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• 1. [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

• 2. [Article] Isotopic Signatures and Concentration Profiles of Nitrous Oxide in a Rice-Based Ecosystem During the Drained Crop-Growing Season

  The stable isotopic composition of nitrous oxide (N₂O) in agricultural soils can improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) ...

  Citation × Citation Citation Abstract Copy Title: Isotopic Signatures and Concentration Profiles of Nitrous Oxide in a Rice-Based Ecosystem During the Drained Crop-Growing Season Author: Xiong, Z. Q., Khalil, M. A. K., Xing, G., Shearer, Martha J., Butenhoff, Christopher Lee Year: 2009 The stable isotopic composition of nitrous oxide (N₂O) in agricultural soils can improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N₂O formation in soils, and can provide constraints for the atmospheric N2O budget. Here, we present soil profiles featuring N₂O concentrations and δ¹⁵N and δ¹⁸O values in N₂O over time, which permit the in situ identification of processes and sites of N₂O production in a rice-based ecosystem seeded with winter wheat. Our δ¹⁵N and δ¹⁸O soil profile values support the conclusion that denitrification is the dominant process behind N₂O production during the winter wheat season. The soil N₂O gas concentrations are higher below 10 cm than above 10 cm, and more depleted in ¹⁵N - N₂O at 10–15 cm depth compared to other soil depths, which indicates that the N₂O production zone is located at a depth of 10–15 cm within 0–20 cm. The δ¹⁵Nair and δ¹⁸OSMOW (δ¹⁸O of standard mean ocean water) values for soil gas N₂O averaged over the entire wheat growing season are +0.90 ± 2.9 (n = 64) and +39.3 ± 3.1 (n = 64), respectively. Given that paddy source N₂O is much heavier in both measured ¹⁵N and ¹⁸O compared to other fertilized soils, agricultural soils have been underestimated as a source of N₂O since isotopic constraints from rice agriculture have not been taken into account. Title: Isotopic Signatures and Concentration Profiles of Nitrous Oxide in a Rice-Based Ecosystem During the Drained Crop-Growing Season Author: Xiong, Z. Q., Khalil, M. A. K., Xing, G., Shearer, Martha J., Butenhoff, Christopher Lee Year: 2009 The stable isotopic composition of nitrous oxide (N₂O) in agricultural soils can improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N₂O formation in soils, and can provide constraints for the atmospheric N2O budget. Here, we present soil profiles featuring N₂O concentrations and δ¹⁵N and δ¹⁸O values in N₂O over time, which permit the in situ identification of processes and sites of N₂O production in a rice-based ecosystem seeded with winter wheat. Our δ¹⁵N and δ¹⁸O soil profile values support the conclusion that denitrification is the dominant process behind N₂O production during the winter wheat season. The soil N₂O gas concentrations are higher below 10 cm than above 10 cm, and more depleted in ¹⁵N - N₂O at 10–15 cm depth compared to other soil depths, which indicates that the N₂O production zone is located at a depth of 10–15 cm within 0–20 cm. The δ¹⁵Nair and δ¹⁸OSMOW (δ¹⁸O of standard mean ocean water) values for soil gas N₂O averaged over the entire wheat growing season are +0.90 ± 2.9 (n = 64) and +39.3 ± 3.1 (n = 64), respectively. Given that paddy source N₂O is much heavier in both measured ¹⁵N and ¹⁸O compared to other fertilized soils, agricultural soils have been underestimated as a source of N₂O since isotopic constraints from rice agriculture have not been taken into account. Close

• 3. [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