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Since nitrogen (N) fertilization is the most efficient practice for increasing rice production, N fertilizer consumptions have continued to increase globally. Therefore, the effects of N fertilization on CH 4 emission characteristics have been extensively studied. However, no consistent conclusions to N fertilization on CH 4 cycles have been drawn so far. In order to evaluate the effect of N fertilization on CH 4 fluxes in rice fields, N fertilizer (urea) was applied at different levels (0–180 kg N ha −1 ) in a typical temperate paddy soil, and CH 4 emissions were characterized under two different soil conditions during cropping seasons (rice plant included and excluded soils). Seasonal CH 4 fluxes responded differently to N fertilization between the rice plants included and excluded soils. In rice plant excluded soils, total CH 4 fluxes significantly increased with increasing N fertilization levels. However, in rice plant included soils, seasonal CH 4 fluxes changed with a quadratic response. Total CH 4 fluxes increased with increasing N fertilization by 115–137 kg N ha −1 and later decreased. The difference in seasonal CH 4 fluxes between the two soils might be caused by rice rhizospheric activities and this difference could be defined as the minimum CH 4 oxidation potentials of rice rhizosphere. This CH 4 oxidation potential significantly increased with increasing N fertilization levels, and is highly correlated with total biomass, straw and root biomass productivities. Therefore, the decrease in CH 4 fluxes by high levels of N fertilization in rice plant included soils might be caused by the increasing N fertilization-induced CH 4 consumption.

The cultivation of a winter cover crop as green manure is strongly recommended to improve soil quality in mono-rice paddy systems; however, the biomass is largely removed to feed cattle in many Asian regions. To determine the minimum recycling ratio of the biomass that can sustain soil organic carbon (SOC) levels and produce more fodder for cattle, the SOC balance, which is the difference between OC input and output during rice cultivation, was evaluated with the various levels of biomass addition. The sources of OC input included cover crop biomass and fertilizer, and the OC outputs were estimated by the losses from the mineralization of C (emissions of CH4 and CO2). A mixture of barley (75% of the recommended dose, RD) and hairy vetch (hereafter, vetch, 25% of the RD) seeds were broadcast after rice harvests in 2011 and 2012, and the aboveground biomass (11.5–12 Mg ha−1, based on dry weight) harvested in the following years was incorporated at different ratios (0–100%) into soils one week before transplantation of rice with the same chemical fertilization. The incorporated OC was lost primarily through emissions of CO2 (73–85% of the OC output). However, the proportion of CH4 loss increased significantly with an increase in the rate of aboveground biomass application, which was caused by the development of anaerobic soils. A negative SOC balance, which implied soil fertility was at risk from a decreasing stock of SOC, was observed with total aboveground biomass removal. However, the balance of SOC increased significantly with an increase in level of biomass recycling and reached a sustainable level at approximately 28–30% recycling of aboveground biomass; thus, the current levels of SOC could be sustained. In conclusion, more than 30% of the aboveground biomass of the cover crop (3.4–3.6 Mg ha−1 dry weight) should be incorporated as a green manure to sustain levels of SOC in mono-rice cultivation systems with chemical fertilization.

Application of organic matter is essential for sustaining the health and productivity of a soil. However, organic amendments produce methane (CH4) emissions from rice (Oryza sativa L.) paddy soils. In this experiment, we evaluated the relative effects of composted and air-dried forms of different manures on CH4 emission from rice paddy soils. Air-dried and composted manures from both Korean cows and dairy cows were applied to evaluate their effects on CH4 emissions in rice paddy soils. Application of organic amendments increased CH4 emissions from soil during rice cultivation. Application of composted manures reduced CH4 emission by up to 50% compared to air-dried manures. The chemical composition of applied cattle manures may also determine the level of CH4 emissions from rice paddy soils. The amount of decomposable organic C, its distribution in lighter soil aggregates and the potential of these soil aggregates to generate labile C compounds in soil were possible influencing factors in the emission of CH4 from organic amended rice paddy soils.

Leguminous cover crops like hairy vetch (Vicia villosa) is generally grown as winter cover crop in the rice paddy fields of tropical countries like South Korea and Japan. The leguminous cover crops are preferably cultivated due to their N-fixing ability by nodule formation. However, the interactions of nodule formation in hairy vetch roots and increased soil nitrate-N content due to hairy vetch cultivation with emissions of greenhouse gases especially nitrous oxide (N2O) during successive rice cultivation were not studied before. For this study, hairy vetch was harvested 1 week (204D), 2 weeks (197D), 3 weeks (190D) and 4 weeks (183D) before succeeding rice seedling transplanting and nodule characteristics of hairy vetch were compared with soil nitrate-N content and N2O emission during rice cultivation. Number and weight of nodules were increased with increasing age of hairy vetch and that in turn enhanced nitrogenase activity of nodules. The highest soil NO3-N content of 204D treatment might be attributed to the highest nitrogenase activity in the roots of 204 days old hairy vetch plants. The enhanced N2O emission from rice paddy soil due to longer hairy vetch cultivation as winter cover crop indicated that the age of leguminous cover crops is an important factor to determine the global warming during succeeding rice cultivation. Cultivation of hairy vetch for 197 days was probably the best practice to minimize adverse effect of N-fixation on N2O emission from rice paddy soils.

Cover crop cultivation during the cold fallow season has been strongly recommended in mono-rice cultivation system to improve soil quality. Its addition as green manure could stimulate methane (CH 4 ) emission in the submerged paddy soil during rice cultivation, but its effect has not been evaluated well. Leguminous Chinese milk vetch (hereafter milk vetch) and non-leguminous rye were seeded after rice harvesting in 2008 and 2009, and total above-ground biomasses were incorporated with the recommended mineral fertilizers (NPK + milk vetch, and NPK + rye, respectively) before rice transplanting in 2009 and 2010. No fertilization (control) and mineral fertilization alone (NPK) plots were installed for comparison. Methane flux which was investigated by the closed chamber method during rice cultivation was significantly increased by chemical fertilization to ca. 80–250% over the control (138–141 kg CH 4 ha −1 ), but increased more markedly by cover crop biomass addition to ca. 28–121% over the NPK treatment. Milk vetch was more effective on stimulating less CH 4 emission (28–61% increase over NPK) and improving rice productivity (18–31% increase over NPK) than rye (86–121% increase of CH 4 emission and −3–6% of yield increase over NPK). As a result, the total CH 4 flux per grain yield was significantly increased by rye addition, but were similar between the NPK + milk vetch and NPK treatments. Conclusively, low C/N ratio cover crop like milk vetch might be more recommendable green manure to minimize CH 4 emission impact and increase rice productivity than high C/N ratio cover crop like rye in paddy soil.

Agricultural practices mostly influence methane (CH4) emissions from rice field, which must be controlled for maintaining the ecosystem balance. No-tillage farming with chemical amendments having electron acceptors could be an effective mitigation strategy in CH4 emissions from irrigated rice (Oryza sativa L.) field. An experiment was conducted in Korean paddy field under tillage and notillage farming practices with silicate iron slag amendments (1–4 Mg ha � 1 ) for suppressing CH4 emissions and maintaining rice productivity. It was found that CH4 emissions from the no-tillage rice field significantly decreased as compared to that of tilled field, irrespective of silicate amendments. The total seasonal CH4 flux from the control tillage and control no-tillage plots were recorded 38.1 and 27.9 g m � 2 , respectively, which were decreased by 20% and 36% with 4 Mg ha � 1 silicate amendment. Silicate fertilization (4 Mg ha � 1 ) with no-tillage system decreased total seasonal CH4 flux by 54% as compared to that of control tillage plot. This is most likely due to the higher concentrations of active iron and free iron oxides in the no-tilled rice field as compared to that of tilled field under silicate fertilization, which acted as electron acceptors and contributed to decrease CH4 emission. In addition, the improved soil porosity and redox potential, rice plant growth parameters such as active tillering rate, root volume and porosity, etc. in combination increased the rhizosphere oxygen concentrations and eventually suppressed CH4 emission during the rice growing season. The leaf photosynthetic rate was significantly increased with 4 Mg ha � 1 silicate amendment, which ultimately increased grain yield by 18% and 13% in the tilled and no-tilled rice field, respectively. CH4 flux showed a strong positive correlation with the availability of soil organic carbon, while there were negative correlations with soil porosity, soil pH, soil Eh, and the content of active iron and free iron oxides in soil.

Application of electron acceptors such as ferric iron oxides and hydroxides for controlling methane (CH4) emission from wetland rice fields deserves special attention due to its dominant role over all other redox species in wetland soils. Silicate iron slag (hereafter, silicate fertilizer), a byproduct of steel industry containing electron acceptors, was applied in paddy field (Agronomy Farm, Gyeongsang National University, South Korea) at the rate of 0, 1, 2 and 4 Mg ha−1 to investigate their effects on reducing CH4 emissions from flood water rice (Oryza sativa, cv. Dongjinbyeo) farming during 2006–2007. CH4 emission rates measured by closed-chamber method decreased significantly (p