146
9.1 Introduction
Lowland rice is a major contributor to greenhouse gas emissions (GHGEs) accounting for 10% of global emissions from agriculture (FAO 2015). This number is even
higher for Southeast Asia (SEA) where 90% of the world’s rice is produced, making
up 10–20% of the region’s total anthropogenic emissions and 40–60% of its agricultural emissions (UNFCC 2019). Rice is one of the largest sources of anthropogenic CH 4 (GWP
1
= 28) and a major contributor of N 2 O (GWP = 265). CO 2
emissions from rice, although large, are considered net-neutral from photosynthesis
according to the IPCC 2006 guidelines. CH 4 accounts for around 65% of global
CO 2 eq emissions from lowland rice; largely from anaerobic decomposition of
straw and crop residue under continuously flooded conditions. The remaining 35%
of emissions from rice can be attributed mostly to N 2 O from soil N cycling of fertilizer and to a smaller extent N from crop residues (EPA 2013). Rice straw management is, therefore, an important factor in controlling GHGEs from lowland
rice-cropping systems.
In addition to emissions, straw management plays an important role in global
carbon cycles through soil organic carbon (SOC) sequestration. SOC is an important indicator of soil quality, which suggests its importance in improving farmer
adaptation to climate change. It is estimated that rice soils contain the largest SOC
stocks among croplands (IPCC 2007; Lal 2004). The potential SOC deposition from
returning rice straw to the soil is significant as almost half of the total carbon in rice
plant residue is within the straw and stubble (although root C contributes most
SOC). The common, yet mostly banned, practice of straw burning reduces the SOC
sequestration potential of fresh straw incorporation.
Although returning fresh straw to the field can increase SOC, its sequestration
benefits may be outweighed by the increase in CH 4 emissions when applied under
flooded conditions due to anaerobic decomposition. Additionally, straw management practices that reduce emissions or improve sequestration are not always
advantageous to crop yields. Striking a balance between emissions reduction, carbon sequestration, and crop yields is challenging, but may be achievable with optimal site-specific straw management. The efficiency of this balance can be quantified
by yield-scaled emissions and mitigation or NGWP and GHGI,
2
more broadly
referred to as climate-smart agriculture (CSA). This chapter discusses in-field/offfield rice straw management options affecting CSA—burning, incorporation, com1 Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the
atmosphere up to a specific time horizon, relative to carbon dioxide (CO 2 ).
2 Net global warming potential (NGWP) can be defined as the radiative properties of all the GHG
emissions plus carbon fixation, expressed as CO 2 eq ha
−1 year
−1 (Robertson and Grace 2004), while
greenhouse gas intensity (GHGI) defines the GWP per unit of crop yield (Mosier et al. 2006)
J. Allen et al.
9.1 Introduction
Lowland rice is a major contributor to greenhouse gas emissions (GHGEs) accounting for 10% of global emissions from agriculture (FAO 2015). This number is even
higher for Southeast Asia (SEA) where 90% of the world’s rice is produced, making
up 10–20% of the region’s total anthropogenic emissions and 40–60% of its agricultural emissions (UNFCC 2019). Rice is one of the largest sources of anthropogenic CH 4 (GWP
1
= 28) and a major contributor of N 2 O (GWP = 265). CO 2
emissions from rice, although large, are considered net-neutral from photosynthesis
according to the IPCC 2006 guidelines. CH 4 accounts for around 65% of global
CO 2 eq emissions from lowland rice; largely from anaerobic decomposition of
straw and crop residue under continuously flooded conditions. The remaining 35%
of emissions from rice can be attributed mostly to N 2 O from soil N cycling of fertilizer and to a smaller extent N from crop residues (EPA 2013). Rice straw management is, therefore, an important factor in controlling GHGEs from lowland
rice-cropping systems.
In addition to emissions, straw management plays an important role in global
carbon cycles through soil organic carbon (SOC) sequestration. SOC is an important indicator of soil quality, which suggests its importance in improving farmer
adaptation to climate change. It is estimated that rice soils contain the largest SOC
stocks among croplands (IPCC 2007; Lal 2004). The potential SOC deposition from
returning rice straw to the soil is significant as almost half of the total carbon in rice
plant residue is within the straw and stubble (although root C contributes most
SOC). The common, yet mostly banned, practice of straw burning reduces the SOC
sequestration potential of fresh straw incorporation.
Although returning fresh straw to the field can increase SOC, its sequestration
benefits may be outweighed by the increase in CH 4 emissions when applied under
flooded conditions due to anaerobic decomposition. Additionally, straw management practices that reduce emissions or improve sequestration are not always
advantageous to crop yields. Striking a balance between emissions reduction, carbon sequestration, and crop yields is challenging, but may be achievable with optimal site-specific straw management. The efficiency of this balance can be quantified
by yield-scaled emissions and mitigation or NGWP and GHGI,
2
more broadly
referred to as climate-smart agriculture (CSA). This chapter discusses in-field/offfield rice straw management options affecting CSA—burning, incorporation, com1 Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the
atmosphere up to a specific time horizon, relative to carbon dioxide (CO 2 ).
2 Net global warming potential (NGWP) can be defined as the radiative properties of all the GHG
emissions plus carbon fixation, expressed as CO 2 eq ha
−1 year
−1 (Robertson and Grace 2004), while
greenhouse gas intensity (GHGI) defines the GWP per unit of crop yield (Mosier et al. 2006)
J. Allen et al.