The intelligent management and utilization of crop residues is essential for the improvement of soil quality and crop productivity under rice-based cropping systems of the tropics. Crop residues, usually considered a problem, when managed correctly can improve soil organic matter dynamics and nutrient cycling, thereby creating a rather favorable environment for plant growth. Crop residues contain large quantities of nutrients, and thus the return of crop residues to the soil can save a considerable quantity of fertilizers. The most viable option is to retain residue in the field; burning should be avoided. The major issue is adapting drills to sow into loose residues. Strategies include chopping and spreading of straw during or after combining or the use of disc-type trash drills. The important conditions that influence crop residue decomposition under field conditions are temperature, moisture, aeration, and N application. Several other factors, such as residue quality, tillage, and soil properties, also affect microbial decomposition of crop residues. Residues rich in lignin and polyphenol contents experience the lowest decay. Decomposition of crop residues occurs at a rapid rate (about 80% of crop residue C is lost in the first year) under the warm and humid conditions of the tropics. Exponential models have often described the process of C decomposition carried out by soil microorganisms, and these have suggested the existence of at least two carbon fractions-labile and resistant. Factors that control C decomposition also affect the N mineralization from crop residues. Decomposition of poor-quality residues with low N contents, high C:N ratios, and high lignin and polyphenol contents generally results in microbial immobilization of soil and fertilizer N. The period of N immobilization varied from 4 to 8 weeks depending on temperature and mineral N content of the soil. The N immobilization potential of cereal residues is very high (26-35 mg N g-1 added C) and is often higher than available mineral N content in soils. Net rates of N mineralization will occur when plant residues with C:N ratios <40 are incorporated. C:N ratios have been criticized because they are species specific and are influenced by soil N supply (site specific). Most of the studies on N mineralization-immobilization have been carried out under laboratory conditions, and it is not precisely known to what extent these can be extrapolated to field conditions. The extent of N immobilization is less in anaerobic than aerobic conditions. Nutrient immobilization caused by the addition of residues will last only a few years before the system adjusts to a new equilibrium, and the rate of mineralization of nutrients in the whole system is increased. The qualitative controls (factors) on the amounts and timing of N release from crop residues are known, but quantification of mineralization-immobilization over both a short- and long-term basis and understanding of the relationship with different types of residues, inputs, and management are not adequate. Little is known about the effect of tillage on mineralization of N from crop residues and mechanisms controlling mineralization in rice-based cropping systems under tropical conditions. Application of crop residues with a high C:N ratio often leads to adverse impacts on available N in soil and growth of crops planted immediately after straw incorporation. A large number of organic compounds, particularly phenolic acid and acetic acid, are released during the decomposition of crop residues under anaerobic conditions. The accumulation of these organic compounds can adversely affect the seedling growth. The accumulation of organic acids in residue-treated soils occurred during the initial 15-20 days of the decomposition period. The accumulation of organic acids is likely to be greater in soils with low percolation rates. The serious decrease in soil available N content can be offset by proper application of N fertilizer in combination with rice straw, and the toxic effects of organic acids and some reducing substances resulting from decomposition of rice straw may be alleviated or eliminated by allowing the rice or wheat straw to decompose for some time (2-4 weeks) before planting the next crop. Crop residue management may affect N cycling and N use efficiency of crops in several ways. Rice has been found to recover up to 25% of the rice straw N. Although the total amount of N contributed by straw from a single application will be relatively small, the long-term effects should be substantial. The effect of crop residues on N losses by leaching and denitrification, and on the availability of fertilizer N, particularly under surface placement of residues, is not conclusive and needs further investigation under field conditions. Both the positive and the negative effects of residues on fertilizer use efficiency have been reported. Incorporation of crop residues markedly increases the activities of urease and many other enzymes in soil. Large NH3 volatilization losses from urea applications to soils amended with crop residues both under flooded and upland conditions have been reported. The application of crop residues can cause short-term immobilization of both P and S, particularly in aerobic soils. Only a small fraction (5%) of the residue P is available to the plants in the first year, and a major fraction is immobilized as microbial biomass. The availability of P in the soil and uptake by rice increased with straw incorporation in flooded soils. Crop residue incorporation generally increased the P adsorption and P sorption maxima in soils but markedly reduced the affinity coefficient or rate of adsorption. Incorporation of crop residues in rice increased the efficiency of P in rock phosphates. Crop residues contain large amounts of K, which upon incorporation increased K availability in soil and helped to reduce K depletion from nonexchangeable K fraction of soil. Long-term application of crop residues increased the organic matter, total N content, and availability of several nutrients (though to a small extent) in soils. The rate of increase in soil organic matter is low due to high turnover rates of C under tropical conditions. The increase in soil organic matter levels due to crop residue recycling was determined by the duration of the study, amount and quality of residue, soil type, climatic conditions, and cropping system followed. Crop residues influence the chemical and biological properties of the soil. In many situations, residue retention may reduce nutrient availability, and additional fertilizer applications may be required to attain yields equal to those previously achieved. Crop residues exerted a favorable, though highly variable, influence on different soil physical parameters. Residue management alters soil properties, mainly by causing a gradual increase in soil organic matter content. The effects of residues on soil physical properties were dependent on soil type, tillage, soil moisture conditions, duration of study, and cropping system followed. The beneficial effects of crop residues on soil physical properties are likely to be greater under the rice-wheat than under the rice-rice cropping system. Crop residues caused marked increases in microbial populations and microbial biomass in soils. The addition of crop residues to flooded soils enhanced biological N fixation by phototrophic and heterotrophic bacteria. The estimate of biological N fixation showed that 15-25 kg ha-1 more N may be fixed per season by amending the soils with crop residues under field conditions. The values of biological N fixation under upland conditions are lower than under flooded conditions. The many reports of investigations into crop residue management and yield showed that results have been variable-no effect, yield increase, or yield decline. Yield decline associated with stubble retention may be due to three main factors: short-term nitrogen immobilization, fungal diseases, and phytotoxicity. The degree of stubble decomposition at the time of planting has a great bearing on the likelihood of problems for the crop. Furthermore, some of these problems are likely to decrease over time. In cases in which rice yield increases occurred with stubble retention, these increases took at least 3-5 years to be expressed. In the rice-wheat cropping system, rice is likely to be more benefited than wheat. The rice straw recycling in wheat will have small or even negative effects on wheat yields over the short term (1-3 years). These negative effects can, however, be effectively mitigated by co-incorporation of cereal residues and leguminous green manures. Long-term recycling of crop residues can have substantial beneficial effects, and high crop yields could be obtained from the combined use of inorganic fertilizers and crop residues. The effects of crop residue recycling on growth and yields of crops are influenced by residue load, time allowed for decomposition, temperature and moisture conditions during decomposition and crop growth, nutrient supply, and soil type. The crop residues are likely to have greater effects on yields of crops grown in fine-textured than in coarse-textured soils. The beneficial effect of a starter dose on crop yields in straw-amended soils has been reported in a few studies, and it needs further evaluation. The time of fertilizer N application is important in enhancing the yields of crops on residue-treated soils. A few studies showed that a dose of fertilizer N higher than recommended for untreated soils may be needed in the initial 1-2 years for straw-amended fields. ...
