Effect of soil water contents in the non-rice growth season on CH4 emission during the following rice-growing period

A pot experiment was carried out to investigate the effect of soil water content in the non-rice growth season (winter season) on CH₄ emission during the following rice-growing period. The results showed that CH₄ fluxes increased significantly with the increase of soil water content in the winter season, except air-dry water condition. The mean CH₄ fluxes of treatments with soil water contents in the winter of 3.89–5.37% (air-dry), 25–35%, 50–60%, 75–85% and 107% (flooded) of field water capacity (FWC) were 13.04, 4.04, 8.61, 13.26 and 20.47 mg m^–2 h^–1, respectively. Antecedent soil water contents also markedly affected temporal variation patterns of CH₄ fluxes and soil redox potential (E_h) during the rice-growing period. The higher soil water contents in the winter season were, the quicker soil E_h decreased, and the earlier CH₄ emission occurred after rice transplanting, except air-dry water condition. Though the seasonal mean CH₄ flux was significantly correlated with the seasonal mean soil E_h, the seasonal variation of CH₄ fluxes was not always significantly correlated with soil E_h. For the treatment flooded in the fallow season, there was no significant correlation between CH₄ flux and soil E_h, but there was significant correlation between CH₄ flux and soil temperature during rice growth season. In contrast, for the other four treatments, it was soil E_h, not soil temperature that significantly affected the temporal variation of CH₄ emissions. Soil water contents in the fallow season significantly influenced concentrations of soil labile organic carbon (including undecomposed plant debris), active Fe and Mn immediately before rice transplanting. The mean CH₄ fluxes during rice-growing period were significantly correlated with soil labile organic carbon contents (positively) and contents of soil active Fe and Mn (negatively).     

Effect of drainage in the fallow season on reduction of CH<Subscript>4</Subscript> production and emission from permanently flooded rice fields

Field and incubation experiments were conducted during 2007–2009 to study the effect of drainage in the fallow season on CH₄ production and emission from permanently flooded rice fields. It was found that drainage in the fallow season significantly affected the temporal variations of CH₄ production and emission from permanently flooded rice fields. CH₄ production and emission from permanently flooded rice fields (Treatment FF) mainly occurred during the rice season, where they were found to be much lower in the late fallow season. No CH₄ flux was detected from drained fields (Treatment DF) in the fallow season. Compared with Treatment FF, Treatment DF was delayed not only its onset of CH₄ production and emission, but also appearance of the highest peak of CH₄ production during the rice season. A significant positive relationship was observed between CH₄ production rates of paddy soil and corresponding CH₄ fluxes (P < 0.01). CH₄ production in rice roots was the highest in rate at the rice booting stage, but was obviously lower at the rice tillering, grain filling and ripening stages, and the highest value reached at the same time as the peak of CH₄ production occurred in the paddy soil. Drainage in the fallow season significantly decreased CH₄ production and emission from Treatment FF. Compared with Treatment FF, Treatment DF was about 42–61% lower in mean CH₄ production rate in the paddy soil during the rice season, and was reduced by approximately 56% in mean CH₄ production rate in rice roots. Accordingly, Treatment DF was 20.6–30.2 g CH₄ m⁻², 39–52% lower than Treatment FF in total CH₄ emission during the rice season, and 44–57% lower in annual total CH₄ emission. Rice yield in Treatment DF tended to be 4–7% lower than that in Treatment FF.     

Using a Crop/Soil Simulation Model and GIS Techniques to Assess Methane Emissions from Rice Fields in Asia. IV. Upscaling to National Levels

The process-based crop/soil model MERES (Methane Emissions from Rice EcoSystems) was used together with daily weather data, spatial soil data, and rice-growing statistics to estimate the annual methane (CH₄) emissions from China, India, Indonesia, Philippines, and Thailand under various crop management scenarios. Four crop management scenarios were considered: (a) a 'baseline' scenario assuming no addition of organic amendments or field drainage during the growing season, (b) addition of 3,000 kg DM ha^–1 of green manure at the start of the season but no field drainage, (c) no organic amendments but drainage of the field for a 14-d period in the middle of the season and again at the end of the season, and (d) addition of 3,000 kg DM ha^–1 of green manure and field drainage in the middle and end of the season. For each scenario, simulations were made at each location for irrigated and rainfed rice ecosystems in the main rice-growing season, and for irrigated rice in the second (or 'dry') season. Overall annual emissions (Tg CH₄ yr^–1) for a province/district were calculated by multiplying the rates of CH₄ emission (kg CH₄ ha^–1 yr^–1) by the area of rice grown in each ecosystem and in each season obtained from the Huke and Huke (1997) database of rice production. Using the baseline scenario, annual CH₄ emissions for China, India, Indonesia, Philippines, and Thailand were calculated to be 3.73, 2.14, 1.65, 0.14, and 0.18 Tg CH₄ yr^–1, respectively. Addition of 3,000 kg DM ha^–1 green manure at the start of the season increased emissions by an average of 128% across the five countries, with a range of 74–259%. Drainage of the field in the middle and at the end of the season reduced emissions by an average of 13% across the five countries, with a range of –10% to –39%. The combination of organic amendments and field drainage resulted in an increase in emissions by an average of 86% across the five countries, with a range of 15–176%. The sum of CH₄ emissions from these five countries, comprising about 70% of the global rice area, ranged from 6.49 to 17.42 Tg CH₄ yr^–1, depending on the crop management scenario.     

Simultaneous Records of Methane and Nitrous Oxide Emissions in Rice-Based Cropping Systems under Rainfed Conditions

Rainfed rice (Oryza sativa L.)-based cropping systems are characterized by alternate wetting and drying cycles as monsoonal rains come and go. The potential for accumulation and denitrification of NO₃ ^– is high in these systems as is the production and emission of CH₄ during the monsoon rice season. Simultaneous measurements of CH₄ and N₂O emissions using automated closed chamber methods have been reported in irrigated rice fields but not in rainfed rice systems. In this field study at the International Rice Research Institute, Philippines, simultaneous and continuous measurements of CH₄ and N₂O were made from the 1994 wet season to the 1996 dry season. During the rice-growing seasons, CH₄ fluxes were observed, with the highest emissions being in organic residue-amended plots. Nitrous oxide fluxes, on the other hand, were generally nonexistent, except after fertilization events where low N₂O fluxes were observed. Slow-release N fertilizer further reduced the already low N₂O emissions compared with prilled urea in the first rice season. During the dry seasons, when the field was planted to the upland crops cowpea [Vigna unguiculata (L.) Walp] and wheat (Triticum aestivum L.), positive CH₄ fluxes were low and insignificant except after the imposition of a permanent flood where high CH₄ fluxes appeared. Evidences of CH₄ uptake were apparent in the first dry season, especially in cowpea plots, indicating that rainfed lowland rice soils can act as sink for CH₄ during the upland crop cycle. Large N₂O fluxes were observed shortly after rainfall events due to denitrification of accumulated NO₃ ^–. Cumulative CH₄ and N₂O fluxes observed during this study in rainfed conditions were lower compared with previous studies on irrigated rice fields.     

Characterization of Methane Emissions from Rice Fields in Asia. II. Differences among Irrigated, Rainfed, and Deepwater Rice

Methane (CH₄) emission rates were recorded automatically using the closed chamber technique in major rice-growing areas of Southeast Asia. The three experimental sites covered different ecosystems of wetland rice--irrigated, rainfed, and deepwater rice--using only mineral fertilizers (for this comparison). In Jakenan (Indonesia), the local water regime in rainfed rice encompassed a gradual increase (wet season) and a gradual decrease (dry season) in floodwater levels. Emission rates accumulated to 52 and 91 kg CH₄ ha^–1 season^–1 corresponding to approximately 40% of emissions from irrigated rice in each season. Distinct drainage periods within the season can drastically reduce CH₄ emissions to less than 30 kg CH₄ ha^–1 season^–1 as shown in Los Baños (Philippines). The reduction effect of this water regime as compared with irrigated rice varied from 20% to 80% from season to season. Methane fluxes from deepwater rice in Prachinburi (Thailand) were lower than from irrigated rice but accumulated to equally high seasonal values, i.e., about 99 kg CH₄ ha^–1 season^–1, due to longer seasons and assured periods of flooding. Rice ecosystems with continuous flooding were characterized by anaerobic conditions in the soil. These conditions commonly found in irrigated and deepwater rice favored CH₄ emissions. Temporary aeration of flooded rice soils, which is generic in rainfed rice, reduced emission rates due to low CH₄ production and high CH₄ oxidation. Based on these findings and the global distribution of rice area, irrigated rice accounts globally for 70–80% of CH₄ from the global rice area. Rainfed rice (about 15%) and deepwater rice (about 10%) have much lower shares. In turn, irrigated rice represents the most promising target for mitigation strategies. Proper water management could reduce CH₄ emission without affecting yields.     

Methane and Nitrous Oxide Emissions from Rice Field and Related Microorganism in Black Soil, Northeastern China

Methane (CH₄) and nitrous oxide (N₂O) emissions from rice field in black soil were measured in situ by using static chamber techniques during crop growth season in 2001. The experiment fields were divided into three plots for three different treatments, one with continuous flooded and applying urea (CU), one with continuous flooded and applying slow-releasing urea (CS), and one with intermittent irrigation and applying urea (IU). Under the same fertilization application, compared with continuous flooded, intermittent irrigation can significantly reduce CH₄ emission and increase N₂O emission. But, integrated global warming potentials (GWP_S) of CH₄ and N₂O emission were reduced greatly, while rice yield was not affected. So, the intermittent irrigation is an effective measure to reduce greenhouse gas emissions from paddy fields. The amount of CH₄ emission during rice-growing season for the three treatments was all much lower than that from any other region in China. There was a trade-off relationship between CH₄ and N₂O emissions. We also measured the numbers of methanogens, methanotrophs, nitrifiers and denitrifers from rice field at various growth stages in 2001. Bacteria populations were estimated by the most probable number (MPN) method. Regression analyses show CH₄ emissions were closely related to methanogens population for all the three treatments. There was a positive correlation between denitrifiers population level and N₂O emission in the treatment of IU.     

Importance of water regime during the non-rice growing period in winter in regional variation of CH4 emissions from rice fields during following rice growing period in China

Rice fields are either continuously flooded or drained in China in the winter (non-rice growth season). Due to great spatial variation of precipitation and temperature, there is a spatial variation of soil moisture in the fields under drained conditions during the winter season. The effect of water regime in winter on CH₄ emissions during the following rice growing period and their regional variation were investigated. Soil moisture in the winter was simulated by DNDC model with daily precipitation and temperature as model inputs. Under the same management during the rice growing period, CH₄ emissions was higher from rice fields flooded, compared to those from fields drained during winter. CH₄ emission from rice fields correlated significantly with simulated soil moisture and with mean precipitation of the preceding winter season. Spatial variation of precipitation in winter and corresponding variations of soil moisture regimes control the regional and annual variation of CH₄ emissions from rice fields in China. Keeping soils drained as much as possible during winter seems to be a feasible option to reduce CH₄ emissions during the following rice growing seasons.     

Overwinter greenhouse gas fluxes in two contrasting agricultural habitats

Mid-day field fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) were measured during late winter/early spring in an arable field and an adjacent fallow in southern Germany. On the arable field, 2 dm high ridges, drawn as seed-beds for potato, were exposed to mild, partly diurnal freezing–thawing. Substantially elevated N₂O emission rates (6–750 µg N₂O-N m^–2 h^–1) were observed throughout the investigation period which coincided with freezing–thawing events in the surface soil (0–5 cm). Soil temperatures in the densely vegetated fallow were more isothermal due to an insulating snow/ice cover, resulting in much lower N₂O emission rates (0–57 µg N₂O-N m^–2 h^–1). CH₄ uptake rates were low in both habitats during soil frost (+2 to –7.5 µg CH₄-C m^–2 h^–1) but increased markedly in the fallow after spring thaw. Our data suggest that N₂O emission peaks may occur recurrently throughout the winter when soils are subjected to diurnal surface thawing. We concluded that microclimatic conditions strongly control N₂O winter loss, thus overriding ecosystem-level differences in off-season nutrient cycling. To further characterize winter-time nutrient cycling and habitat functioning in our sites, we determined NO₃ ^– and NH₄ ⁺ contents, fumigation-extractable carbon (C_mic) and nitrogen (N_mic) and enumerated protozoa and nematoda throughout the investigation period. C_mic and microbial C:N ratios in the fallow were higher in winter than during the rest of the year as indicated by a 2-year study, reflecting favorable conditions for microbial C assimilation at low temperatures in the absence of freeze–thaw perturbation. In the arable soil, C_mic contents were significantly reduced during soil freezing but recovered quickly upon warming of the soil. Dynamics of C_mic in the arable soil were paralleled by protozoan biomass and transient shifts in functional composition of the nematode community, indicating that microfaunal predation played an important role in nutrient cycling after freeze–thaw perturbation. Only minor microfaunal dynamics were observed in the climatically more stable fallow, essentially confirming the absence of perturbation at this site. Our findings provide strong evidence that overwinter N₂O formation is regulated by both the physical freeze–thaw susceptibility of the soil and the ecological functioning of the habitat.     

Nitrous oxide emissions from paddy soil in three rice-based cropping systems in China

Rice-flooding fallow, rice-wheat, and double rice-wheat systems were adopted in pot experiment in an annual rotation to investigate the effects of cropping system on N₂O emission from rice-based cropping systems. The annual N₂O emission from the rice-wheat and the double rice-wheat cropping systems were 4.3 kg N ha^–1 and 3.9 kg N ha^–1, respectively, higher than that from rice-flooding fallow cropping system, 1.4 kg N ha^–1. The average N₂O flux was 115 and 118 g N m^–2 h^–1 for rice season in rice-wheat system and early rice season in double rice-wheat system, respectively, 68.6 and 35.3 g N m^–2 h^–1 for the late rice season in double rice-wheat system and rice season in rice-flooding fallow, respectively, and only 3.1–5.3 g N m^–2 h^–1 for winter wheat or flooding fallow season. Temporal variations of N₂O emission during rice growing seasons differed and high N₂O emission occurred when soil conditions changed from upland crop to flooded rice.     

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^–2 d^–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 tillering 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 emissions could 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 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.     

Methane production from anaerobic soil amended with rice straw and nitrogen fertilizers

Laboratory experiments were conducted on the effects of rice straw application and N fertilization on methane (CH₄) production from a flooded Louisiana, USA, rice soil incubated under anaerobic conditions. Rice straw application significantly increased CH₄ production; CH₄ production increased in proportion to the application rate. Urea fertilization also enhanced CH₄ production. The maximum production rate was 17% higher, and occurred 1 week earlier, than that of soil samples which did not receive urea, possibly due to the increase in soil pH following urea hydrolysis. The increase in soil pH following urea hydrolysis may have stimulated CH₄-generating bacteria by providing more optimal soil pH conditions or contributed to the drop in redox potential (Eh). The significant decrease in both the production rate and the total amount of CH₄ by application of NH₄NO₃ was associated with increases in soil Eh after addition of this oxidant. Addition of 300 mg. kg^–1 NO ₃ ^- -N increased soil Eh by 220 mV and almost completely inhibited CH₄ production. However, this inhibitory effect was short-termed. Soon after the applied NO ₃ ^- -N was reduced through denitrification, CH₄ production increased. When (NH₄)₂SO₄ was applied, the inhibition of CH₄ production was not associated with an increase in soil Eh which did not change significantly. A direct inhibitory effect of sulphate on methanogenesis might have been more important.     

Nitrogen mineralization,nitrate leaching and crop growth following cultivation of a temporary leguminous pasture in autumn and winter

A field experiment was conducted to investigate the effect of timing and method of cultivation of a 3-year old ryegrass/white clover pasture on subsequent N mineralization, NO ₃ ^- -N leaching, and growth and N uptake of a wheat crop in the following season. The size of various N pools and decomposition of¹⁴C-labelled ryegrass material were also investigated. Cultivation method (mouldboard or chisel ploughing) generally had no significant effect on the accumulation of mineral N in the profile in the autumn or on the amount of NO ₃ ^- -N leached over winter.¹⁴C measurements suggested that initial decomposition rate of plant material was faster from May than March cultivation treatments. Despite this, overall net mineralization of organic N (of soil plus plant origin) increased with increasing fallow period between cultivation and leaching. The total amounts of mineral N accumulated in the soil profile before the start of leaching were 139, 119 and 22 kg N ha^–1 for the March, May and July cultivated soils respectively. Cumulative leaching losses over the trial calculated from soil solution samples were 78, 40 and 5 kg N ha^–1 for the March, May and July cultivated soils respectively. Differences in N mineralization over the season were generally not reflected by changes in amounts of potentially-mineralizable soil N (as measured by extraction or laboratory incubation) or levels of microbial biomass during the season. The amount of mineral N in the profile in spring increased with decreasing fallow period. This was reflected in an approximately 15% and 25% greater grain yield and N uptake respectively by the following wheat crop in plots cultivated in July rather than in March.     

The Effects of Cultural Practices on Methane Emission from Rice Fields

A field experiment was conducted in a clayey soil to determine the effects of cultural practices on methane (CH₄) emissions from rice fields. The factors evaluated were a) direct seeding on dry vs wet soil, b) age of transplanted seedlings (8 d old and 30 d old), and c) fall vs spring plowing. Methane emissions were measured weekly throughout the rice-growing season using a closed static chamber technique. Transplanted 8-d-old seedlings showed the highest emission of 42.4 g CH₄ m^–2 season^–1, followed by transplanted 30-d-seedlings (40.3 g CH₄ m^–2 season^–1), and direct seeding on wet soil (37.1 g CH₄ m^–2 season^–1). Direct seeding on dry soil registered the least emission of 26.9 g CH₄ m^–2 season^–1. Thus transplanting 30-d-old seedlings, direct seeding on wet soil, and direct seeding on dry soil reduced CH₄ emission by 5%, 13%, and 37%, respectively, when compared with transplanting 8-d-old seedlings. Methane emission under spring plowing was 42.0 g CH₄ m^–2 season^–1 and that under fall plowing was 31.3 g CH₄ m^–2 seasons^–1. The 26% lower emission in the field plowed in spring was caused by degradation of organic matter over the winter.     

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 trend 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^–2 d^–1 in the dry season (DS) and 269–503 mg CH₄ m^–2 d^–1 in the wet season (WS). The higher emission 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 16–54% 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 to 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.     

Land management between crops affects soil inorganic nitrogen balance in a tropical rice system

Sustainable production of lowland rice (Oryza sativa L.) requires minimising undesirable soil nitrogen (N) losses via nitrate (NO₃ ^?) leaching and denitrification. However, information is limited on the N transformations that occur between rice crops (fallow and land preparation), which control indigenous N availability for the subsequent crop. In order to redress this knowledge gap, changes in NO₃ ^? isotopic composition (δ¹⁵N and δ¹⁸O) in soil and water were measured from harvest through fallow, land preparation, and crop establishment in a 7 year old field trial in the Philippines. During the period between rice crops, plots were maintained either, continuously flooded, dry, or alternately wet and dry from rainfall. Plots were split with addition or removal of residue from the previous rice crop. No N fertilizer was applied during the experimental period. Nitrogen accumulated during the fallow (20 kg NH₄ ⁺–N ha^?1 in flooded treatments and 10 kg NO₃ ^?–N ha^?1 in treatments with drying), but did not influence N availability for the subsequent crop. Nitrate isotope fractionation patterns indicated that denitrification drove this homogenisation: during land preparation ~50 % of inorganic N in the soil (top 10 cm) was denitrified, and by 2 weeks after transplanting this increased to >80 % of inorganic N, regardless of fallow management. The 17 days between fallow and crop establishment controlled not only N attenuation (3–7 kg NO₃ ^?–N ha^?1 denitrified), but also N inputs (3–14 kg NO₃ ^?–N ha^?1 from nitrification), meaning denitrification was dependent on soil nitrification rates. While crop residue incorporation delayed the timing of N attenuation, it ultimately did not impact indigenous N supply. By measuring NO₃ ^? isotopic composition over depth and time, this study provides unique in situ measurements of the pivotal role of land preparation in determining paddy soil indigenous N supply.     

Crop Management Affecting Methane Emissions from Irrigated and Rainfed Rice in Central Java (Indonesia)

Methane (CH₄) emissions were determined from 1993 to 1998 using an automated closed chamber technique in irrigated and rainfed rice. In Jakenan (Central Java), the two consecutive crops encompass a gradient from low to heavy rainfall (wet season crop) and from heavy to low rainfall (dry season crop), respectively. Rainfed rice was characterized by very low emission at the onset of the wet season and the end of the dry season. Persistent flooding in irrigated fields resulted in relatively high emission rates throughout the two seasons. Average emission in rainfed rice varied between 19 and 123 mg CH₄ m^–2 d^–1, whereas averages in irrigated rice ranged from 71 to 217 mg CH₄ m^–2 d^–1. The impact of organic manure was relatively small in rainfed rice. In the wet season, farmyard manure (FYM) was completely decomposed before CH₄ emission was initiated; rice straw resulted in 40% increase in emission rates during this cropping season. In the dry season, intensive flooding in the early stage promoted high emissions from organically fertilized plots; seasonal emissions of FYM and rice straw increased by 72% and 37%, respectively, as compared with mineral fertilizer. Four different rice cultivars were tested in irrigated rice. Average emission rates differed from season to season, but the total emissions showed a consistent ranking in wet and dry season, depending on season length. The early-maturing Dodokan had the lowest emissions (101 and 52 kg CH₄ ha^–1) and the late-maturing Cisadane had the highest emissions (142 and 116 kg CH₄ ha^–1). The high-yielding varieties IR64 and Memberamo had moderately high emission rates. These findings provide important clues for developing specific mitigation strategies for irrigated and rainfed rice.     

Methane Emissions from Irrigated Rice Fields in Northern India (New Delhi)

Methane (CH₄) emission fluxes from rice fields as affected by water regime, organic amendment, and rice cultivar were measured at the Indian Agricultural Research Institute, New Delhi, using manual and automatic sampling techniques of the closed chamber method. Measurements were conducted during four consecutive cropping seasons (July to October) from 1994 to 1997. Emission rates were very low (between 16 and 40 kg CH₄ m^–2 season^–1) when the field was flooded permanently. These low emissions were indirectly caused by the high percolation rates of the soil; frequent water replenishment resulted in constant inflow of oxygen in the soil. The local practice of intermittent flooding, which encompasses short periods without standing water in the field, further reduced emission rates. Over the course of four seasons, the total CH₄ emission from intermittently irrigated fields was found to be 22% lower as compared with continuous flooding. The CH₄ flux was invariably affected by rice cultivar. The experiments conducted during 1995 with one cultivar developed by IRRI (IR72) and two local cultivars (Pusa 169 and Pusa Basmati) showed that the average CH₄ flux from the intermittently irrigated plots without any organic amendment ranged between 10.2 and 14.2 mg m^–2 d^–1. The impact of organic manure was tested in 1996 and 1997 with varieties IR72 and Pusa 169. Application of organic manure (FYM + wheat straw) in combination with urea (1:1 N basis) enhanced CH₄ emission by 12–20% as compared with fields treated with urea only. The site in New Delhi represents one example of very low CH₄ emissions from rice fields. Emissions from other sites in northern India may be higher than those in New Delhi, but they are still lower than in other rice-growing regions in India. The practice of intermittent irrigation--in combination with low organic inputs--is commonly found in northern India and will virtually impede further mitigation of CH₄ emissions in significant quantities. In turn, the results of this study may provide clues to reduce emissions in other parts of India with higher baseline emissions.     

Methane Emission from Rice Fields at Cuttack, India

Methane (CH₄) emission from rice fields at Cuttack (State of Orissa, eastern India) has been recorded using an automatic measurement system (closed chamber method) from 1995–1998. Experiments were laid out to test the impact of water regime, organic amendment, inorganic amendment and rice cultivars. Organic amendments in conjunction with chemical N (urea) effected higher CH₄ flux over that of chemical N alone. Application of Sesbania, Azolla and compost resulted in 132, 65 and 68 kg CH₄ ha^–1 in the wet season of 1996 when pure urea application resulted in 42 kg CH₄ ha^–1. Intermittent irrigation reduced emissions by 15% as compared to continuous flooding in the dry season of 1996. In the wet season of 1995, four cultivars were tested under rainfed conditions resulting in a range of emissions from 20 to 44 kg CH₄ ha^–1. Application of nitrification inhibitor dicyandiamide (DCD) inhibited while Nimin stimulated CH₄ flux from flooded rice compared to that of urea N alone. Wide variation in CH₄ production and oxidation potentials was observed in rice soils tested. Methane oxidation decreased with soil depth, fertilizer-N and nitrification inhibitors while organic amendment stimulated it. The results indicate that CH₄ emission from the representative rainfed ecosystem at the experimental site averaged to 32 kg CH₄ ha^–1 yr^–1.     

Emissions of nitrous oxide from boreal agricultural clay and loamy sand soils

Long-term studies of greenhouse gas fluxes from agricultural soils in different climate regions are needed to improve the existing calculation models used in greenhouse gas inventories. The aim of this study was to obtain more information on nitrous oxide (N₂O) emissions from agricultural mineral soils in the boreal region. N₂O emissions were studied during 2000–2002 on two soil types in Finland, a loamy sand and a clay with plots of grass, barley and fallow. N₂O fluxes were measured with static chambers throughout the year. Other parameters measured were water filled pore space (WFPS), soil mineral nitrogen concentration, soil porosity, soil temperature and depth of soil frost. The annual fluxes from the clay soil ranged from 3.7 to 7.8 kg N ha^–1 and those from sandy loam from 1.5 to 7.5 kg N ha^–1. On average 60% of the annual fluxes occurred outside the growing season, from October to April. Increasing the number of freeze-thaw events was found to increase the fluxes during winter and during the thawing period in spring. The results suggest that N₂O fluxes from these boreal mineral soils do not vary much as a function of applied fertiliser N and could probably be better estimated from soil physical properties, including soil porosity.