Publisher Summary 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. 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. 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. 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. Factors that control C decomposition also affect the N mineralization from the 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. Nutrient cycling in the soil–plant ecosystem is an essential component of sustainable productive agricultural enterprise. Although during the last three decades, fertilization practices have played a dominant role in the rice-based cropping systems, crop residues—the harvest remnants of the previous crop still play an essential role in the cycling of nutrients. Incorporation of crop residues alters the soil environment that in turn influences the microbial population and activity in the soil and subsequent nutrient transformations.

Crop residue management for nutrient cycling and improving soil productivity in rice-based cropping systems in the tropics

The management of crop residues has become an important aspect of sustaining long-term fertility in cropping systems. Incorporation of crop residues can change microbial processes, which aAect nutrient availability and hence crop yield. Carbon (C) use eAciency by soil microorganisms during rice straw decomposition was determined in a rice paddy soil, under aerobic and anaerobic (flooded) conditions at diAerent temperatures (5, 15, and 258C). Flooding had a tendency to reduce C mineralization and enhance methane (CH4) production; however, with decreasing temperature CH4 production became negligible. Our study showed that anaerobes recycled fermentation waste products during the long-term incubation resulting in a lower net residue-C mineralization in flooded systems compared to non-flooded conditions. As a result, we observed similar microbial production under flooded and non-flooded conditions even though anaerobes decomposed less straw-C than aerobes. These results indicate that a significant amount of decomposition occurred under flooded conditions, but because substrate use eAciency was higher, less straw-C was mineralized compared to aerobic conditions. Kinetic analyses of C mineralization curves confirmed that the C mineralized in the flooded treatment was mainly from labile pools with significant amounts coming from more recalcitrant pools, such as cellulose and lignin depending on temperature. The results are discussed in relation to nutrient availability in rice cropping systems. 7 2000 Elsevier Science Ltd. All rights reserved.

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Decomposition of rice straw and microbial carbon use efficiency under different soil temperatures and moistures

In rice cultivation, there are controversial reports on net impacts of nitrogen (N) fertilizers on methane (CH 4 ) emissions. Nitrogen fertilizers increase crop growth as well as alter CH 4 producing (Methanogens) and consuming (Methanotrophs) microbes, and thereby produce complex effects on CH 4 emissions. Objectives of this study were to determine net impact of N fertilizers on CH 4 emissions and to identify their underlying mechanisms in the rice soils. Database was obtained from 33 published papers that contained CH 4 emissions observations from N fertilizer (28-406 kg N ha-1 ) treatment and its control. Results have indicated that N fertilizers increased CH 4 emissions in 98 of 155 data pairs in rice soils. Response of CH 4 emissions per kg N fertilizer was significantly (P   140 kg N ha-1 indicating that substrate switch from CH 4 to ammonia by Methanotrophs may not be a dominant mechanism for increased CH 4 emissions. On the contrary, decreased CH 4 emission in intermittent drainage by N fertilizers has suggested the stimulation of Methanotrophs in rice soils. Effects of N fertilizer stimulated Methanotrophs in reducing CH 4 emissions were modified by the continuous flood irrigation due to limitation of oxygen to Methanotrophs. Greater response of CH 4 emissions per kg N fertilizer in urea than ammonia sulfate probably indicated the interference of sulfate in the CH 4 production process. Overall, response of CH 4 emissions to N fertilizers was correlated with N-induced crop yield (r = +0.39; P < 0.01), probably due to increased carbon substrates for Methanogens. Using CH 4 emission observations, this meta-analysis has identified dominant microbial processes that control net effects of N fertilizers on CH 4 emissions in rice soils. Finally, we have provided a conceptual model that included microbial processes and controlling factors to predict effects of N fertilizers on CH 4 emissions in rice soils.

Do nitrogen fertilizers stimulate or inhibit methane emissions from rice fields

Methane (CH4) emission from Philippine rice paddies was monitored with a closed chamber technique during the 1992 dry and wet season. CH4 emissions were significantly higher in the dry season. Application of green manure stimulated CH4 emissions. In plots that received more than 11 t ha−1 of fresh green manure, CH4 emission was highest during the first half of the growing season. Significant amounts of CH4 may evolve during or immediately after transplanting, if the organic amendments are incorporated 1 to 3 weeks before transplanting. Laboratory incubations of soil cores show that CH4 production is highest near the soil surface. CH4 production in green manure treated fields is higher than in urea-fertilized fields, but toward the end of the season this difference is less pronounced. Around panicle initiation, the fraction of CH4 produced, which was emitted to the atmosphere, is lower than at tillering or ripening. The impact of organic amendments on CH4 emissions at different locations of the world can be described by a dose response curve, if CH4 emission from organically amended plots is expressed relative to CH4 emission from mineral fertilized plots of the same location and season. Various organic amendments (e.g., straw, fermented residues) have a similar effect on CH4 emissions after correction for differences in easily decomposable carbon content.

Influence of organic matter incorporation on the methane emission from a wetland rice field

Methane (CH4) emissions were measured with an automated system in Central Luzon, the major rice producing area of the Philippines. Emission records covered nine consecutive seasons from 1994 to 1998 and showed a distinct seasonal pattern: an early flush of CH4 before transplanting, an increasing mend in emission rates reaching maximum toward grain ripening, and a second flush after water is withdrawn prior to harvesting. The local practice of crop management, which consists of continuous flooding and urea application, resulted in 79-184 mg CH4 m’ d-’ in the dry season (DS) and 269–503 mg CH4 m-2 d-1 in the wet season (WS). The higher emissions in the WS may be attributed to more labile carbon accumulation during the dry fallow period before the WS cropping as shown by higher % organic C. incorporation of sulfate into the soil reduced CH4 emission rates. The use of ammonium sulfate as N fertilizer in place of urea resulted in a 25-36% reduction in CH4 emissions. Phosphogypsum reduced CH4 emissions by 72% when applied in combination with urea fertilizer. Midseason drainage reduced CH4 , emission by 43%, which can be explained by the influx of oxygen into the soil. The practice of direct seeding instead of transplanting resulted in a 16–54ik reduction in CH4 emission, but the mechanisms for the reducing effect are not clear. Addition of rice straw compost increased CH4 emission by only 23-30% as compared with the 162-250% increase in emissions with the use of fresh rice straw. Chicken manure combined with urea did not increase CH4 emission. Fresh rice straw has wider C/N (25 to 45) while rice straw compost has C/N = 6 to 10 and chicken manure has C/N = 5 (o 8. Modifications in inorganic and organic fertilizer management and water regime did not adversely affect grain yield and are therefore potential mitigation options. Direct seeding has a lower yield potential than transplanting but is getting increasingly popular among farmers due to labor savings. Combined with a package of technologies, CH4 emission can best be reduced by (1)the practice of midseason drainage instead of continuous flooding,(2) the use of sulfate-containing fertilizers such as ammonium sulfate and phosphogypsum combined with urea; (3) direct seeding crop establishment; and (4) use of low C/N organic fertilizer such as chicken manure and rice straw compost.

Methane emission from irrigated and intensively managed rice fields in Central Luzon (Philippines)

To understand why atmospheric methane is increasing worldwide, accurate estimates are needed of the global input from rice fields. We report greenhouse and laboratory studies over three growing seasons to isolate and control factors that might affect methane emission from rice paddies, including soil texture, added exogenous organic matter (OM), nitrogen and sulfate ion, and water management. Without added OM, methane production was relatively low, increasing during the growing season, and continuing after harvest, provided the soil remained water-logged. If ground rice straw was added to the soil prior to planting, methane production began shortly after flooding, with an initial burst of the gas after 3 to 5 weeks, and then a gradual increase to a second peak later in the season (and after harvest), with rates considerably higher than in treatments without added OM.

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C. C. Delwiche

Papers

Factors affecting methane production under rice