Organic rice farming is acknowledged as a safe and environmentally friendly rice production method. However, the application of fresh organic matter as fertilizer can increase methane (CH₄) emissions during the rice-growing period because it is a carbon (C) source for CH₄ under anaerobic conditions. In this study, we evaluated the CH₄ emissions and net greenhouse gas emissions (NGHGE) from rice paddy fields managed by long-term organic farming. We also assessed the effect of early incorporation of green manure and mid-season drainage (to reduce CH₄ emission and NGHGE) on the rice grain yield. In the first year, we measured CH₄ and nitrous oxide (N₂O) emissions, C budget, and yield in conventionally managed (CF) and organically managed (GM) rice paddy fields, in which white clover was grown in the fallow period. In the second year, we set up four treatments with dried hairy vetch (Vicia villosa) as green manure, incorporated in the conventional season (G), one week (GE1), and three weeks (GE3) earlier, and early mid-season drainage (GED), in which green manure was incorporated in the same way as in G. In the second year, we measured the same factors as in the first year. In the first year, C was accumulated in GM due to the application of C from green manure, despite C being lost in CF. On the other hand, the large amount of CH₄ emission induced in GM contributed to global warming due to high NGHGE compared to CF. Early incorporation of green manure in rice paddy fields did not increase CO₂ or decrease CH₄ emissions before rice transplanting in the second year. A significant reduction in grain yield in GE3 suggested that three weeks earlier incorporation of green manure is not appropriate in terms of stable agricultural management. Meanwhile, early mid-season drainage reduced CH₄ emissions, which contributed to both soil C sequestration and NGHGE reduction. These results indicate that mid-season drainage a week earlier than conventional practice could be a workable way of maintaining the brown rice yield and soil C sequestration and mitigating global warming in rice paddy fields receiving green manure.

A long-term study on the effect of nitrogen (N) fertilization on soil carbon dioxide (CO₂) fluxes in tropical peatland was conducted to (1) quantify the annual CO₂ emissions from an oil palm plantation under different N application rates and (2) evaluate the temporal effects of groundwater level (GWL) and water-filled pore space (WFPS) on soil organic carbon (SOC) and CO₂ fluxes. Monthly measurement of soil CO₂ fluxes using a closed chamber method was carried out from January 2010 until December 2013 and from January 2016 to December 2017 in an oil palm plantation on tropical peat in Sarawak, Malaysia. Besides the control (T1, without N fertilization), there were three N treatments: low N (T2, 31.1 kg N ha⁻¹ year⁻¹), moderate N (T3, 62.2 kg N ha⁻¹ year⁻¹), and high N (T4, 124.3 kg N ha⁻¹ year⁻¹). The annual CO₂ emissions ranged from 7.7 ± 1.2 (mean ± SE) to 16.6 ± 1.0 t C ha⁻¹ year⁻¹, 9.8 ± 0.5 to 14.8 ± 1.4 t C ha⁻¹ year⁻¹, 10.5 ± 1.8 to 16.8 ± 0.6 t C ha⁻¹ year⁻¹, and 10.4 ± 1.8 to 17.1 ± 3.9 t C ha⁻¹ year⁻¹ for T1, T2, T3, and T4, respectively. Application of N fertilizer had no significant effect on annual cumulative CO₂ emissions in each year (p = 0.448), which was probably due to the formation of large quantities of inorganic N when GWL was temporarily lowered from January 2010 to June 2010 (−80.9 to −103.4 cm below the peat surface), and partly due to low soil organic matter (SOM) quality. A negative relationship between GWL and CO₂ fluxes (p < 0.05) and a positive relationship between GWL and WFPS (p < 0.001) were found only when the oil palm was young (2010 and 2011) (p < 0.05), indicating that lowering of GWL increased CO₂ fluxes and decreased WFPS when the oil palm was young. This was possibly due to the fact that parameters such as root activity might be more predominant than GWL in governing soil respiration in older oil palm plantations when GWL was maintained near or within the rooting zone (0–50 cm). This study highlights the importance of roots and WFPS over GWL in governing soil respiration in older oil palm plantations. A proper understanding of the interaction between the direct or indirect effect of root activity on CO₂ fluxes and balancing its roles in nutrient and water management strategies is critical for sustainable use of tropical peatland.

Miscanthus-dominated semi-natural grasslands in Japan appear to store considerable amounts of soil C. To estimate the long-term effect of Miscanthus vegetation on the accumulation of soil carbon by soil biota degradation in its native range, we measured total soil C from the surface to a 1.2 m depth along a catena toposequence in three annually burned grasslands in Japan: Kawatabi, Soni, and Aso. Soil C stock was estimated using a radiocarbon age and depth model, resulting in a net soil C accumulation rate in the soil. C₄-plant contribution to soil C accumulation was further estimated by δ¹³C of soil C. The range of total soil C varied among the sites (i.e., Kawatabi: 379–638 Mg, Soni: 249–484, and Aso: 372–408 Mg C ha⁻¹). Catena position was a significant factor at Kawatabi and Soni, where the toe slope soil C accumulation exceeded that of the summit. The soil C accumulation rate of the whole horizon in the grasslands, derived C mainly from C₄ plant species, was 0.05 ± 0.02 (Average ± SE), 0.04 ± 0.00, and 0.24 ± 0.04 Mg C ha⁻¹ yr⁻¹ in Kawatabi, Soni, and Aso, respectively. Potential exists for long-term sequestration of C under M. sinensis, but the difference in the C accumulation rate can be influenced by the catena position and the amount of vegetation.

Organic matter (OM) application is a possible solution to carbon (C) sequestration. Among the soil pedogenic factors for volcanic ash, time of exposure to weathering affects formation of short-range ordered minerals and soil organic carbon (SOC) accumulation. This study aimed to evaluate the effect of long-term OM application with the vertical infiltration of OM on different ages of Andosols in Hokkaido, Japan. Twelve fields of Vitric Andosols and 22 fields of Silandic Andosols were selected. Vitric and Silandic Andosols are characterized by newly erupted volcanic ash with weak weathering, and formation of short-range ordered minerals by weathering, respectively. The C application rate via OM was determined using questionnaires given to farmers, and soil physicochemical properties in the 0–0.15 m and 0.15–0.30 m layers were analyzed. The average C application rate in Vitric and Silandic Andosols were 1.71 ± 1.18 and 1.31 ± 1.28 (Mg C ha⁻¹ yr⁻¹), respectively (mean ± sd). The sand content was significantly higher in Vitric Andosols (55.7% ± 4.0%) than in Silandic Andosol (51.2% ± 2.0%). Although there was no significant difference in micro-porosity, Silandic Andosols had significantly higher total porosity and lower bulk density than Vitric Andosols. These results indicate better soil structure development in Silandic Andosols. The SOC stock in the 0–0.30 m layer in Vitric Andosols [71.5 ± 15.3 (Mg C ha⁻¹ 0.30 m⁻¹)] was significantly lower than in Silandic Andosols [86.4 ± 12.7 (Mg C ha⁻¹ 0.30 m⁻¹)]. The SOC stock in Vitric and Silandic Andosols showed a different response to the OM application. An increased C application rate significantly increased the SOC stock in the 0–0.15 and 0–0.30 m layers in Vitric Andosols, but the relations were not significant in Silandic Andosols. In contrast, in Silandic Adosols, weak trends (p = 0.07) were observed in the 0.15–0.30 m layer. In Silandic Andosols, SOC stock in the 0–0.15 m layer showed a z significant positive correlation with soil micro-porosity, suggesting physical protection of OM inside the aggregate. These results suggest that there are different factors affecting SOC stock between Vitric and Silandic Andosols.

Organic matter, fertilizers, and soil amendments are essential for sustainable agricultural practices to guarantee soil productivity. However, these materials can increase the emission of greenhouse gases (GHGs) such as CH₄ and N₂O. Thus, technologies for reducing GHG emissions in concert with the increase in rice production from rice fields are needed. The objectives of this study were to determine the best chicken manure (CM) and steel slag (SS) combination to mitigate CH₄, N₂O, and CO₂ emissions in an incubation experiment, to identify the best CM:SS ameliorant mixture to mitigate CH₄ and N₂O, and to evaluate dry biomass and grain yield in a pot experiment. A randomized block design was established with four treatments, namely conventional (chemical fertilizer only) and three combinations of different ratios of CM and SS (1:1, 1:1.5, and 1:2.5), with five replications in a pot experiment. CM:SS (1:2.5) was identified as the best treatment for mitigating CH₄, N₂O, and CO₂ in the incubation experiment. However, CM:SS (1:1.5) was the best CM and SS ameliorant for mitigating CH₄ and N₂O in the pot experiment. The global warming potential of CH₄ and N₂O revealed that CM:SS (1:1.5) had the lowest value. None of the combinations of CM and SS significantly increased dry biomass and grain yield.