Biomass Management for Fodder & Energy

Lesson 6. Residue management for surface mulch and soil incorporation

6.2. Residue burning

Traditionally, rice straw is removed from the fields for use as cattle feed and for other purposes in India. Recently, with the advent of mechanised harvesting, farmers have been burning in-situ large quantities of crop residues left in the field as crop residues interfere with tillage and seeding operations for the subsequent crop, causing loss of nutrients and soil organic matter. When burnt, the residues instantly generate as much as 13 tonnes of CO₂ per ha, contaminating the air and killing of beneficial soil insects and microorganisms. This practice also kills soil borne deleterious pests and pathogens. One of the advantages of burning is that it clears the land quickly of residues before the next crop is established, thus facilitating seed germination and establishment. So there is a need to adopt ways and means to manage this valuable resource.

 

Nutrient content of rice straw and amounts removed with 1 tonne of straw residue

	N	P2O5	K2O	S	Si
Content in straw, % dry matter	0.5-0.8	0.16-0.27	1.4-2.0	0.05-0.10	4-7
Removal with 1 tonne straw, kg/ha	5-8	1.6-2.7	14-20	0.5-1.0	40-70

6.3. Surface retention and mulching

Direct drilling in surface mulched residues is a practice that leaves straw residues from a previous crop on the soil surface without any form of incorporation. Surface retention of residues helps in protecting the fertile surface soil against wind and water erosion. The large volume of residues remaining on the surface often leads to machinery failures, thus affecting sowing of seeds of the following crop. Farmers usually follow this method where no till or conservation tillage practices are prevalent.

Surface retention of some or all of the residues may be the best option in many situations. Residues decompose slowly on the surface, increasing the organic carbon and total nitrogen in the top 5-15 cm of soil, while protecting the surface soil from erosion. Retention of residues on the surface increased soil nitrate concentration by 46%, nitrogen uptake by 29% and yield by 37% compared to burning. Retention, however, provides habitat for both harmful as well as useful organisms in one hand and also it provides carbon substrate for heterotrophic nitrogen fixation, increase microbial activity, soil carbon and nitrogen and reduce fertilizer nitrogen requirements for rice. The faster decomposition and release of nitrogen to soil is possible if it is treated with urea and applied during field preparation.

6.4. Straw incorporation

Crop residues may be incorporated partially or completely into the soil depending upon methods of cultivation. Ploughing is the most efficient residue incorporation method. Incorporation of the remaining stubble and straw into the soil returns most of the nutrients and helps to conserve soil nutrient reserves in the long term. Short-term effects on grain yield are often small (compared with straw removal or burning) but long-term benefits are significant. Where mineral fertilizers are used and straw is incorporated, reserves of soil N, P, K, and Si are maintained and may even be increased. Incorporation of straw and stubble into wet soil results in temporary immobilization of N and a significant increase in methane emission from rice paddy,
a practice that contributes to greenhouse gases. Incorporation of large amounts of fresh straw is either labour-intensive or requires suitable machinery for land preparation and may result in the build-up of disease problems. Transplanting should be carried out two to three weeks after straw incorporation.

Recent research results from experimental farms indicate that early, dry shallow tillage at 5 to 10 cm depths to incorporate crop residues and enhance soil aeration during fallow periods has beneficial effects on soil fertility in intensive rice-rice systems. Shallow tillage of dry soil should be carried out up to two to three weeks after harvest in cropping systems where the
dry-moist fallow period between two crops is at least 30 days.

Beneficial effects include:

• A more complete carbon turnover is achieved by aerobic decomposition of crop residues (about 50 percent of the C within 30 to 40 days), thereby minimizing negative effects (e.g., phytotoxicity) of the products of anaerobic decomposition on early rice growth.

• Improved soil aeration i.e., reoxidation of iron and other reduced substances that accumulate during the flooding period.

• Increased N mineralization and soil P release to the succeeding crop, up to the panicle initiation stage.

• Reduced weed growth during the fallow period.

• Reduced irrigation water requirement during land preparation (i.e., less soil cracking and bypass flow water losses in heavy clay soils).

• Easier wetland preparation (i.e., there is often no need for a second plowing operation).

• Smaller CH₄ emissions compared with straw incorporation during land preparation for the crop.

The major disadvantage of incorporation of cereal straw is the immobilization of inorganic nitrogen and its adverse effect due to nitrogen deficiency. Incorporation of rice straw into the soil after its harvest leads to slow down the decomposition and soil nitrate is immobilised, reducing the nitrogen uptake and yield of subsequent crops. Of course, proper fertilizer management practices can reduce N immobilization due to incorporation of crop residues into the soil.

These practices include appropriate method, time and rate of fertilizer (Nitrogen) application: i) placement of nitrogen fertilizer below the surface soil layer that is enriched with carbon after incorporation of crop residue, ii) application of nitrogen fertilizer at a higher rate than the recommended rate and iii) application of nitrogen 15-20 kg/ha as starter dose with straw incorporation increases yields of following crop compared to either burning of straw or its incorporation in the soil.

6.5. Residue Management Effects on Soil Properties

Soil physical, chemical and biological properties, though interrelated are affected by the residue management practices.

6.5.1. Soil physical properties

Residue management practices affect soil physical properties such as soil moisture content, temperature, aggregate formation, bulk density, soil porosity and hydraulic conductivity. Increasing amounts of rice residues on the soil surface reduce evaporation rates and increased duration of first-stage drying. Thus, residue-covered soils tend to have greater soil moisture content than bare soil except after extended drought. Soil temperature is affected through the mechanism of change in radiant energy balance and insulation. The radiation balance is influenced by heating of air and soil, evaporation of soil water and reflection of incoming radiation by surface residue. The insulation effect of residues is controlled by the amount and associated thickness of residue cover. Soil aggregation refers to the cementing or binding together of several primary soil particles into secondary units. The binding substances include oxides and hydroxides of iron, organic substances directly from plants, decomposition products of crop residues, microbial cells, excretory products of microorganisms and gelatinous substances secreted by earthworms. Decomposition of crop residues is controlled by the chemical composition of the residues, soil temperature, soil water, and associated macro and micro flora. The effect of residue management on soil bulk density and cone index has been found to be variable. Crop residues increase soil hydraulic conductivity and infiltration by modifying mainly soil structure, proportion of macropores, and aggregate stability. Hydraulic conductivity under straw-retained directly drilled treatments was around four times greater than that of straw-burnt conventional tillage treatments. The improvement in physical properties coupled with supply of nutrients from FYM and rice straw resulted in consistently greater grain yields than burning.

6.5.2. Soil chemical properties

One of the most important factors determining soil fertility is pH, which may however be influenced strongly by crop residue management. Research has shown that if these organic residues are returned to the soil, soil pH can be increased due to the decarboxylation of organic anions on decomposition by microorganisms. Silica rich plant material has the potential of transforming the electrochemical properties of acidic soils that reduces phosphorous fixation; improves base retention and increase the soil pH. Therefore, retention or incorporation of particularly the rice residues can manifest all the benefits of liming acidic soils. This is a common practice with most Indian farmers in the hills where acidic soils are found. Benefits of rice crop residues incorporation as soil amendments (as substitute for liming material) can also be tested in high rainfall regions of eastern India and Bangladesh, where acidic soils are commonly found and deficiencies of zinc, boron and phosphorous are at times quite acute. Silicates and organics (rice straw) improve iso-electric soils by way of improving the net negative charge, neutralizing acidity/detoxification of aluminium through manipulation of soil pH and point of zero charge of soil sediments having variable charge contributing materials; reduce phosphorous fixation and increase Si content in plants.

 

References

1. Kadam, K.L., L.H. Forrest and W. A. Jacobson. 2000, Rice straw as a lignocellulosic resource: collection, processing, transportation and environmental aspects, Biomass and Bioenergy, Vol. 18; 369-389
2. Dobermann, A and T.H. Fairhurst. 2002, Rice Straw Management, Better Crops International Vol. 16, Special Supplement