, volcanic ash soil (Andisol), and that the observed difference in N 2 O flux between normal and pelleted manure could have resulted from a similar mechanism with different rates of gross production and gross consumption. The isotopomeric analysis is based on data from pure culture bacteria and would be improved by further studies on in situ biological processes in soils including those by fungi. When flux/concentration-weighted average isotopomer ratios of N 2 O from various fertilized soils were examined, linear correlations were found between d 15 N bulk and d 18 O, and between SP and d 15 N bulk . These relationships would be useful to parameterize isotopomer ratios of soil-emitted N 2 O for the modeling of the global N 2 O isotopomer budget. The results obtained in this study and those from previous firn/ice core studies confirm that the principal source of anthropogenic N 2 O is fertilized soils.Citation: Toyoda, S., et al. (2011), Characterization and production and consumption processes of N 2 O emitted from temperate agricultural soils determined via isotopomer ratio analysis, Global Biogeochem. Cycles, 25, GB2008,

Agricultural fields are significant sources of anthropogenic atmospheric nitrous oxide (N 2 O). We compiled and analyzed data on N 2 O emissions from Japanese agricultural fields (246 measurements from 36 sites) reported in peer-reviewed journals and research reports. Agricultural fields were classified into three categories: upland fields, tea fields and rice paddy fields. In this analysis, data measured over a period of more than 90 days for upland fields and 209 days for tea fields were used to estimate annual fertilizer-induced emission factors (EF) because of limitations in the available data. The EF is defined as the emission from fertilized plots minus the background emission (emission from a zero-N control plot), and is expressed as a percentage of the N applied. The mean of N 2 O emissions from upland fields with well-drained soils was significantly lower than that from poorly drained soils. Mean (± standard deviation) N 2 O emissions measured over a period of more than 90 days from fertilized upland fields were 1.03 ± 1.14 kg N ha −1 and 4.78 ± 5.36 kg N ha −1 for well-drained and poorly drained soils, respectively. Because the ratio of the total areas of well-drained soils and poorly drained soils was different from the ratio of the number of available EF data for each soil category, we used a weighted mean to estimate EF for all upland fields. The EF was estimated to be 0.62 ± 0.48% for all fertilized upland fields. Mean N 2 O emissions and the estimated EF for fertilized tea fields measured over a period of more than 209 days were 24.3 ± 16.3 kg N ha −1 and 2.82 ± 1.80%, respectively. The mean N 2 O emission and estimated EF from Japanese rice paddy fields were 0.36 kg N ha −1 and 0.31 ± 0.31% for the cropping season, respectively. Significant uncertainties remain in these results because of limitations in the available data.

[1] Rice cultivation is an important anthropogenic source of atmospheric nitrous oxide (N 2 O) and methane. We compiled and analyzed data on N 2 O emissions from rice fields (113 measurements from 17 sites) reported in peer-reviewed journals. Mean N 2 O emission ± standard deviation and mean fertilizer-induced emission factor during the rice-cropping season were, respectively, 341 ± 474 g N ha À1 season À1 and 0.22 ± 0.24% for fertilized fields continuously flooded, 993 ± 1075 g N ha À1 season À1 and 0.37 ± 0.35% for fertilized fields with midseason drainage, and 667 ± 885 g N ha À1 season À1and 0.31 ± 0.31% for all water regimes. The estimated whole-year background emission was 1820 g N ha À1 yr À1 . A large uncertainty remains, especially for background emission because of limited data availability. Although midseason drainage generally reduces CH 4 and increases N 2 O emissions, it may be an effective option for mitigating the net global warming potential of rice fields.

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