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A 3-year field experiment was conducted to simultaneously measure methane (CH4) and nitrous oxide (N2O) emissions from rice paddies under various agricultural managements including water regime, crop residue incorporation, and synthetic fertilizer application. In contrast with continuous flooding, midseason drainage incurred a drop in CH4 fluxes while triggering substantial N2O emission. Moreover, N2O emissions after midseason drainage depended strongly on whether or not fields were waterlogged due to intermittent irrigation. Urea application tended to reduce CH4 emissions but significantly increased N2O emissions. Under a water regime of flooding-midseason drainage-reflooding-moist intermittent irrigation but without water logging (F-D-F-M), both wheat straw and rapeseed cake incorporation increased CH4 emissions by 25 2%, and rapeseed cake increased N2O by 17% while wheat straw reduced N2O by 19% compared to controls. Seasonal average fluxes of CH4 ranged from 25.4 mg m-2 d-1 when no additional residue was applied under the water regime of flooding-midseason drainage- reflooding to 116.9 mg m-2 d-1 when wheat straw was applied at 2.25 t ha-1 under continuous irrigation flooding. Seasonal average fluxes of N2O varied between 0.03 mg N2O-N m-2 d-1 under continuous flooding and 5.23 mg N2O-N m-2 d-1 under the water regime of F-D-F-M. Both crop residue-induced CH4, ranging from 9 to 15% of the incorporated residue C, and N2O, ranging from 0.01 to 1.78% of the applied N, were dependent on water regime in rice paddies. Estimations of net global warming potentials (GWPs) indicate that water management by flooding with midseason drainage and frequent water logging without the use of organic amendments is an effective option for mitigating the combined climatic impacts from CH4 and N2O in paddy rice production. [ABSTRACT FROM AUTHOR]

Abstract A model developed by the authors (Huang et al. 1998) was further validated against field measurements from various regions of the world and calibrated to estimate methane emission from irrigated rice cultivation of China. On the basis of available information on rice cultivated area, growth duration, grain yield, soil texture and temperature, methane emission from Chinese rice paddies was estimated for 28 rice cultivated provinces in mainland. The calculated daily methane emission rates, on a provincial scale, ranged from 0.15 to 0.86 g m–2 with an average of 0.32 g m–2. Five of the top six locations with higher daily methane emissions are located at a latitude between 28° and 31° N. A total amount of 9.66 Tg (1 Tg = 1012 g) CH4 per year, ranging from 7.19 to 13.62, was estimated to be released from Chinese rice paddy soils. Of the total, 45% is emitted from the single-rice growing season, and 19% and 36% are from the early-rice and the late-rice growing seasons, respectively. Approximately 70% of the total is emitted in the region located at latitude between 25° and 32° N. The emissions from rice fields in Sichuan and Hunan Province were calculated to be 2.85 Tg y–1, accounting for ≈ 30% of the total. Comparisons of the estimated and the observed emission rates show that the estimates were, in general, close to the measurements at most locations. [ABSTRACT FROM AUTHOR]

Abstract Reliable regional or global estimates of methane emissions from flooded rice paddy soils depend on an examination of methodologies by which the current high variability in the estimates might be reduced. One potential way to do this is the development of predictive models. With an understanding of the processes of methane production, oxidation and emission, a semi-empirical model, focused on the contributions of rice plants to the processes and also the influence of environmental factors, was developed to predict methane emission from flooded rice fields. A simplified version of the model was also derived to predict methane emission in a more practical manner. In this study, it was hypothesized that methanogenic substrates are primarily derived from rice plants and added organic matter. Rates of methane production in flooded rice soils are determined by the availability of methanogenic substrates and the influence of environmental factors. Rice growth and development control the fraction of methane emitted. The amount of methane transported from the soil to the atmosphere is determined by the rates of production and the emitted fraction. Model validation against observations from single rice growing seasons in Texas, USA demonstrated that the seasonal variation of methane emission is regulated by rice growth and development. A further validation of the model against measurements from irrigated rice paddy soils in various regions of the world, including Italy, China, Indonesia, Philippines and the United States, suggests that methane emission can be predicted from rice net productivity, cultivar character, soil texture and temperature, and organic matter amendments. [ABSTRACT FROM AUTHOR]

Methane emissions at different rice productivity levels were observed from Texas rice paddy soils during the years 1991--95. Analysis of field measurements showed that seasonal methane emission (E) was strongly dependent on soil, cultivar, and rice grain yield. The relationship can be quantitatively described by E (g m-2) = 0.048 xSI x VI x GY. SI is a soil index to characterize the relative effectof soil texture on emission and is linked with soil sand percentage.VI is a variety index to identify the intervarietal difference in methane emission and is related to the amount of methane emission per unit grain yield. GY is grain yield (g m-2). Constant 0.048was derived from the measurements of 10 cultivars planted in 1993. Computed emission applying the relationship is well matched with measured data. The comparison of computed with measured seasonal methane emission over an 80-day period using a total of 32 data sets yields a correlation coefficient r2 of 0.800. In addition, the ratio of seasonal methane emission to net primary productivity was calculated on a carbon to carbon basis, which produces an average value of 2.8%, ranging from 1.2% to 5.4%. A further investigation indicated that the ratio is soil and variety dependent and can be quantitatively explained by C[CH4]/C[NPP] (%) = 3.21 x SI x VI + 0.12 (r2 = 0.738, n = 32). Under the condition of 30% soil sand, this ratio is (approx.) 3% for the majority of cultivars. [ABSTRACT FROM AUTHOR]

Measurements focused on seasonal contribution of rice productivity to methane emission were made in three experiments conducted in Texas flooded paddy soils during 1994 and 1995 growing seasons. A total of five rice cultivars representing two distinct groups in methane emission were involved. Over a 10-week period after permanent flooding, total seasonal methane emission was positively correlated with rice above-ground biomass (r2 = 0.845, n = 11). A very strong dependence of daily methane emission on above-ground vegetative biomass (r2 = 0.887, n = 93) and on root biomass (r2 = 0.816, n = 33) was also observed. Calculation from three developmentalperiods (vegetative, reproductive and ripening) of rice plant indicated that more than 75% of total seasonal methane was emitted during the last 5-week period in concert with reproductive and ripening stages, while rice biomass production during the same period-amounted to (approx.) 50% of the seasonal total. According to the correlation of cumulative methane emission with above-ground biomass increment between every two-week interval (r2 = 0.490, n = 93, P = 0.000),the carbon released as methane is approximately equivalent to 3% and4.5% of photosynthetically fixed carbon in the biomass for low and high emission cultivars, respectively. A further investigation showed that these fractions are related to plant growth and development. Thecarbon ratio of methane emitted to net photosynthetic production during vegetative, reproductive, and ripening periods averaged 0.9%, 3.6% and 7.9%, respectively, for low emission cultivars, and 2.0%, 5.0% and 8.3%, respectively, for high emission cultivars. Moreover, the ratio was strongly dependent on plant biomass, resulting in r2 values from 0.775 to 0.907. [ABSTRACT FROM AUTHOR]

Outdoor pot experiments with various paddy soils representing five soil types were conducted at Nanjing Agricultural University during the 2000 and 2001 rice-growing seasons. Eighteen soils and ten out of the eighteen soils were involved in the 2000 and the 2001 experiment, respectively. Two treatments were designed as mineral fertilization (MF) and mineral fertilizer + wheat straw incorporation (MF + WS) for the 2001 experiment. Seasonal average rate of CH4 emission from different soils ranged from 1.96 to 11.06 mg m-2 h-1 in the 2000 experiment, and from 0.89 to 5.92 mg m-2 h-1 for the MF treatment in the 2001 experiment, respectively. Incorporation of wheat straw enhanced considerably CH4 emission with an average increment of 7.09 mg m-2 h-1. CH4 emissions from the two-year experiment were negatively correlated to soil available and total copper concentration. A further investigation showed that CH4 emission from the MF treatment was positively related to the dissolved organic carbon (DOC) in the soil (r=0.904, p<0.001), and that the DOC was negatively correlated to the concentrations of available copper (r=-0.844, p<0.01) and total copper (r=-0.833, p<0.01), respectively. Nevertheless, the incorporation of wheat straw did not enhance the soil DOC, and the relationship between CH4 emission and soil DOC was not statistically significant (r=0.470, p<0.20). It was concluded that higher concentration of copper in the soils resulted in lower soil DOC and thus reduced CH4 emission when there was no additional organic matter input. Incorporation of wheat straw did not affect soil DOC and available copper concentration but enhanced CH4 emission. [Copyright &y& Elsevier]