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9.2.2 Incorporation Rates and Environmental Factors
9.2.2.1 Water Management
CH 4 emissions from rice are highly dependent on the amount of straw or crop residue returned under continuously flooded conditions (Liu et al. 2014). Because of
this, removing rice straw in flooded rice is considered a mitigation strategy that
could theoretically reduce the GWP of emissions from rice by 45% (Wang et al.
2016). The benefits of complete straw removal on reducing emissions, however, are
offset by reduced SOC sequestration, soil quality, and long-term yields. Maximum
emission reductions and yield (and SOC deposition) may be best achieved by partial
straw return/removal in most continuous rice systems (Romasanta et al. 2017). This
balance can still increase SOC storage over time and provide adequate crop nutrients. Because straw decomposition rates, and thus emissions, depend on climate,
cropping system, and soil type, these factors can help determine the appropriate
percentage of straw to return. Generally, soils that are well-drained or have low
SOC with aerobic periods benefit from increased straw return to maximize SOC
sequestration and increase yields with minimal CH 4 emissions, i.e., the percentage
of straw returned should be approximately proportional to the percentage of time
under aerobic conditions (Monteleone et al. 2015).
Controlling the aerobic condition of paddy soil is primarily achieved by irrigation management. The use of non-flooded, aerobic periods to reduce CH 4 from
organic matter decomposition in rice is a well-established mitigation strategy called
alternate wetting and drying (AWD) that can reduce emissions in lowland irrigated
rice by 48% on average (IRRI 2016). AWD will be an increasingly important strategy to mitigate future emissions of CH 4 as expanding combine harvester use promotes straw incorporation. Reduced flooding can also be achieved with the use of
laser land-levelling, dry direct-seeded rice, and short-duration rice varieties. These
methods are well established water-saving practices described in previous studies
(Monteleone et al. 2015; Bouman et al. 2007). Reduced flooding affects emissions
by shifting from anaerobic to aerobic microbial respiration to produce CO 2 in place
of CH 4 . Although CO 2 emissions increase under aerobic conditions, the effect on
GWP is much lower than CH 4 . Additionally, aerobic decomposition of residue
improves SOM conversion to more stabilized forms of SOC that have a lower additive effect on CH 4 once flooded (Jiang et al. 2019).
Despite the benefits of aerobic regimes on emissions from rice straw, it comes
with an increased risk of SOC loss compared to continuous flooding. Additionally,
N 2 O emissions may be significant during dry conditions— although N 2 0 emissions
are largely an effect of fertilizer, as straw supplies only around 10% of N in intensive systems (Yadvinder-Singh et al. 2004; Eagle et al. 2001). In more aerobic rice
systems, N 2 O emissions can be mitigated by proper nutrient management, and SOC
losses can be compensated for by increasing the rate of straw return.
J. Allen et al.
9.2.2 Incorporation Rates and Environmental Factors
9.2.2.1 Water Management
CH 4 emissions from rice are highly dependent on the amount of straw or crop residue returned under continuously flooded conditions (Liu et al. 2014). Because of
this, removing rice straw in flooded rice is considered a mitigation strategy that
could theoretically reduce the GWP of emissions from rice by 45% (Wang et al.
2016). The benefits of complete straw removal on reducing emissions, however, are
offset by reduced SOC sequestration, soil quality, and long-term yields. Maximum
emission reductions and yield (and SOC deposition) may be best achieved by partial
straw return/removal in most continuous rice systems (Romasanta et al. 2017). This
balance can still increase SOC storage over time and provide adequate crop nutrients. Because straw decomposition rates, and thus emissions, depend on climate,
cropping system, and soil type, these factors can help determine the appropriate
percentage of straw to return. Generally, soils that are well-drained or have low
SOC with aerobic periods benefit from increased straw return to maximize SOC
sequestration and increase yields with minimal CH 4 emissions, i.e., the percentage
of straw returned should be approximately proportional to the percentage of time
under aerobic conditions (Monteleone et al. 2015).
Controlling the aerobic condition of paddy soil is primarily achieved by irrigation management. The use of non-flooded, aerobic periods to reduce CH 4 from
organic matter decomposition in rice is a well-established mitigation strategy called
alternate wetting and drying (AWD) that can reduce emissions in lowland irrigated
rice by 48% on average (IRRI 2016). AWD will be an increasingly important strategy to mitigate future emissions of CH 4 as expanding combine harvester use promotes straw incorporation. Reduced flooding can also be achieved with the use of
laser land-levelling, dry direct-seeded rice, and short-duration rice varieties. These
methods are well established water-saving practices described in previous studies
(Monteleone et al. 2015; Bouman et al. 2007). Reduced flooding affects emissions
by shifting from anaerobic to aerobic microbial respiration to produce CO 2 in place
of CH 4 . Although CO 2 emissions increase under aerobic conditions, the effect on
GWP is much lower than CH 4 . Additionally, aerobic decomposition of residue
improves SOM conversion to more stabilized forms of SOC that have a lower additive effect on CH 4 once flooded (Jiang et al. 2019).
Despite the benefits of aerobic regimes on emissions from rice straw, it comes
with an increased risk of SOC loss compared to continuous flooding. Additionally,
N 2 O emissions may be significant during dry conditions— although N 2 0 emissions
are largely an effect of fertilizer, as straw supplies only around 10% of N in intensive systems (Yadvinder-Singh et al. 2004; Eagle et al. 2001). In more aerobic rice
systems, N 2 O emissions can be mitigated by proper nutrient management, and SOC
losses can be compensated for by increasing the rate of straw return.
J. Allen et al.