Midseason Drainage Research Articles

Bacterial Communities Responsible for the Decomposition of Rice Straw Compost in a Japanese Rice Paddy Field Estimated by DGGE Analysis of Amplified 16S rDNA and 16S rRNA Fragments

To estimate the succession and phylogenetic composition of the bacterial communities responsible for the decomposition of rice straw compost under flooded conditions during the cultivation period of paddy rice, denaturing gradient gel electrophoresis (DGGE) analyses targeting 16S rDNA and 16S rRNA, followed by sequencing were conducted in a Japanese paddy field. The DGGE bands of the bacterial communities in the rice straw compost were significantly more numerous in the DNA samples than in the RNA samples. Although the band number of the DNA samples was almost constant throughout the period, RNA samples showed fewer DGGE bands after mid-season drainage than before it. Thus, about 81% of the bacteria present in rice straw compost were considered to be metabolically “active” before mid-season drainage and about 62% after it. The changes in the DGGE patterns of bacterial DNA and RNA before and after mid-season drainage, respectively, were also revealed by cluster analysis and principal component analysis of the DGGE patterns. These results indicated that the bacterial communities of rice straw compost incorporated into flooded paddy fields changed gradually along with the decomposition, except for the period of mid-season drainage, but that they were influenced by mid-season drainage. Members of β-, γ- and δ-Proteobacteria, Cytophaga-Flavobacterium-Bacteroides (CFB) group, Chlorobia, Verrucomicrobia, Chloroflexi, Spirochaetes, Firmicutes (clostridia) and Actinobacteria were present during the decomposition of rice straw compost. Characteristic “active” bacteria among them were as follows: Clostridium, Acinetobacter (γ-Proteobacteria) and β-Proteobacteria before mid-season drainage, Flavobacterium, Chondromyces, Chlorflexi and δ-Proteobacteria after mid-season drainage, and Spirochaeta and myxobacteria throughout the period.

A 3‐year field measurement of methane and nitrous oxide emissions from rice paddies in China: Effects of water regime, crop residue, and fertilizer application

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 252%, 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.

The effect of water management based on soil redox potential on methane emission from two kinds of paddy soils in Japan

Abstract Paddy fields are major sources of the global methane (CH4) emission. Water management of paddy fields affects rice yield and CH4 emission. There is need for improvement in the management to achieve a balance between them. We suggest the water management based on soil redox potential (Eh), which was named Eh control. The effects of Eh control on rice yield and CH4 emission, and applicability to field conditions were considered by pot experiments. This study was carried out at a plastic film house in the Agricultural and Forestry Research Center, University of Tsukuba, Japan, in 2002. The effects of Eh control (EH) were compared with conventional management methods (continuous flooding [CF], midseason drainage [MD], and MD plus intermittent irrigation [IM]) for two kinds of soils. The drainage for Eh control, aim of which was not to reach −200 mV, was carried out when the soil Eh decreased to −150 mV. Then one or two days later, the soil was reflooded. The CH4 emissions of MD, IM, and EH were 64, 26, and 17% of CF, respectively. However, the order of brown rice yield was as follows: IM > MD > CF > EH. With the drainage for Eh control, soil Eh increased to higher than 400 mV several times, and this condition was maintained for up to one day. This excessive drying of the soil affected rice growth and yield of EH. To apply Eh control to field conditions, the upper limit of soil Eh must be controlled to avoid the excessive drying. Moreover, water demand of rice plants is necessary to be considered.

Mitigation of methane and nitrous oxide emissions from drained irrigated rice fields

One of the important cultural practices that affect methane and nitrous oxide emissions from tropical rice plantations is the water drainage system. While drainage can reduce methane emissions, it can also increase nitrous oxide emissions, as well as reduce yields. In this experiment, four different water drainage systems were compared in a rice field in central Thailand including: (1) continuous flooding, (2) mid-season drainage, (3) multiple drainage and (4) a local method (drainage was done according to local cultural practice) in order to find a system of drainage that would optimize yields while simultaneously limiting methane and nitrous oxide emissions. Methane and nitrous oxide emission were observed and compared with rice yield and physical changes of rice plants. It was found that drainage during the flowering period could reduce methane emission. Interestingly, nitrous oxide emission was related to number of drain days rather than the frequency of draining. Fewer drain days can help reduce nitrous oxide emission. The mid-season drainage and the multiple drainage, with 6.9% and 11.4% reduction in rice yield, respectively, had an average methane emission per crop 27% and 35% lower when compared to the local method. Draining with fewer drain days during the flowering period was recommended as a compromise between emissions and yield. The field drainage can be used as an option to reduce methane and nitrous oxide emissions from rice fields with acceptable yield reduction. Mid-season drainage during the rice flowering period, with a shortened drainage period (3 days), is suggested as a compromise between the need to reduce global warming and current socio-economic realities.

Direct N2O emissions from rice paddy fields: Summary of available data

Rice cultivation is an important anthropogenic source of atmospheric nitrous oxide (N2O) and methane. We compiled and analyzed data on N2O emissions from rice fields (113 measurements from 17 sites) reported in peer‐reviewed journals. Mean N2O emission ± standard deviation and mean fertilizer‐induced emission factor during the rice‐cropping season were, respectively, 341 ± 474 g N ha−1 season−1 and 0.22 ± 0.24% for fertilized fields continuously flooded, 993 ± 1075 g N ha−1 season−1 and 0.37 ± 0.35% for fertilized fields with midseason drainage, and 667 ± 885 g N ha−1 season−1 and 0.31 ± 0.31% for all water regimes. The estimated whole‐year background emission was 1820 g N ha−1 yr−1. A large uncertainty remains, especially for background emission because of limited data availability. Although midseason drainage generally reduces CH4 and increases N2O emissions, it may be an effective option for mitigating the net global warming potential of rice fields.

Effect of Irrigation Regimes, mid season drainage and time of Application of Nitrogen on Hybrid Rice

Field experiments were conducted during kharif seasons of 1999-2000 and 20002001 to study the effect of irrigation, drainage and nitrogen management on CoRH 2 Hybrid rice at Agricultural college and Research Institute, Madurai. The results revealed that irrigation to 5cm depth one day after disappearance of ponded water with mid season drainage and application of N in four splits viz., 16.7% at 10 DAT, 33.3% at AT, 33.3% at PI and 16.7% at Heading registered the higher growth attributes, yield attributes, grain yields, WUE and economics. The above treatment combinations registered the maximum grain yield of 7533 and 8078 kg ha -1 during 1999-2000 and 20002001 respectively.

Effect of irrigation regimes, mid-season drainage and time of application of nitrogen on growth and yield of hybrid rice

Field experiments were conducted at the Central Farm of the Agricultural College and Research Institute, Madurai, India during the rabi (October-January) seasons of 1999-2000 and 2000-2001 in a split plot design with three replications. The soil of the experiments was sandy clay loam with a neutral reaction. The main plot consisted of six irrigation schedules with mid-season drainage, while four N splits were taken as sub- plots. The experimental results revealed that irrigation to a depth of 5 cm one day after the disappearance of ponded water and mid-season drainage, along with N applied in four splits, with 16.7% at 10 days after transplanting, 33.3% at active tillering, 33.3% at panicle initiation and 16.7% at the heading stage, produced significantly higher growth, yield attributes, grain and straw yields in hybrid rice. A combination of the above treatments led to maximum grain yields of 7533 and 8078 kg ha-1 (45.53 and 45.86% in excess of the control) in 1999-2000 and 2000-2001, respectively.

Modeling greenhouse gas emissions from rice‐based production systems: Sensitivity and upscaling

A biogeochemical model, Denitrification‐Decomposition (DNDC), was modified to enhance its capacity of predicting greenhouse gas (GHG) emissions from paddy rice ecosystems. The major modifications focused on simulations of anaerobic biogeochemistry and rice growth as well as parameterization of paddy rice management. The new model was tested for its sensitivities to management alternatives and variations in natural conditions including weather and soil properties. The test results indicated that (1) varying management practices could substantially affect carbon dioxide (CO2), methane (CH4), or nitrous oxide (N2O) emissions from rice paddies; (2) soil properties affected the impacts of management alternatives on GHG emissions; and (3) the most sensitive management practices or soil factors varied for different GHGs. For estimating GHG emissions under certain management conditions at regional scale, the spatial heterogeneity of soil properties (e.g., texture, SOC content, pH) are the major source of uncertainty. An approach, the most sensitive factor (MSF) method, was developed for DNDC to bring the uncertainty under control. According to the approach, DNDC was run twice for each grid cell with the maximum and minimum values of the most sensitive soil factors commonly observed in the grid cell. The simulated two fluxes formed a range, which was wide enough to include the “real” flux from the grid cell with a high probability. This approach was verified against a traditional statistical approach, the Monte Carlo analysis, for three selected counties or provinces in China, Thailand, and United States. Comparison between the results from the two methods indicated that 61‐99% of the Monte Carlo‐produced GHG fluxes were located within the MSA‐produced flux ranges. The result implies that the MSF method is feasible and reliable to quantify the uncertainties produced in the upscaling processes. Equipped with the MSF method, DNDC modeled emissions of CO2, CH4, and N2O from all of the rice paddies in China with two different water management practices, i.e., continuous flooding and midseason drainage, which were the dominant practices before 1980 and in 2000, respectively. The modeled results indicated that total CH4 flux from the simulated 30 million ha of Chinese rice fields ranged from 6.4 to 12.0 Tg CH4‐C per year under the continuous flooding conditions. With the midseason drainage scenario, the national CH4 flux from rice agriculture reduced to 1.7–7.9 Tg CH4‐C. It implied that the water management change in China reduced CH4 fluxes by 4.2–4.7 Tg CH4‐C per year. Shifting the water management from continuous flooding to midseason drainage increased N2O fluxes by 0.13–0.20 Tg N2O‐N/yr, although CO2 fluxes were only slightly altered. Since N2O possesses a radiative forcing more than 10 times higher than CH4, the increase in N2O offset about 65% of the benefit gained by the decrease in CH4 emissions.

Microbial communities responsible for the decomposition of rice straw compost in a Japanese rice paddy field determined by phospholipid fatty acid (PLFA) analysis

To identify the microbial communities responsible for the decomposition of rice straw compost in soil during the rice cultivation period, phospholipid fatty acid (PLFA) composition of rice straw compost was determined by periodically sampling the compost from a Japanese rice field under flooded conditions. About 21% of the compost was decomposed within a period of 3 months. The total amount of PLFAs, as an indicator of microbial biomass, was significantly lower under drained conditions than under flooded conditions and was relatively constant during the flooding period. This indicates that the microbial biomass in the compost samples did not increase during the gradual decomposition of rice straw compost under flooded conditions. The proportion of branched-chain PLFAs (biomarker of Grampositive and anaerobic Gram-negative bacteria) slightly decreased during the early period after placement, and increased gradually afterwards. Among the branched-chain PLFAs, i15:0, ail5:0, i16:0 and i17:0 PLFAs predominated and their proportions increased gradually except for i16:0. The proportion of straight mono-unsaturated PLFAs (biomarker of Gramnegative bacteria) was almost constant throughout the period, and 18:1ω9 and 18:1ω7 PLFAs predominated. The proportion of straight poly-unsaturated PLFAs as a biomarker of eukaryotes including fungi was also constant throughout the period, except for a decrease under drained conditions. Straight poly-unsaturated PLFAs consisted mainly of 18:2ω6c PLFA. Therefore, these results suggest that the proportions of Gram-positive and anaerobic Gram-negative bacteria increased during the decomposition of rice straw compost in flooded paddy field. Statistical analyses enabled to divide PLFA patterns of microbiota in the rice straw compost into two groups, one group consisting of rice straw compost samples collected before mid-season drainage and the other of samples collected after mid-season drainage. Small squared distances among samples in cluster analysis indicated that the community structure of microbiota was similar to each other as a whole. These results suggest that the microbial communities changed gradually during the period of placement, and that mid-season drainage may have affected the community structure of microbiota. Principal component analysis of the PLFA composition suggested that the succession of microbiota along with the decomposition in flooded soil was similar between rice straw compost and rice straw and that the changes in the community structure during the decomposition in flooded soil were more conspicuous for rice straw than for rice straw compost.

Changes in the community structure of aquatic organisms after midseason drainage in the floodwater of Japanese paddy fields

The purpose of this study was to analyze the seasonal variations in diversity and community structure of aquatic organisms in paddy fields. The varieties and the number of aquatic organisms ranging in size from 30 mm to 2 cm in the floodwater of five Japanese paddy fields were surveyed every 10 d during the growth period of rice plant. Aquatic organisms were classified mainly at the order level. As a result, forty four varieties (classified groups) of aquatic organisms were detected in the present survey. The number of varieties significantly increased and the community structure of the aquatic organisms changed after midseason drainage. Principal component analysis showed that three varieties (Volvocida, Oligotrichida, Cladocera) and four varieties (Lemnaceae, Pharyngophorida, Chaetonotida, Turbellaria) characterized the community structure of aquatic organisms before and after midseason drainage, respectively.

Control of Grain Number by Midseason Drainage and Nitrogen Topdressing at the Panicle Formation Stage of Rice Cultivar "Haenuki" Effected by Air-Dried Paddy Soil before Submergence

Control of Grain Number by Midseason Drainage and Nitrogen Topdressing at the Panicle Formation Stage of Rice Cultivar "Haenuki" Effected by Air-Dried Paddy Soil before Submergence

Reduced methane emissions from large‐scale changes in water management of China's rice paddies during 1980–2000

Decreased methane emissions from paddy rice may have contributed to the decline in the rate of increase of global atmospheric methane (CH4) concentration over the last 20 years. In China, midseason paddy drainage, which reduces growing season CH4 fluxes, was first implemented in the early 1980s, and has gradually replaced continuous flooding in much of the paddy area. We constructed a regional prediction for China's rice paddy methane emissions using the DNDC biogeochemical model. Results of continuous flooding and midseason drainage simulations for all paddy fields in China were combined with regional scenarios for the timing of the transition from continuous flooding to predominantly mid‐season drainage to generate estimates of total methane flux for 1980–2000. CH4 emissions from China's paddy fields were reduced over that period by ∼5 Tg CH4 yr−1.

ALCOA AND LANDSVIRKJUN SIGN MOU ON NEW SMELTER

Elevated greenhouse gas (GHG) emissions, particularly of methane (CH4) from flooded rice production systems contribute to global warming. Different crop management strategies, such as drainage of paddy soils and climate-smart residue management, are essential in order to mitigate GHG emissions from flooded rice systems, but they often conflict with practical management preferences.The aim of this study was to assess the potential of early-season drainage for mitigating CH4 and N2O emissions from soils with and without added organic amendments in relation to native soil organic carbon (SOC). Rice plants were grown in pots under controlled conditions in a growth chamber with different treatments in a 2 × 2 × 3 factorial design. The treatments included an arable soil with two different carbon levels: 1.4% (low carbon, [L]) and 2.2% (high carbon [H]); two water regimes: midseason drainage (M) and early plus midseason drainage (EM); and three nutrient treatments: one inorganic control (nitrogen fertiliser only [N]), and two organic: maize straw + N fertiliser (S) and maize compost + N fertiliser (C). An equal amount of mineral N fertiliser was applied in all treatments. Straw and compost were applied to the soils on the basis of an equivalent amount of C added in each organic treatment.The results revealed rapid mineralization of organic C in the double-drained system, resulting in lower total CH4 emissions in treatments under early plus midseason drainage compared to those under midseason drainage only. Total CH4 emissions were reduced by 89% and 92% in the S + EM treatments in low C soils and high C soils respectively, as compared to S + M. The drainage effects on CH4 emissions from compost amendments were only significant in the low C soil, with a 61% reduction in EM compared to M drainage. N2O emissions from non-organic treatments in EM were 87% higher than in M under low C soils. The concentrations of dissolved organic carbon (DOC) were higher in organic treatments and decreased by the end of growth period. This experiment demonstrated an interaction between water and straw management to achieve both sustainable soil quality and low-emission rice production.

Seasonal variations in the community structure of aquatic organisms in a paddy field under a long-term fertilizer trial

To estimate the effect of fertilizers on aquatic organisms ranging in size from 30 µm to 2 cm in a Japanese paddy field under a long-term fertilizer trial (the plots without fertilizers, with chemical fertilizers only, and with chemical fertilizers as well as two levels (7.5 and 22.5 tons ha-1) of rice straw compost), we collected the floodwater samples once every 10 d during the cropping season throughout 3 y and mainly classified aquatic organisms in the floodwater at the order level. The frequency of the presence of Chlorococcales, Lemanaceae, Euglenida, Oligotrichida, Pharyngophorida, Sessilida, Turbellaria, Podocopida, and Cyclopoida was significantly different among the plots, while the maximum population density was significantly different for Cladocera and Cyclopoida among the plots. The effects of the sampling year on the frequency of presence and the maximum population density were also observed. The floodwater samples after midseason drainage in three plots with fertilizer application differed from the samples in the plot without fertilizer application in the community structure of aquatic organisms. Principal component analysis showed that Lemanaceae, Pharyngophorida, and Turbellaria were important components in the floodwater samples after midseason drainage in the three plots with fertilizer application. Although Cladocera appeared more abundantly after midseason drainage in the plot without fertilizers and the plot with chemical fertilizers only, it appeared more abundantly before midseason drainage in the two plots with composts. Shannon-Weaver's index of diversity (H′) was higher in the three plots with fertilizers than in the plot without fertilizers. The effect of fertilizer application on the community structure and the diversity of aquatic organisms was more conspicuous than that of the sampling year.

平野部における畦畔除去による区画拡大水田の作業性と収量性

The working efficiency and rice yield of large-scale paddies were investigated in a flat plane in Niigata. Large-scale paddies (40 a and 60 a) were created by removal of levees, without further mechanical land consolidation, and cv. Koshihikari was produced from May to September in 1998 and 1999. The newly created large-scale paddies were flat, as the standard deviation (SD) of ground height in the original paddies was within the standard level set by Ministry of Agriculture, Forestry and Fisheries. Transplantation of rice seedlings and irrigation were performed smoothly. The efficiency of human labor such as fertilizer application decreased in the large-scale paddies. On the other hand, the efficiency of mechanical work such as tillage, pudding and harvest increased and fuel consumption decreased clearly in the large-scale paddies. The direction in which the drain was made in the paddy did not affect the efficiency of irrigation during midseason drainage. There was not a significant difference in the rice yield among the 20 a, 40 a and 60 a-paddies.

Comparison of phospholipid fatty acid composition in floodwater and plow layer soil during the rice cultivation period in a Japanese paddy field

The bacterial numbers and the composition of phospholipid fatty acids (PLF As) in the floodwater and the plow layer soil in a Japanese paddy field were compared during the rice cultivation period. The numbers of total and culturable bacteria ranged from 8.41×105 to 1.72×106 cells mL−1 and 1.47×104 to 8.49 × 104 CFU mL−1 in the floodwater, and 1.49×109 to 4.31×109 cells gy−1 and 1.26 × 107 to 3.89 × 107 CFU g−1 in the plow layer soil, respectively. There was no appreciable seasonal variation in the number of bacteria either in the floodwater or in the plow layer soil. The amount of PLF As in the plow layer soil ranged from 25.8 to 55.5 μg g−1 soil. The amount of PLFAs in the plow layer soil tended to increase just after midseason drainage. Dominant PLF As in the floodwater were 16 : 1ω7c, 16 : 0, and 18 : 1ω7, and those in the plow layer soil were 16 : 0,16 : 1ω7c, 18 : 1ω7,18 : 1ω9, il5 : 0, ail5 : 0, and 10Me17 : 0, respectively. The proportion of straight, mono-unsaturated PLFAs and branched PLFAs ranged from 33 to 48% and from 9 to 21% in the floodwater and from 28 to 33% and from 30 to 44% in the plow layer soil, respectively, suggesting the predominance of Gram-negative bacteria in the floodwater and Gram-positive bacteria in the plow layer soil. Principal component analysis of PLFA composition clearly revealed the difference in microbiota between the floodwater and plow layer soil. An indicator of the environmental stress imposed upon microbiota expressed by the trans vs. cis ratio of 16 : 1ω7 was consistently lower in the floodwater than in the plow layer soil.

Seasonal variations in vertical distribution of methanogenic activity and Fe(II) content and relationship between them in wetland rice field soil

Methanogenic activity and Fe(II) content in soil samples collected at different depths of the plow layer in a wetland rice field were determined over a period of 1 y. The methanogenic activity increased at first in the top layer of soil (0–1 cm in depth) during the continuous flooding period of the field and the activity decreased after the period of midseason drainage. In contrast, in the deeper soil layers, the increase in the activity persisted during the period of intermittent irrigation after midseason drainage and the period of the highest methanogenic activity at each depth in the year varied for about 1 month depending on the depth. The Fe(II) content in soil did not change appreciably during the continuous flooding period regardless of the soil depth. However, after midseason drainage, the content significantly varied with depth. Thus, until the midseason drainage, a sharp increase in the methanogenic activity preceded the increase in the Fe(II) content in the surface (0–1 cm) soil layer, while the opposite was observed in the deeper soil layers. Based on these data, the relationship between the methanogenic activity and Fe(II) content in soil was investigated by calculating the correlation coefficients between them and comparing the slopes and constants of regression lines obtained. It was found that although during the cropping season of rice the methanogenic activity depended mainly on the Fe(II) content in soil as a whole, the relationship between the two parameters was different before and after the period of midseason drainage. The relationship between them before the midseason drainage was distinctly different depending on the depth, while after the midseason drainage the relationship was almost the same irrespective of the depth. During the non-cropping season, no correlation was obtained between the two parameters. Most of the correlation coefficients between the methanogenic activity and the soil temperature were low for all the data sets examined except for the deepest soil layer (13–15 cm) during the cropping season. Correlation coefficients between the methanogenic activity and water content in soil were also low.

Methane Emissions and Mitigation Options in Irrigated Rice Fields in Southeast China

Methane (CH4) emissions from rice fields were monitored in Hangzhou, China, from 1995 to 1998 by an automatic measurement system based on the "closed chamber technique." The impacts of water management, organic inputs, and cultivars on CH4 emission were evaluated. Under the local crop management system, seasonal emissions ranging from 53 to 557 kg CH4 ha−1 were observed with an average value of 182 kg CH4 ha−1. Methane emission patterns differed among rice seasons and were generally governed by temperature changes. Emissions showed an increasing trend in early rice and a decreasing trend in late rice. In a single rice field, CH4 emissions increased during the first half of the growing period and decreased during the second half. Drainage was a major modifier of seasonal CH4 emission pattern. The local practice of midseason drainage reduced CH4 emissions by 44% as compared with continuous flooding; CH4 emissions could further be reduced by intermittent irrigation, yielding a 30% reduction as compared with midseason drainage. The incorporation of organic amendments promoted CH4 emission, but the amount of emission varied with the type of organic material and application method. Methane emission from fields where biogas residue was applied was 10–16% lower than those given the same quantity (based on N content) of pig manure. Rice straw applied before the winter fallow period reduced CH4 emission by 11% as compared with that obtained from fields to which the same amount of rice straw was applied during field preparation. Broadcasting of straw instead of incorporation into the soil showed less emission (by 12%). Cultivar selection influenced CH4 emission, but the differences were smaller than those among organic treatments and water regimes. Modifications in water regime and organic inputs were identified as promising mitigation options in southeast China.

A Four-Year Record of Methane Emissions from Irrigated Rice Fields in the Beijing Region of China

Methane (CH4) emissions from irrigated rice fields were measured using an automatic sampling-measuring system with a closed chamber method in 1995-98. Average emission rates ranged from 11 to 364 mg m-2 d-1 depending on season, water regime, and fertilizer application. Crop management typical for this region (i.e., midseason drainage and organic/mineral fertilizer application) resulted in emission of 279 and 139 mg CH4 m-2d-1 in 1995 and 1997, respectively. This roughly corresponds to emissions observed in other rice-growing areas of China. Emissions were very intense during the tittering stage, which accounted for 85% of total annual emission, but these were suppressed by low temperature in the late stage of the season. The local irrigation practice of drying at mid-season reduced emission rates by 23%, as compared with continuous flooding. Further reduction of CH4 emissionscould be attained by (1) alternate flooding/drying, (2) shifting the drainage period to an earlier stage, or (3) splitting drainage into two phases (of which one is in an earlier stage). Emission rates were extremely sensitive to organic amendments: seasonal emissions from fields treated with pig manure were 15–35 times higher than those treated with ammonium sulfate in the corresponding season. On the basis of identical carbon inputs, CH4 emission potential varied among organic amendments. Rice straw had higher emissions than cattle manure but lower emissions than pig manure. Use of cultivar Zhongzhuo (modern japonica) reduced CH4 emission by 56% and 50%, in 1995 and 1997, respectively, as compared with Jingyou (japonica hybrid) and Zhonghua (tall japonica). The results give evidence that CH4 emissions from rice fields in northern China can be reduced by a package of crop management options without affecting yields.

Methane emission from irrigated and intensively managed rice fields in Central Luzon (Philippines)

Methane (CH4) emissions were measured with an automated system in Central Luzon, the major rice producing area of the Philippines. Emission records covered nine consecutive seasons from 1994 to 1998 and showed a distinct seasonal pattern: an early flush of CH4 before transplanting, an increasing mend in emission rates reaching maximum toward grain ripening, and a second flush after water is withdrawn prior to harvesting. The local practice of crop management, which consists of continuous flooding and urea application, resulted in 79-184 mg CH4 m’ d-’ in the dry season (DS) and 269–503 mg CH4 m-2 d-1 in the wet season (WS). The higher emissions in the WS may be attributed to more labile carbon accumulation during the dry fallow period before the WS cropping as shown by higher % organic C. incorporation of sulfate into the soil reduced CH4 emission rates. The use of ammonium sulfate as N fertilizer in place of urea resulted in a 25-36% reduction in CH4 emissions. Phosphogypsum reduced CH4 emissions by 72% when applied in combination with urea fertilizer. Midseason drainage reduced CH4 , emission by 43%, which can be explained by the influx of oxygen into the soil. The practice of direct seeding instead of transplanting resulted in a 16–54ik reduction in CH4 emission, but the mechanisms for the reducing effect are not clear. Addition of rice straw compost increased CH4 emission by only 23-30% as compared with the 162-250% increase in emissions with the use of fresh rice straw. Chicken manure combined with urea did not increase CH4 emission. Fresh rice straw has wider C/N (25 to 45) while rice straw compost has C/N = 6 to 10 and chicken manure has C/N = 5 (o 8. Modifications in inorganic and organic fertilizer management and water regime did not adversely affect grain yield and are therefore potential mitigation options. Direct seeding has a lower yield potential than transplanting but is getting increasingly popular among farmers due to labor savings. Combined with a package of technologies, CH4 emission can best be reduced by (1)the practice of midseason drainage instead of continuous flooding,(2) the use of sulfate-containing fertilizers such as ammonium sulfate and phosphogypsum combined with urea; (3) direct seeding crop establishment; and (4) use of low C/N organic fertilizer such as chicken manure and rice straw compost.