F. T. Turner

Methane emission from rice fields : the effect of floodwater management

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.

Methane emission from rice fields as influenced by solar radiation, temperature, and straw incorporation

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.

Methane emissions from rice fields: Effect of soil properties

Flooded rice fields emit methane and are important contributors to the increasing atmospheric methane concentration. Various estimates of global release rates of methane from rice paddies range from a low of 20 Tg per year to a high of 200 Tg per year. Global estimates of methane emissions from rice fields depend upon obtaining reliable data from a variety of soil types. We have compared a variety of methane emission data sets obtained over a four-year period from three different soil types found at the Texas Agricultural Experiment Station near Beaumont, Texas, with several physical and chemical properties of the soils. We find that seasonal methane emissions directly correlate with the percent sand in the soils. Along a transect with soil sand content ranging from 18.8% to 32.5%, seasonal methane emissions ranged from 15.1 g m −2 to 36.3 g m−2.

Increased P diffusion as an explanation of increased p availability in flooded rice soils

SummaryPhosphorus supply factors (capacity, kinetic, intensity, and diffusivity) and plant growth were the approaches used to assess P supply of flooded rice soils. Increases in the capacity, intensity, and kinetic factors, as measured by E-value, solution P concentration, and soil P release rate to a distilled water ‘sink’ respectively, were unpronouced and infrequent upon water-saturation of ten soils. However, increases in the diffusitivity factor, as measured by 32P diffusion coefficients, were at least ten-fold as soil moisture increased. The greatest increases in P diffusion occurred as soil moisture increased beyond one-third bar.Using a P fertilized soil or P treated powdered cellulose as the P source and a minus P nutrient solution to nourish a split root system with water and nutrients, data were obtained which suggested that P uptake and rice-shoot growth (indicators of P availability) increased with increasing moisture level. Phosphorus uptake and rice-shoot growth were greatst when the soil or P treated cellulose were water-saturated. These data indicate that increased soil P availability upon flooding can be attributed to an increase in the diffusivity factor.

Phosphorus relationships to manganese and iron in rice soils

Rice plants (Oryza sativa L.) grown on soils containing low soil testextractable P frequently do not respond to fertilizer P application under reduced conditions. The lack of rice response to fertilizer P in soils with low extractable P has been attributed to increased solubility of Fe-associated P upon flooding. The increased solubility of Mn oxides and release of Mn-associated P in flooded soils may also increase P availability to rice plants. The effect of Mn oxides on P availability has not been adequately described or quantified. In this experiment we used rice soils with low soil test P and high or low Mn or Fe oxides to measure the effects of Mn oxides on P availability under field, greenhouse, and laboratory conditions. Soils were incubated under oxidized or reduced conditions for 35 days. At the end of 0, 2, 5, 8, 11, 15, 20, 25, 30, and 35 days, soil solution pH and Eh and soluble and extractable P, Fe, and Mn were determined. Greenhouse and field experiments showed that rice plants did not respond to P application in three of four soils despite low extractable P. A large increase in P concentration following reduction was observed and was correlated positively with reactive Fe and Mn oxides in soils. During incubation water-soluble P in soils increased from 0.06 to 0.37 mg P kg−1, Fe from 0.2 to 22.0 mg Fe kg−1, and Mn from 0.08 to 6.43 mg Mn kg−1. Linear stepwise regression analysis indicated that 65 to 91% of solution P variation was attributable to Fe oxides and about 14 to 28% was attributable to Mn oxides. The contribution of Fe reduction to increased P availability was highly significant and related to large amounts of poorly crystalline Fe oxides in soils extracted by oxalate. The manganese oxide contribution to P availability was important, especially in the early stages of soil reduction in soils with large amounts of reactive Mn. In soils dominated by Mn oxides, the reduction of Mn-associated P can be a significant source of P for plants.

Dicyandiamide (DCD) as a nitrification inhibitor for rice culture in the United States

Abstract Efficient nitrogen (N) fertilizer management for paddy rice production is difficult because of potentially high N losses from denitrification, NH3 volatilization, and leaching. The use of a nitrification inhibitor, by slowing the rate of nitrification of NH4 +‐N sources prior to flooding, offers the potential to reduce denitrification losses that occur after flooding. Dicyandiamide (DCD) is one such nitrification inhibitor. The objective of this series of studies was to evaluate DCD for its effectiveness as a nitrification inhibitor in paddy rice production across an array of soils, management systems, and climate conditions. Studies were conducted on fine‐ and medium‐textured soils in Arkansas, California, Louisiana, Mississippi, and Texas. Dicyandiamide was coated onto or formulated with urea (7 or 10% of total N as DCD‐N) and applied either broadcast pre‐plant incorporated or broadcast as a topdress application prior to flooding at the 4‐ to 5‐leaf development stage of the rice plant. These tr...

Water balance of flooded rice paddies

Abstract Rice ( Oryza sativa L., var. Labelle) was grown in 300 m 2 paddies of Beaumont clay soil (Typic pelludert) and subjected to two management schemes of flooded rice culture. These schemes were continuous irrigation and intermittent irrigation. Careful measurements of irrigation, precipitation, evapotranspiration, deep percolation and runoff were made, and the total water balance for the two water management schemes was calculated. The results show continuous irrigation to be very wasteful of water with slightly over 1 m of irrigation water applied to supply an evapotranspirational need of 0.5–0.6 m. The intermittent irrigation management is less wasteful but still could be improved upon. Suggestions are presented for techniques to help improve the water use efficiency and reduce runoff losses.

“Starter nitrogen”; fertilization in soybean culture 1

Abstract Proponents of the “starter nitrogen”; concept believe that a small amount of nitrogen fertilizer stimulates early vegetative growth of soybeans, thereby facilitating cultivation and weed control. To evaluate this concept, soybean leaf area, plant height, fresh weight, and yield were measured over three growing seasons. Growth parameters were measured at weekly intervals during the 7 weeks following planting of ‘Bragg’ soybeans (Glycine max (L.) Merr.) which had received 0, 16.8, and 50.4 kg N ha‐1 banded pre‐plant into the soil. The analysis, a multiple linear regression maximum R2 (multiple correlation coefficient) improvement procedure, showed that starter nitrogen did not significantly (.05) influence leaf area, plant height, fresh weight, or yield of Bragg soybeans.

SCHEDULING INPUTS WITH PRODUCTION FUNCTIONS: OPTIMAL NITROGEN PROGRAMS FOR RICE

The problem of scheduling input applications can be examined by extending conventional production function analysis. Using appropriately designed agricultural experiments, it is possible to estimate production function parameters with alternative specifications for input timing (and amount). A study of nitrogen applications to rice is employed to illustrate scheduling via production functions. Alternative specifications and functional forms are simultaneously examined to determine the sensitivity of economic results to these factors. Sensitivity is found to be high, and this finding is hypothesized to be critical for other approaches to input scheduling as well.

Effect of moisture and oxidation status of alkaline rice soils on the adsorption of soil phosphorus by an anion resin

SummaryConsiderable effort was devoted to experimentally explaining the greater amount of anion resin-adsorbable P (ARAP) in water-saturated alkaline soils relative to moist soils with the purpose of explaining the phenomenon of increased P availability in flooded rice soils. ARAP increased when waterlogged conditions were imposed on soils, but the increase in ARAP occurred before reducing conditions were obtained. Reducing conditions did not increase ARAP. The increase in ARAP in the water-saturated alkaline soils was attributed to the enhanced P diffusion resulting from a decrease in tortuosity, thus indicating that increased P availability upon flooding could be due to increased P diffusion.