Methane and nitrous oxide emissions from flooded rice fields as affected by water and straw management between rice crops
Introduction
The agricultural sector contributes about 10–12% of the total global anthropogenic emissions, and of these total anthropogenic emissions, 47% of CH4 and 58% of N2O emissions have been attributed to agriculture (Smith et al., 2007). Methane emissions from rice production have been predicted to increase substantially by 42% with an increase of atmospheric CO2 concentration to levels between 550 and 743 ppm (van Groenigen et al., 2013), predicted to be reached between 2050 and 2080 (IPCC, 2001).
Rice production is one of the major agricultural undertakings responsible for increased CH4 emissions. Many studies, as reviewed by Linquist et al. (2012), have measured high fluxes of CH4 and relatively low fluxes of N2O in production of rice on flooded soil. However, CH4 emissions are reduced when the rice field undergoes drying periods such as mid-season drainage, but N2O fluxes are increased (Bronson et al., 1997a, Cai et al., 1997, Chen et al., 1997, Zou et al., 2007). The application of nitrogen (N) fertilizers can increase N2O emissions (Denmead et al., 1979, Ma et al., 2007, Mosier et al., 1989, Zou et al., 2009). Based on similar studies of Mosier et al. (1989) and Zou et al. (2007) about 0.02% to 0.42% of the N applied at 100 to 200 kg N ha− 1 was emitted as N2O.
Several mitigation strategies to decrease CH4 and N2O emissions especially from irrigated rice systems have been studied and proposed (Ahmad et al., 2009, Epule et al., 2011, Horwath, 2011, Ma et al., 2010, Majumdar, 2003, Wassman et al., 2000, Yan et al., 2005). Such strategies include the use of no tillage, single or multiple mid-season drainage events, application of rice straw as compost, off-season straw incorporation, alternative rice cultivars, and modified fertilizers (i.e., nitrification inhibitors, urease inhibitors, and slow-release fertilizers).
In a double-rice cropping system, a fallow period exists between the two cropping seasons. A limited number of studies (Bronson et al., 1997b, Zhang et al., 2011) have measured the magnitude of CH4 and N2O fluxes during the fallow period. In most cases, the rice field is untended during this period with no interventions. The field can be exposed to variable weather conditions, which can influence soil water regime and hence CH4 and N2O emissions during the period between crops. Few studies have related the condition of the field during the fallow period to the CH4 and N2O emissions in the following rice cropping season (Cai et al., 2003, Kang et al., 2002, Xu et al., 2000, Zhang et al., 2011). In addition, the management of the rice residue also influences CH4 and N2O emissions.
Rice residues remaining in the field after harvest are generally incorporated into the soil during land preparation. According to Watanabe et al. (1994) rice straw is the primary source of C for CH4 production during the early growth period of rice plants. Hence when rice straw is incorporated to the soil, the emissions of CH4 can increase during the rice-growing period. But incorporated rice residues also provide benefits such as a source of nutrients, especially potassium, to subsequent rice crops. The timing of residue incorporation, however, can be managed to reduce CH4 emissions during the rice growing season (Xu et al., 2000).
The CH4 and N2O emissions in rice production as affected by mitigation strategies can be assessed by GWP relative to grain yield, which is important in order to account for differences in management practices on productivity of rice. This study was conducted to assess the influence of water management and crop residue management between rice crops (i.e., fallow period) on CH4 and N2O emissions in the succeeding cropping season and to quantify the yield-scaled GWP of rice as affected by fallow and residue management practices.
Section snippets
Site characteristics, treatment, and experiment description
Measurements were taken from a field experiment in the Experiment Station of the International Rice Research Institute, Los Baños, Philippines (14° 10′ 07.1″ N, 121° 15′ 23.9′ E), with an elevation of 21 m above msl and mean yearly rainfall of 1992 mm (2000–2012). The field was managed from 2003 to 2012 with two rice crops per year. The study was conducted during the 2011 wet season (WS) and the 2012 dry season (DS). The soil was classified as Aquandic Epiaquoll (Soil Survey Staff, 1994) with a
Methane flux
Water regime and tillage management during the fallow period, regardless of residue management, significantly affected CH4 flux during the growing period of rice in both seasons (Fig. 1). Higher CH4 fluxes were measured in the 2011 WS than in the 2012 DS. In all cases, the flooded treatment had the highest CH4 flux compared to the other treatments. For without residue incorporation in both seasons, the flooded treatment had the highest CH4 flux throughout the season while the other treatments
Discussion
The study confirmed that management of the field during the fallow period preceding the cropping season has a large effect on CH4 emissions during the rice-growing period. In this study, continuously flooding the field during the fallow led to high CH4 emissions in the cropping season. This agrees with the results of studies from China (Cai et al., 2003, Kang et al., 2002, Zhang et al., 2011) in which higher CH4 fluxes were observed during the rice-growing season in fields that were flooded
Conclusion
Water management between rice crops has a substantial influence on GHG emissions during the rice growth period. Flooding of the field during the fallow increases CH4 emissions in the growing season, whereas drying of the field during the fallow decreases CH4 emissions in the growing season. Straw incorporation strongly increased seasonal methane emissions and the total GWP as well as the yield-scaled GWP. It is therefore highly recommended to consider residue and water management during the
Acknowledgment
The Kellogg Company provided support for this research through a grant to IRRI (grant No. DPPC-2009-116). The position of B.O. Sander at IRRI was funded by the Federal Ministry for Economic Cooperation and Development, Germany in 2011–2012 (grant No. 08.7860.3-001.00) and the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS) in 2013 (grant No. A-2012-264). We thank Ms. Sheryll Elaine Rigua for the assistance with the statistical analyses.
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