135
incorporation of the straw is affected by the accumulation of phenolic compounds
that are formed under the straw’s anaerobic decomposition (Olk et al. 2006). These
phenolic compounds tend to bind the N in the soil making it unavailable for
plant uptake.
Nonetheless, the long-term incorporation of crop residues in flooded rice soil can
increase soil organic matter, total N, and soil biological activity (Yadvinder-Singh
et al. 2004). Continuous incorporation of crop residues after each crop can eventually increase the N-supplying capacity of rice soils (Eagle et al. 2000). In a study in
Vietnam, soil N increased from 0.65% to 0.085% following 9 years of cropping
with incorporation of rice straw while straw removal caused a decline in soil N
(Thuan and Long 2010). The benefits of incorporated residues on soil organic matter and soil N supply, however, seldom translate into increased yield or profit for
flooded rice (Bijay-Singh et al. 2008). However, Thanh et al. (2016) observed that
N fertilizer requirement was reduced by about 20% in a long-term study with rice
straw incorporation.
Timings of straw incorporation and water management are important considerations for effective use of straw as a nutrient resource (Dobermann and Fairhurst
2002; Witt et al. 2000). Rice straw should be incorporated in dry soil at least 3 weeks
before sowing or transplanting the next crop to allow the straw to decompose aerobically. This minimizes the negative effects of anaerobic decomposition, which
results in release of phenolic compounds (Olk et al. 2006), and methane emissions
(Sander et al. 2014), while allowing for decomposition and mineralization of the
nutrients, making them available during plant growth. In a cropping system comparing rice–rice to rice–maize, N supply was greater when residue incorporation took
place 63 rather than 14 days before planting wet season rice; this was associated
with greater rice yields (Witt et al. 2000). This points to the need for time for straw
decomposition before the nutrients become available for plant uptake. Rice straw
compost, on the other hand, because it is more decomposed before its application in
the soil, has greater nutrient availability compared to raw rice straw. However, farmers prioritize the use of compost on higher-value crops such as vegetables than
on rice.
Rice straw can serve as an important source of S, which is particularly important
in situations where S-free fertilizers are used (Dobermann and Fairhurst 2002). It
also serves as an important source of micronutrients including zinc (Zn), but its
long-term application can decrease the availability of Zn (Yadvinder-Singh et al.
2005). Rice straw is also important for P recycling. For example, in a 4-year study
in India, P balances were negative where rice and wheat straw were removed or
burned (Gupta et al. 2007). The mineral P dynamics were improved where P fertilizers were added. Similarly, Gangwar et al. (2006) observed greater concentrations
of plant available P when rice straw was incorporated compared to when it was
removed in a 3-year rice–wheat study. Continuous addition of biochar made from
rice straw on a degraded soil in Soc Son District of Hanoi resulted in an increase in
soil pH, cation exchange capacity, and soil organic C after four seasons (Table 8.2A)
(Trinh et al. 2011). Biochar has high soil pH and tends to have a liming effect in soil,
while it is also stable and decomposes slowly, resulting in an increase in soil organic
8 Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter
incorporation of the straw is affected by the accumulation of phenolic compounds
that are formed under the straw’s anaerobic decomposition (Olk et al. 2006). These
phenolic compounds tend to bind the N in the soil making it unavailable for
plant uptake.
Nonetheless, the long-term incorporation of crop residues in flooded rice soil can
increase soil organic matter, total N, and soil biological activity (Yadvinder-Singh
et al. 2004). Continuous incorporation of crop residues after each crop can eventually increase the N-supplying capacity of rice soils (Eagle et al. 2000). In a study in
Vietnam, soil N increased from 0.65% to 0.085% following 9 years of cropping
with incorporation of rice straw while straw removal caused a decline in soil N
(Thuan and Long 2010). The benefits of incorporated residues on soil organic matter and soil N supply, however, seldom translate into increased yield or profit for
flooded rice (Bijay-Singh et al. 2008). However, Thanh et al. (2016) observed that
N fertilizer requirement was reduced by about 20% in a long-term study with rice
straw incorporation.
Timings of straw incorporation and water management are important considerations for effective use of straw as a nutrient resource (Dobermann and Fairhurst
2002; Witt et al. 2000). Rice straw should be incorporated in dry soil at least 3 weeks
before sowing or transplanting the next crop to allow the straw to decompose aerobically. This minimizes the negative effects of anaerobic decomposition, which
results in release of phenolic compounds (Olk et al. 2006), and methane emissions
(Sander et al. 2014), while allowing for decomposition and mineralization of the
nutrients, making them available during plant growth. In a cropping system comparing rice–rice to rice–maize, N supply was greater when residue incorporation took
place 63 rather than 14 days before planting wet season rice; this was associated
with greater rice yields (Witt et al. 2000). This points to the need for time for straw
decomposition before the nutrients become available for plant uptake. Rice straw
compost, on the other hand, because it is more decomposed before its application in
the soil, has greater nutrient availability compared to raw rice straw. However, farmers prioritize the use of compost on higher-value crops such as vegetables than
on rice.
Rice straw can serve as an important source of S, which is particularly important
in situations where S-free fertilizers are used (Dobermann and Fairhurst 2002). It
also serves as an important source of micronutrients including zinc (Zn), but its
long-term application can decrease the availability of Zn (Yadvinder-Singh et al.
2005). Rice straw is also important for P recycling. For example, in a 4-year study
in India, P balances were negative where rice and wheat straw were removed or
burned (Gupta et al. 2007). The mineral P dynamics were improved where P fertilizers were added. Similarly, Gangwar et al. (2006) observed greater concentrations
of plant available P when rice straw was incorporated compared to when it was
removed in a 3-year rice–wheat study. Continuous addition of biochar made from
rice straw on a degraded soil in Soc Son District of Hanoi resulted in an increase in
soil pH, cation exchange capacity, and soil organic C after four seasons (Table 8.2A)
(Trinh et al. 2011). Biochar has high soil pH and tends to have a liming effect in soil,
while it is also stable and decomposes slowly, resulting in an increase in soil organic
8 Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter