137
supply of nutrients and improves biological and physical properties of the soil. Rice
straw incorporation has been shown to increase soil organic C (Tables 8.1 and 8.2).
However, the role of soil organic C in rice soils remains debatable. While soil
organic C is considered important on one hand, the increased GHG emissions associated with increased soil organic C can contribute to climate change. Thus, there is
need to evaluate the tradeoffs and synergies of soil organic C sequestration in
rice soils.
Soil organic C has been shown to be stable under intensive rice cropping, even
when straw is removed from the field. Soil organic C was shown not to change in a
50-year, long-term continuous cropping experiment at the International Rice
Research (IRRI) in the Philippines where three rice crops were grown annually with
the removal of all aboveground biomass even without the addition of N fertilizer
(Pampolino et al. 2008). This is in contrast to systems where rice is rotated with an
upland crop, e.g., Majumder et al. (2008) observed a decline in soil organic C when
no residues were added in a rice–wheat cropping system in India. In a 9-year study
in Bac Giang Province in Vietnam, soil organic C did not change with straw removal,
but the addition of straw increased soil organic C from 1.28% to 1.65% (Thuan and
Long 2010). Alberto et al. (2015) showed a cumulative effect of continuous straw
incorporation in a lowland rice soil, likely due to slower organic matter decomposition. However, the addition of straw increases soil organic C (Bi et al. 2009;
Yadvinder-Singh et al. 2005), particularly in rainfed upland rice systems (Naklang
et al. 1999) or where lowland rice is rotated with an upland crop. In a rice–wheat
system, Gangwar et al. (2006) observed greater soil organic C and infiltration when
5 t ha
−1
rice straw was incorporated in the soil than when it was removed or burned.
8.5 Rice Straw Effects on Yield
While yield increases are expected with the retention of crop residues in upland
cropping systems, in lowland rice the benefits when compared to straw removal are
small particularly in the short-term. Under continuous flooded rice, the retention of
rice straw has not been shown to increase rice yield. This might be due to the lowquality nature of rice straw with a high C:N ratio, which results in N immobilization
and hence poor availability for plant uptake. Additionally, anaerobic decomposition
of organic materials has been shown to trigger production of phenolic compounds
that also renders N to be unavailable and affect crop growth. Incorporation of rice
straw on three different soil types did not increase rice yield and this was attributed
to an increase in toxic substances and organic acids (Hoi et al. 2009). This is particularly important when the straw has not been given adequate time for decomposition. However, long-term benefits of straw incorporation on rice yield can be
significant. A summary of some studies conducted in the Philippines and Vietnam
shows yield benefits from straw incorporation (Table 8.3).
In a long-term study in the Mekong Delta in Vietnam (Watanabe et al. 2009)
observed that the application of 6 Mg ha
−1
rice straw compost (fresh weight)
8 Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter
supply of nutrients and improves biological and physical properties of the soil. Rice
straw incorporation has been shown to increase soil organic C (Tables 8.1 and 8.2).
However, the role of soil organic C in rice soils remains debatable. While soil
organic C is considered important on one hand, the increased GHG emissions associated with increased soil organic C can contribute to climate change. Thus, there is
need to evaluate the tradeoffs and synergies of soil organic C sequestration in
rice soils.
Soil organic C has been shown to be stable under intensive rice cropping, even
when straw is removed from the field. Soil organic C was shown not to change in a
50-year, long-term continuous cropping experiment at the International Rice
Research (IRRI) in the Philippines where three rice crops were grown annually with
the removal of all aboveground biomass even without the addition of N fertilizer
(Pampolino et al. 2008). This is in contrast to systems where rice is rotated with an
upland crop, e.g., Majumder et al. (2008) observed a decline in soil organic C when
no residues were added in a rice–wheat cropping system in India. In a 9-year study
in Bac Giang Province in Vietnam, soil organic C did not change with straw removal,
but the addition of straw increased soil organic C from 1.28% to 1.65% (Thuan and
Long 2010). Alberto et al. (2015) showed a cumulative effect of continuous straw
incorporation in a lowland rice soil, likely due to slower organic matter decomposition. However, the addition of straw increases soil organic C (Bi et al. 2009;
Yadvinder-Singh et al. 2005), particularly in rainfed upland rice systems (Naklang
et al. 1999) or where lowland rice is rotated with an upland crop. In a rice–wheat
system, Gangwar et al. (2006) observed greater soil organic C and infiltration when
5 t ha
−1
rice straw was incorporated in the soil than when it was removed or burned.
8.5 Rice Straw Effects on Yield
While yield increases are expected with the retention of crop residues in upland
cropping systems, in lowland rice the benefits when compared to straw removal are
small particularly in the short-term. Under continuous flooded rice, the retention of
rice straw has not been shown to increase rice yield. This might be due to the lowquality nature of rice straw with a high C:N ratio, which results in N immobilization
and hence poor availability for plant uptake. Additionally, anaerobic decomposition
of organic materials has been shown to trigger production of phenolic compounds
that also renders N to be unavailable and affect crop growth. Incorporation of rice
straw on three different soil types did not increase rice yield and this was attributed
to an increase in toxic substances and organic acids (Hoi et al. 2009). This is particularly important when the straw has not been given adequate time for decomposition. However, long-term benefits of straw incorporation on rice yield can be
significant. A summary of some studies conducted in the Philippines and Vietnam
shows yield benefits from straw incorporation (Table 8.3).
In a long-term study in the Mekong Delta in Vietnam (Watanabe et al. 2009)
observed that the application of 6 Mg ha
−1
rice straw compost (fresh weight)
8 Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter