Search

Search Results

• 1. [Article] Emissions of Anaerobically Produced Methane by Trees

  Recent studies indicate that plants may be a previously overlooked but significant source of atmospheric CH₄, though there is considerable disagreement on the mechanism of production. Our work sought to ...

  Citation × Citation Citation Abstract Copy Title: Emissions of Anaerobically Produced Methane by Trees Author: Rice, Andrew L., Butenhoff, Christopher Lee, Shearer, Martha J., Teama, Doaa, Rosenstiel, Todd N., Khalil, M. A. K. Year: 2010 Recent studies indicate that plants may be a previously overlooked but significant source of atmospheric CH₄, though there is considerable disagreement on the mechanism of production. Our work sought to verify that woody deciduous trees grown under inundated conditions had the capacity for transporting CH₄ from an anaerobic subsurface to the atmosphere and to consider if such a source could be important globally. Here, we report results from a greenhouse mesocosm study that indicate significant emissions of anaerobically produced CH₄ transmitted to the atmosphere through broadleaf riparian tree species grown under flooded conditions. Using a leaf area normalized mean emission rate (0.7 ± 0.3 μg cm⁻² hr⁻¹), results were scaled globally for flooded forest regions and estimated to be 60 ± 20 Tg year⁻¹, ~10% of the global CH₄ source. The carbon isotopic composition of CH4 emitted was found to be significantly enriched compared with expectations (δ13C ~ −54‰) and provided an important isotopic constraint on the global source which coincides with the mean of the globally scaled greenhouse-based estimate. Title: Emissions of Anaerobically Produced Methane by Trees Author: Rice, Andrew L., Butenhoff, Christopher Lee, Shearer, Martha J., Teama, Doaa, Rosenstiel, Todd N., Khalil, M. A. K. Year: 2010 Recent studies indicate that plants may be a previously overlooked but significant source of atmospheric CH₄, though there is considerable disagreement on the mechanism of production. Our work sought to verify that woody deciduous trees grown under inundated conditions had the capacity for transporting CH₄ from an anaerobic subsurface to the atmosphere and to consider if such a source could be important globally. Here, we report results from a greenhouse mesocosm study that indicate significant emissions of anaerobically produced CH₄ transmitted to the atmosphere through broadleaf riparian tree species grown under flooded conditions. Using a leaf area normalized mean emission rate (0.7 ± 0.3 μg cm⁻² hr⁻¹), results were scaled globally for flooded forest regions and estimated to be 60 ± 20 Tg year⁻¹, ~10% of the global CH₄ source. The carbon isotopic composition of CH4 emitted was found to be significantly enriched compared with expectations (δ13C ~ −54‰) and provided an important isotopic constraint on the global source which coincides with the mean of the globally scaled greenhouse-based estimate. Close

• 2. [Article] Production, Oxidation, and Emissions of Methane from Rice Fields in China

  Detailed field studies of methane emissions from rice fields show that when nitrogen fertilizers are used with intermittent irrigation the seasonal average flux is about 3 mg/m²/hr. When continuous flooding ...

  Citation × Citation Citation Abstract Copy Title: Production, Oxidation, and Emissions of Methane from Rice Fields in China Author: Khalil, M. A. K., Shearer, Martha J., Rasmussen, R. A., Duan, Chang-Lin, Ren, Lixin Year: 2008 Detailed field studies of methane emissions from rice fields show that when nitrogen fertilizers are used with intermittent irrigation the seasonal average flux is about 3 mg/m²/hr. When continuous flooding and organic material is added to the same fields, the emissions rise to nearly 30 mg/m²/hr. Production rates measured during the two years when both organic material and water were applied to these fields were found to be 60 and 90 mg/m²/hr in consecutive years. The fraction oxidized is calculated to be about 80% one year and 43% the next year. The results suggest that high organic fertilizer increases production, but may affect the emissions more by reducing oxidation. The data show that seasonally averaged emissions vary by up to factor of 2 among adjacent fields but the variability of production is only about 20%. Information obtained from the farmers suggests that there is a trend towards less use of organic manure as nitrogen fertilizers have become available and the occurrence of intermittent flooding has increased for various reasons. These conditions have likely led to decreasing emissions of methane from rice fields in China in the recent decades. Title: Production, Oxidation, and Emissions of Methane from Rice Fields in China Author: Khalil, M. A. K., Shearer, Martha J., Rasmussen, R. A., Duan, Chang-Lin, Ren, Lixin Year: 2008 Detailed field studies of methane emissions from rice fields show that when nitrogen fertilizers are used with intermittent irrigation the seasonal average flux is about 3 mg/m²/hr. When continuous flooding and organic material is added to the same fields, the emissions rise to nearly 30 mg/m²/hr. Production rates measured during the two years when both organic material and water were applied to these fields were found to be 60 and 90 mg/m²/hr in consecutive years. The fraction oxidized is calculated to be about 80% one year and 43% the next year. The results suggest that high organic fertilizer increases production, but may affect the emissions more by reducing oxidation. The data show that seasonally averaged emissions vary by up to factor of 2 among adjacent fields but the variability of production is only about 20%. Information obtained from the farmers suggests that there is a trend towards less use of organic manure as nitrogen fertilizers have become available and the occurrence of intermittent flooding has increased for various reasons. These conditions have likely led to decreasing emissions of methane from rice fields in China in the recent decades. Close

• 3. [Article] Methane and Nitrous Oxide Emissions From Subtropical Rice Agriculture in China

  Emissions of methane and nitrous oxide, two significant greenhouse gases, were measured from rice fields at Qingyuan in Guangdong Province, China. The region has a subtropical climate which allows two ...

  Citation × Citation Citation Abstract Copy Title: Methane and Nitrous Oxide Emissions From Subtropical Rice Agriculture in China Author: Khalil, M. A. K., Shearer, Martha J., Rasmussen, R. A., Xu, Li, Liu, Jin-Luan Year: 2008 Emissions of methane and nitrous oxide, two significant greenhouse gases, were measured from rice fields at Qingyuan in Guangdong Province, China. The region has a subtropical climate which allows two crops of rice to be grown every year. The prevailing agricultural practices create a complex interaction between factors known to have a major effect on methane and nitrous oxide emissions from rice fields, namely, intermittent flooding and use of organic fertilizers. In this region, the farmers depend on nitrogen fertilizers and, at least in recent years, have only intermittently flooded their fields during the growing seasons. These factors tend to reduce methane emissions. But the rice straw and crop residues from the first crop of the year are plowed into the fields, providing a large addition of organic material under hot weather conditions favorable to quick decomposition during the second crop period. This, and the addition of farmyard manure, increases emissions of methane emissions from these fields. The results of the present study show that the effect of these competing factors and their timing lead to an average rate of emissions of 5 ± 2 and 6 ± 2 mg/m2/h from the first crops for the 2 years when measurements were taken (2003 and 2004), and 12 ± 8 and 13 ± 8 mg/m2/h from the second crop. Further, production measurements showed that during the 2 years of these experiments, the average production rates were about 27 mg/m2/h for the first crop and 22–34 mg/m2/h for the second crop, resulting in estimated oxidation rates of about 80% for the first crop and 50–60% for the second crop. The higher fluxes in the second crop therefore appear to be caused more by reduced oxidation than higher production. Nitrous oxide emissions, when they were detected, usually occurred within a few days after the application of nitrogen fertilizers. The seasonally averaged emissions were between 0.01 and 0.02 mg/m2/h except in the first year when large emissions over one short period pushed the average upward Title: Methane and Nitrous Oxide Emissions From Subtropical Rice Agriculture in China Author: Khalil, M. A. K., Shearer, Martha J., Rasmussen, R. A., Xu, Li, Liu, Jin-Luan Year: 2008 Emissions of methane and nitrous oxide, two significant greenhouse gases, were measured from rice fields at Qingyuan in Guangdong Province, China. The region has a subtropical climate which allows two crops of rice to be grown every year. The prevailing agricultural practices create a complex interaction between factors known to have a major effect on methane and nitrous oxide emissions from rice fields, namely, intermittent flooding and use of organic fertilizers. In this region, the farmers depend on nitrogen fertilizers and, at least in recent years, have only intermittently flooded their fields during the growing seasons. These factors tend to reduce methane emissions. But the rice straw and crop residues from the first crop of the year are plowed into the fields, providing a large addition of organic material under hot weather conditions favorable to quick decomposition during the second crop period. This, and the addition of farmyard manure, increases emissions of methane emissions from these fields. The results of the present study show that the effect of these competing factors and their timing lead to an average rate of emissions of 5 ± 2 and 6 ± 2 mg/m2/h from the first crops for the 2 years when measurements were taken (2003 and 2004), and 12 ± 8 and 13 ± 8 mg/m2/h from the second crop. Further, production measurements showed that during the 2 years of these experiments, the average production rates were about 27 mg/m2/h for the first crop and 22–34 mg/m2/h for the second crop, resulting in estimated oxidation rates of about 80% for the first crop and 50–60% for the second crop. The higher fluxes in the second crop therefore appear to be caused more by reduced oxidation than higher production. Nitrous oxide emissions, when they were detected, usually occurred within a few days after the application of nitrogen fertilizers. The seasonally averaged emissions were between 0.01 and 0.02 mg/m2/h except in the first year when large emissions over one short period pushed the average upward Close

• 4. [Article] Decreasing Emissions of Methane from Rice Agriculture

  Methane emissions from rice fields are controlled by several key factors. The most important are the application of organic soil amendments and water management: whether the fields are flooded intermittently ...

  Citation × Citation Citation Abstract Copy Title: Decreasing Emissions of Methane from Rice Agriculture Author: Khalil, M. A. K., Shearer, Martha J. Year: 2006 Methane emissions from rice fields are controlled by several key factors. The most important are the application of organic soil amendments and water management: whether the fields are flooded intermittently or continuously. The total annual emissions from any country or the world are thus affected by the area of rice harvested, and a composite emission factor that takes into account how the various agricultural practices are distributed in terms of fertilizer use and water management, as well as other less important variables. Over decades the area of rice harvested changes but so does the composite emission factor. The product therefore, may change substantially over decadal time scales. We will argue that for China at least, there are downward trends of both area and emission factor leading to a substantial reduction of annual emissions over the last two decades from some 30 Tg/yr to perhaps less than 10 Tg/yr. The changes that cause these reductions would increase nitrous oxide emissions from rice fields as organic fertilizers are replaced with nitrogen based fertilizers. Similar changes are likely to be occurring world-wide. These trends of emissions are probably represented in the observed slowdown of methane trend in the atmosphere and the continuing steady increase of nitrous oxide. Title: Decreasing Emissions of Methane from Rice Agriculture Author: Khalil, M. A. K., Shearer, Martha J. Year: 2006 Methane emissions from rice fields are controlled by several key factors. The most important are the application of organic soil amendments and water management: whether the fields are flooded intermittently or continuously. The total annual emissions from any country or the world are thus affected by the area of rice harvested, and a composite emission factor that takes into account how the various agricultural practices are distributed in terms of fertilizer use and water management, as well as other less important variables. Over decades the area of rice harvested changes but so does the composite emission factor. The product therefore, may change substantially over decadal time scales. We will argue that for China at least, there are downward trends of both area and emission factor leading to a substantial reduction of annual emissions over the last two decades from some 30 Tg/yr to perhaps less than 10 Tg/yr. The changes that cause these reductions would increase nitrous oxide emissions from rice fields as organic fertilizers are replaced with nitrogen based fertilizers. Similar changes are likely to be occurring world-wide. These trends of emissions are probably represented in the observed slowdown of methane trend in the atmosphere and the continuing steady increase of nitrous oxide. Close

• 5. [Article] Effects of Production and Oxidation Processes on Methane Emissions from Rice Fields

  The emission of methane from rice fields is the difference between the amount produced in the anaerobic zone below the soil and the amount oxidized in the root zone. Plants can also contribute to methane ...

  Citation × Citation Citation Abstract Copy Title: Effects of Production and Oxidation Processes on Methane Emissions from Rice Fields Author: Khalil, M. A. K., Rasmussen, R. A., Shearer, Martha J. Year: 1998 The emission of methane from rice fields is the difference between the amount produced in the anaerobic zone below the soil and the amount oxidized in the root zone. Plants can also contribute to methane production by exuding organic compounds that may be utilized by methanogenic bacteria. We measured methane emissions from rice fields at Tu Zu in China between 1988 and 1994, which gave average emissions of about 30 mg m⁻² h⁻¹. We estimate that 45-60% of the methane produced was oxidized before reaching the atmosphere; and root exudates may have contributed of the order of 10% of the methane that was produced. The fraction of methane oxidized is low compared to experimental studies at other locations (60-85%). At Tu Zu, methane production is enhanced by continuously flooded fields and the use of large amounts of organic fertilizers; in addition, the lower oxidation rate may also contribute to the higher methane emissions observed compared to other locations. In the past, most of the attention has been devoted to the factors that affect methane production and transport, but it seems that the factors that affect methane oxidation are equally important in determining the flux, if not more so. The comparison of methane fluxes observed at different locations and the extrapolation of field measurements to accurately estimate global emissions will require a better understanding of the rate of methane oxidation in the soils and the factors that control it. Title: Effects of Production and Oxidation Processes on Methane Emissions from Rice Fields Author: Khalil, M. A. K., Rasmussen, R. A., Shearer, Martha J. Year: 1998 The emission of methane from rice fields is the difference between the amount produced in the anaerobic zone below the soil and the amount oxidized in the root zone. Plants can also contribute to methane production by exuding organic compounds that may be utilized by methanogenic bacteria. We measured methane emissions from rice fields at Tu Zu in China between 1988 and 1994, which gave average emissions of about 30 mg m⁻² h⁻¹. We estimate that 45-60% of the methane produced was oxidized before reaching the atmosphere; and root exudates may have contributed of the order of 10% of the methane that was produced. The fraction of methane oxidized is low compared to experimental studies at other locations (60-85%). At Tu Zu, methane production is enhanced by continuously flooded fields and the use of large amounts of organic fertilizers; in addition, the lower oxidation rate may also contribute to the higher methane emissions observed compared to other locations. In the past, most of the attention has been devoted to the factors that affect methane production and transport, but it seems that the factors that affect methane oxidation are equally important in determining the flux, if not more so. The comparison of methane fluxes observed at different locations and the extrapolation of field measurements to accurately estimate global emissions will require a better understanding of the rate of methane oxidation in the soils and the factors that control it. Close

• 6. [Article] Measurements of Methane Emissions from Rice Fields in China

  Rice fields have always been regarded as one of the largest anthropogenic sources of atmospheric methane. Here we report the results of a 7-year study of methane emissions from rice fields in the Sichuan ...

  Citation × Citation Citation Abstract Copy Title: Measurements of Methane Emissions from Rice Fields in China Author: Khalil, M. A. K., Rasmussen, R. A., Shearer, Martha J., Dalluge, R. W., Ren, Lixin, Duan, Chang-Lin Year: 1998 Rice fields have always been regarded as one of the largest anthropogenic sources of atmospheric methane. Here we report the results of a 7-year study of methane emissions from rice fields in the Sichuan Province of China. In this region, there is one crop of rice per year, the fields are continuously flooded from transplanting to harvest, and there is heavy use of organic fertilizers. Emissions over the entire growing season were measured from each of up to 24 plots. Environmental variables were measured and relevant supporting data on the agricultural practices were recorded. The fields were studied under prevailing agricultural practices of the local farmers. The results represent emissions under standard agricultural practices and the year to year variability of climate, fertilizers, available irrigation water, and cultivars. Based on some 5000 flux measurements, the average emission rates between 1988 and 1994 were 30 mg/m²/h for a growing season of between 100 and 120 days. This emission rate is comparable to other published data from similar rice fields but somewhat on the high side of the range. There were no systematic trends of emissions during the 7 years of our experiment, but there was substantial year to year variability. The data have been subjected to exhaustive analyses for validity, accuracy, and reliability. From this, a high-quality, spatially averaged data set has been constructed representing average emissions from the rice fields for each day when measurements were taken. We describe here the main observational results and document the spatial and temporal variability observed on timescales ranging from a day to several years and on spatial scales ranging from 0.5 m² to 16 m². Title: Measurements of Methane Emissions from Rice Fields in China Author: Khalil, M. A. K., Rasmussen, R. A., Shearer, Martha J., Dalluge, R. W., Ren, Lixin, Duan, Chang-Lin Year: 1998 Rice fields have always been regarded as one of the largest anthropogenic sources of atmospheric methane. Here we report the results of a 7-year study of methane emissions from rice fields in the Sichuan Province of China. In this region, there is one crop of rice per year, the fields are continuously flooded from transplanting to harvest, and there is heavy use of organic fertilizers. Emissions over the entire growing season were measured from each of up to 24 plots. Environmental variables were measured and relevant supporting data on the agricultural practices were recorded. The fields were studied under prevailing agricultural practices of the local farmers. The results represent emissions under standard agricultural practices and the year to year variability of climate, fertilizers, available irrigation water, and cultivars. Based on some 5000 flux measurements, the average emission rates between 1988 and 1994 were 30 mg/m²/h for a growing season of between 100 and 120 days. This emission rate is comparable to other published data from similar rice fields but somewhat on the high side of the range. There were no systematic trends of emissions during the 7 years of our experiment, but there was substantial year to year variability. The data have been subjected to exhaustive analyses for validity, accuracy, and reliability. From this, a high-quality, spatially averaged data set has been constructed representing average emissions from the rice fields for each day when measurements were taken. We describe here the main observational results and document the spatial and temporal variability observed on timescales ranging from a day to several years and on spatial scales ranging from 0.5 m² to 16 m². Close