Frank M. Fisher

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 oxidation and pathways of production in a Texas paddy field deduced from measurements of flux, δl3C, and δD of CH4

Irrigated rice paddies are one of the few methane (CH4) sources where the management of its emissions may be possible. Before that can be initiated, however, the relationship between production, oxidation, and emission of CH4 and the processes controlling them must be better known. To that end we have made measurements of concentration and stable carbon and hydrogen isotopes of CH4 and CO2 in paddy fields along the Gulf Coast of Texas. Although only small differences in total CH4 flux (∼46.5 g m−2 clayey and ∼43 g m−2 sandy) and average δ13CH4 (seasonal averages of −56.11±1.21‰ clayey and −53.57±0.97‰ sandy) from emitted CH4 were observed in two plots with different soil textures, by making additional measurements of belowground CH4 and CO2 we learned much about processes occurring in the paddy field. We estimated that roughly 98% of the CH4 released was transported through the plant and that residence times for belowground CH4 were from about 1 to 5 hours during most of the season, indicating fast processing of both organic carbon and current photosynthesized carbon to make CH4. The percentage of CH4 made from acetate fermentation calculated using isotope data was strongly dependent on the value of the fractionation factor (α) associated with the CO2/H2 reduction pathway for CH4 formation. Using a range of reasonable values for α, we calculated that acetate fermentation was from 67 to 80% early in the season to 29 to 60% late in the season (generally decreasing as the season progressed). Most importantly, we have strong evidence that rhizospheric CH4 oxidation occurs in paddy fields. We have developed a semiempirical equation and used it to calculate the percent of CH4 oxidized as a function of total CH4 produced from field measurements of δ13CH4 under natural conditions. Because most emitted CH4 is transported by the rice plant, it was necessary to determine the isotopic fractionation CH4 underwent during its transport through the plant. This value, 12±l‰, was used to calculate oxidation percent using belowground and emitted δ13CH4 values. In Texas, oxidation of CH4 in the soil increased from ∼20 to ∼60% over the 6 week period just prior to harvest.

Comparison of soil acetate concentrations and methane producton, transport, and emission in two rice cultivars

The amount of methane emitted from irrigated rice paddies is dependent on the variety of rice grown. In this study we examined two varieties of rice with differing methane emission rates to determine if the primary mechanism for these differences was related to transport processes or the rate of methane production. The cultivars used were Mars and Lemont, with 1994 seasonal emissions of 34 and 18 g m−2, respectively. Seasonal methane emission and soil acetate concentration data were measured weekly over two seasons in both varieties. In addition, gas transport through the two rice varieties was investigated in both field and laboratory experiments in 1995. We found no significant differences in gas transport between the two varieties. However, significant differences between the two varieties were detected in the soil acetate concentrations during the vegetative growth stage. Mars exhibited higher seasonal methane emissions and higher soil acetate concentrations than Lemont. This suggests that the intervarietal differences in methane emissions are the result of different soil substrate levels and hence different rates of methane production. The turnover time of soil acetate was found to be small, about 1 hour in the last half of the season. Calculations of methane oxidation, using two methods, support previous findings that the fraction of methane oxidized in the soil prior to emission increases from 10 to 30% before heading to 30–70% after heading.

Differences in CH4 oxidation and pathways of production between rice cultivars deduced from measurements of CH4 flux and δ13C of CH4 and CO2

We report measurements of CH4 flux and δ13C and δD values of emitted CH4 and sediment CH4 and CO2 during the 1995 rice growing season in Beaumont, Texas. Four rice plant cultivars, Lemont, Mars, Cypress, and Della, and an unplanted plot were studied to provide possible explanations for the differences in CH4 emissions between cultivars. Using the measured isotope values, along with data of CH4 and CO2 concentrations and other ecosystem data, we determined differences between cultivars in the processes of oxidation and production throughout the growing season. For instance, rhizospheric CH4 oxidation increased as the season progressed in both Mars and Lemont cultivars. Late in the season, however, 71±10% of CH4 produced in the Mars plot was oxidized compared to only 39±10% in the Lemont plot. The contribution of acetate fermentation to methanogenesis at specific times during the season was calculated using measured isotopic values and assuming identical isotopic fractionation factors in methanogenic pathways for the cultivars. In these calculations a range of values for the contribution to CH4 production from acetate fermentation and CO2 reduction with H2 was estimated by considering different fractionation factors for the methanogenic CO2 reduction pathway and the possibility of a 10% contribution to CH4 production from acetate produced by homoacetogenesis. In general, a steady increase in the CH4 portion produced by acetate fermentation was noted in the Lemont cultivar, while an increase followed by a decrease near the end of the season was observed for the Mars cultivar.

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.

Exchange of methane from rice fields: National, regional, and global budgets

Over the past decade, several estimates have been developed to establish source strengths of various atmospheric trace gases. Interest is, in part, driven by international agreements wherein governments are required to establish emission inventories and to develop means to stabilize or reduce national emissions. Although techniques and models for quantifying gas fluxes have improved considerably for some gases and sources, the uncertainties in the national, regional, and global budgets for a number of atmospheric compounds remain. In this paper, we review estimates of the emission of methane from rice fields both on a global scale and on a country scale, using China as an example. Several reported methods of obtaining an annual methane emission value for China are reviewed. A comparison among them yields a suggested emission value of 13.0±3.3 Tg yr−1 and a range from 9.7 to 16.2 Tg yr−1. Several strategies are suggested for future requirements to obtain greater accuracy in regional and country emission values.

Vertical migration as a refuge from predation in intertidal marsh snails: A field test

Marsh periwinkles Littorina irrorata Say are common gastropod inhabitants of tidal marshes along the East and Gulf coasts of the U.S.A. These snails migrate up cordgrass Spartina alterniflora Loisel stems with tidal inundation. We conducted a field experiment in a Texas coastal marsh to determine if climbing grass stalks provides a refuge from predation for L. irrorata from blue crabs Callinectes sapidus Rathbun and other predators which enter and feed in the marsh at high tide. Treatments consisted of cages with short or tall grass heights from which predators were either excluded (closed cage) or allowed to enter and leave with the tides (open cage). A folded aluminum edging prevented most L. irrorata emigration from open treatments. Our results show that snails not allowed to climb grass stalks suffered significantly higher predation-related mortality than climbing snails indicating that vertical migration does provide a refuge from predation. Analysis of population size structure data from this and other studies show that juvenile L. irrorata are less abundant than larger conspecifics during the summer reproductive peak when abundances should be high. We attribute this to size selective predation by spring-recruited blue crabs and other predators.

Dispersion of the salt-marsh periwinkle Littoraria irrorata: Effects of water level, size, and season

This paper documents horizontal and vertical dispersion patterns of a Texas population of the saltmarsh periwinkle, Littoraria irrorata, over a 15-month period. The study was conducted within a tidal marsh on the Anahuac National Wildlife Refuge in Galveston Bay. Two mark-recapture experiments demonstrated that L. irrorata rarely move more than 2 m from their release point over long periods of time and do not home to individual Spartina plants. Adult L. irrorata forage farther away from the base of Spartina stalks at low tide than do juvenile snails. Remaining near the plant base may decrease both temperature and desiccation stress on juveniles. During warm months, L. irrorata climb grass stalks with tidal inundation and forage on the substratum at low tide. Snails are inactive and aggregate in detrital debris at the base of Spartina clumps during the winter. *** DIRECT SUPPORT *** A01BY058 00016

Lead concentrations in tissues of marsh birds: relationship of feeding habits and grit preference to spent shot ingestion.

The toxic effects of lead shot ingestion on waterfowl are well documented. During the summer of 1982, a random sample of marsh birds from the upper Texas coast indicated a relatively high incidence of shot ingestion in several species. In addition, lead and arsenic levels in bones of these species were correlated with the presence of ingested shot. This paper addresses possible reasons why some species are affected and others are not, in particular the relationship between feeding habits and size of grit which is found in the gizzards of each species.