To refine estimates of source strengths from agricultural wetlands and to study the process of methane production and emission, this study was carried out in rice fields at the

Scanning electron microscopy and X‐ray microanalysis were employed to characterize the morphological properties of iron coatings on rice roots at different growth stages. This information is needed for further understanding of the influence of Fe coatings on rice plant development. Rice root coatings are visible about 1 week after flooding as a brownish discoloration which thickens with age of the root. No coating was found on younger parts of major roots near their tips or on young secondary roots which are critical regions of nutrient uptake. Roots of ‘Brazos’ cultivar rice (Oryza sativa L.) plants grown in Beaumont clay soil had a relatively thin coating of FeOOH mixed with soil particles before panicle differentiation. As a rice plant approached maturity and the outermost cell wall of the root decomposed, a mixture of FeOOH and soil particles began to fill the rectangular spaces that had once been occupied by epidermal cells. Casts in open cell cavities are porous and rough on the exterior side of the root. There were many shapes of casts and they generally matched the varied shapes of the outer layer of epidermal cells of rice roots. Roots of Brazos cultivar rice grown to maturity in Katy fine sandy loam soil have completely developed polyhedral casts. Precipitation of relatively pure FeOOH on cell walls formed hollow casts with the shapes of the original cells. The models presented describe hypothetical steps in the formation of the two types of casts observed by the oxidation of Fe2+ by O2 and precipitating of iron on the walls of closed and open cell cavities.

Since rice fields emit methane, an important contributor to the increasing greenhouse effect, one of our goals is to characterize factors that influence this emission. To create a range in plant and soil temperature, solar radiation, and microbial substrate, rice fields were planted on April 13, May 18 and June 18 of 1990 on silty clay soils near Beaumont, Texas. Immediately prior to planting, one half of each field was supplemented with 6000 kg ha−1 of disc‐incorporated grass straw (Paspalum spp.). Methane emission rates were measured throughout the cultivation period. Methane emission rates varied markedly with planting date and straw addition. The highest emission rate originated from the earliest planted straw‐supplemented field. In general, methane emission decreased with the later plantings that received less solar radiation. Annual emission rates of methane and rice grain yield from individual fields were positively correlated with accumulated solar radiation for both straw‐incorporated and control plots. Straw incorporation resulted in decreased grain yield and increased methane emission in all three fields. Diel variation of methane emission strongly correlated with temperature. The activation energies for methane production, obtained from laboratory soil incubations, and methane emission, obtained from diel field measurements, were approximately the same: 88–98 kJ mol−1 for production and 87 kJ mol−1 for emission.

Rice fields emit methane and are important contributors to the increasing atmospheric CH4 concentration. Manipulation of rice floodwater may offer a means of mitigating methane emission from rice fields without reducing rice yields. To test methods for reducing methane emission, we applied four water management methods to rice fields planted on silty‐clay soils near Beaumont, Texas. The four water treatments investigated were: normal permanent flood (46 days post planting), normal flood with mid‐ season drainage aeration, normal flood with multiple drainage aeration, and late flood (76 days post planting). Methane emission rates varied markedly with water regime, showing the lowest seasonal total emission (1.2 g m−2) with a multiple‐aeration treatment and the highest (14.9 g m−2) with a late flood. Although the multiple‐ aeration water management treatment emitted 88% less methane than the normal irrigation treatment and did not reduce rice yields, the multiple‐aeration treatment did require 2.7 times more water than the 202 mm required by the normal floodwater treatment. A comparison of measured methane emission and production rates obtained from incubated soil cores indicated that, depending on time of season and flood condition, from zero to over 90% of the methane produced was oxidized. The average amount of methane which was oxidized during times of high emission was 73.1 ± 13.7 percent of that produced.

Rice (Oryza sativa L.) root coatings were investigated to determine the amounts and mineralogical properties of the coatings during plant growth and in different soils under field conditions. X‐ray diffraction, electron microscopy, high gradient magnetic separation, and chemical methods were employed. Determinations of oxygen transport by different rice cultivars were made in vitro.Rice cultivars, growth stage, and soil type were significantly related to the amount of iron oxyhydroxide (FeOOH) on field‐grown rice roots. Accumulation of FeOOH was < 2% of dry root weight 7 days after flooding of the soil, increased to a maximum of about 10% at plant maturity and decreased slightly thereafter. Of the four soils studied, two Vertisols had less FeOOH accumulation on root surfaces than two Alfisols. Of the four cultivars characterized (‘Brazos’, ‘Labelle’, ‘Lebonnet’, and ‘Bluebelle’), Brazos had the largest amount of O2 released from the roots and precipitated the largest amount of FeOOH on the roots when field grown in three of four soils. Lake Charles clay was the only soil in which Brazos rice did not rank highest in percent FeOOH on roots and in grain yield.Goethite (α‐FeOOH) and lepidocrocite (τ‐FeOOH) were identified in rice root coatings that were ultrasonically dispersed and concentrated by high gradient magnetic separation.