133
8.2 Different Components of Rice Straw
At harvest, rice biomass includes rice straw and paddy grain, which is partitioned
into milled grain, bran, and husks after milling. On average, particularly for modern
high-yielding varieties, the harvest index (i.e., grain dry weight: total plant biomass
dry weight) of rice is between 0.45 and 0.50 (Dobermann and Fairhurst 2000).
Thus, roughly for every ton of rice grain produced, a ton of rice straw is also produced. Rice straw usually has higher moisture content than the husks and bran
because it is obtained during harvest whereas the bran and husks are drier because
they are obtained after dried paddy is milled. When rice is harvested, the straw is
removed from the field for threshing to separate grains while the stalk and the stubble remain in the field. The amount of stubble depends on the height at which the
straw is cut. The stubble is usually retained in situ and is either burned or incorporated into the soil during land preparation. Rice husks are the parts that are removed
when milling paddy to make brown rice while the bran is the layer that is then
removed when brown rice is polished to make white rice. Depending on the processing, the different rice plant components other than the grain can be collected
and used for composting, biofuel production, or other purposes. Composted materials can be returned to the same soil or applied to the fields of other crops, usually of
high value, resulting in the export of nutrients.
8.3 Forms in Which Rice Straw Is Returned to the Soil
Due to ease of management, the main form in which rice straw is currently returned
to the soil is by incorporating burned ash after open-field burning of stubble following harvest. However, with the banning of straw burning in most rice-growing countries, incorporating fresh rice residue serves as the alternative for returning rice
straw to rice fields. While this is associated with benefits of improving soil quality
and recycling nutrients, it can have negative impacts on the environment through
release of GHGs, depending on management. Alternatively, the straw is collected
from the field and mixed with other inputs such as livestock manure, green manure,
or household waste to make compost, which is more decomposed and of higher
quality compared to fresh straw. In other instances, earthworms and microorganisms are added to the straw to enhance decomposition to make vermicompost (see
Chap. 3). Compost and vermicompost can be made from fresh or spent straw from
mushroom production, but farmers generally prefer to use them for high-value crops
such as vegetables rather than for rice production. The straw can be used in energy
production and the carbonized rice straw that remains can be used as a soil amendment (Haefele et al. 2011).
8 Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter
8.2 Different Components of Rice Straw
At harvest, rice biomass includes rice straw and paddy grain, which is partitioned
into milled grain, bran, and husks after milling. On average, particularly for modern
high-yielding varieties, the harvest index (i.e., grain dry weight: total plant biomass
dry weight) of rice is between 0.45 and 0.50 (Dobermann and Fairhurst 2000).
Thus, roughly for every ton of rice grain produced, a ton of rice straw is also produced. Rice straw usually has higher moisture content than the husks and bran
because it is obtained during harvest whereas the bran and husks are drier because
they are obtained after dried paddy is milled. When rice is harvested, the straw is
removed from the field for threshing to separate grains while the stalk and the stubble remain in the field. The amount of stubble depends on the height at which the
straw is cut. The stubble is usually retained in situ and is either burned or incorporated into the soil during land preparation. Rice husks are the parts that are removed
when milling paddy to make brown rice while the bran is the layer that is then
removed when brown rice is polished to make white rice. Depending on the processing, the different rice plant components other than the grain can be collected
and used for composting, biofuel production, or other purposes. Composted materials can be returned to the same soil or applied to the fields of other crops, usually of
high value, resulting in the export of nutrients.
8.3 Forms in Which Rice Straw Is Returned to the Soil
Due to ease of management, the main form in which rice straw is currently returned
to the soil is by incorporating burned ash after open-field burning of stubble following harvest. However, with the banning of straw burning in most rice-growing countries, incorporating fresh rice residue serves as the alternative for returning rice
straw to rice fields. While this is associated with benefits of improving soil quality
and recycling nutrients, it can have negative impacts on the environment through
release of GHGs, depending on management. Alternatively, the straw is collected
from the field and mixed with other inputs such as livestock manure, green manure,
or household waste to make compost, which is more decomposed and of higher
quality compared to fresh straw. In other instances, earthworms and microorganisms are added to the straw to enhance decomposition to make vermicompost (see
Chap. 3). Compost and vermicompost can be made from fresh or spent straw from
mushroom production, but farmers generally prefer to use them for high-value crops
such as vegetables rather than for rice production. The straw can be used in energy
production and the carbonized rice straw that remains can be used as a soil amendment (Haefele et al. 2011).
8 Rice Straw Incorporation Influences Nutrient Cycling and Soil Organic Matter