Midseason Drainage Research Articles

Effect of Water-Saving Irrigation on CH<sub>4</sub> Emissions from Rice Fields

Although water saving irrigation (WSI) has been widely used in China, there is limited understanding on the effect of such a practice on the CH4 emission in rice fields. Consequently it is difficult to estimate the regionSubscript textal distribution of CH4 emissions in space and time from rice fields across China. Two water treatments (controlled irrigation (CI), a routine WSI practice in China, and a traditional continuous flooded irrigation (FI)) were used to examine diurnal and seasonal variations of CH4 emissions in field experiments in Kunshan, east China. The heavy loams in the site have organic matter content of 30.3 g/kg while percolation rates in the shallow groundwater range from 2 to 10 mm per day. Gas samples were collected and analyzed using a static chamber technique and a Gas Chromatograph system respectively. The results show that under WSI conditions, the diurnal variation of CH4 emissions presented regular afternoon-maximum mode during the initial and middle tillering stage, which mainly depends on air temperature. Only one peak of CH4 emission occurred in initial/middle tillering growth stage of rice season under CI conditions, which is mainly regulated by drainage or water layer receding. For CI, seasonal CH4 emission is 2.4 g•m-2~24.5g•m-2, and the seasonal average flux is 0.8 mg•m-2•h-1~8.15 mg•m-2•h-1, which is 39-85% lower than that for FI. CI has more mitigation potential than midseason drainage. Furthermore, CI significantly reduces irrigation water use while maintains rice yields, even increases yields under atrocious weather conditions. A hydrologic characterization and spatial distribution of rice field in China is needed to assess the extent and magnitude of potential emission reduction in the region.

Water regime–nitrogen fertilizer–straw incorporation interaction: Field study on nitrous oxide emissions from a rice agroecosystem in Nanjing, China

The comprehensive impacts of agricultural management on nitrous oxide (N 2O) emissions are not well documented. Field experiments with 2 3 factorial designs were conducted to investigate the influence of water regimes, nitrogen fertilizer, and straw incorporation on N 2O emissions from rice paddies in Nanjing, China. In addition to the main factorial design, three single factor designs were included: water regime, N rate, and mid-season drainage duration, each with three levels. The results demonstrate that there were significant differences in the responses of soil N 2O emissions to water regime, nitrogen fertilizer, and straw amendment as well as interaction between straw and nitrogen fertilizer. The cumulative seasonal N 2O emissions from the treatments with mid-season drainage averaged 0.41 kg N ha −1, ranging from 0.20 to 0.73 kg N ha −1. These emissions were higher than those from continuously flooded treatments, which averaged 0.28 kg N ha −1 and ranged from 0.13 to 0.55 kg N ha −1. The integrated application of straw and nitrogen fertilizer mitigated N 2O emissions by approximately 50% under both water regimes. N 2O emissions were mainly promoted by the transition period from the upland crop season to the flooded rice season, by nitrogen application, and by depression with straw amendment. Three groups were formed according to a polynomial relationship between seasonal N 2O emissions and rice production. The results of this study suggest that the integrated application of straw and nitrogen fertilizer can mitigate N 2O emissions from rice agriculture without a decrease in rice production.

Mitigation of methane emissions from paddy fields by prolonging midseason drainage

In order to analyze the mitigation of methane (CH 4) emissions and the global warming potentials (GWPs) of CH 4 and nitrous oxide (N 2O) emissions from paddy fields by modifying the adopted water-management technique, we conducted field experiments to measure the CH 4 and N 2O fluxes at nine sites across Japan. Over 2 years, we tested different water-management strategies such as prolonged midseason drainage (MD) in each site. The CH 4 emission rates at each site varied considerably; the rates were dependent on the ratio of reductive and oxidative capacities of the fields. Seasonal CH 4 emission was effectively reduced at most sites by prolonging MD beyond its conventional duration, especially at sites where organic matter was added to the soil before the cultivation. We attribute this result to the effective suppression of the CH 4 emission peak that occurs early in the cultivation period. Despite the large variation in seasonal CH 4 emissions among the sites, the rate of CH 4 emission resulting from alternative water-management strategies relative to that resulting from conventional water-management strategies is highly dependent on the degree of drainage during the MD period. N 2O emission at most sites, in terms of GWP-based CO 2-equivalent, was much smaller than that of CH 4 emission. Compared to conventional water-management strategies, the seasonal CH 4 emissions and the net 100-year GWPs (CH 4 + N 2O) can be suppressed to 69.5 ± 3.4 (SE)% and 72.0 ± 3.1% while maintaining grain yields as high as 96.2 ± 2.0% by prolonging MD on average by employing the selected alternative water-management strategies that satisfied the following conditions: the percent of CH 4 emission of alternative water-management strategies was less than 90% and the grain yield was greater than 85% relative to conventional water-management strategies.

Methane and nitrous oxide emissions from organic and conventional rice cropping systems in Southeast China

To evaluate the impacts of organic cropping system on global warming potentials (GWPs), field measurements of CH4 and N2O were taken in conventional and organic rice (Oryza sativa L.) cropping systems in southeast China. Rice paddies were under various water regimes, including continuous flooding (F), flooding–midseason drainage–reflooding (F-D-F), and flooding–midseason drainage–reflooding and moisture but without waterlogging (F-D-F-M). Nitrogen was applied at the rate of 100 kg N ha−1, as urea-N or pelletized, dehydrated manure product in conventional or organic rice paddies, respectively. Seasonal fluxes of CH4 averaged 4.44, 2.14, and 1.75 mg m−2 h−1 for the organic paddy plots under the water regimes of F, F-D-F and F-D-F-M, respectively. Relative to conventional rice paddies, organic cropping systems increased seasonal CH4 emissions by 20%, 23%, and 35% for the plots under the water regimes of F, F-D-F, and F-D-F-M, respectively. Under the water regimes of F-D-F and F-D-F-M, seasonal N2O-N emissions averaged 10.85 and 13.66 μg m−2 h−1 in organic rice paddies, respectively, which were significantly lower than those in conventional rice paddies. The net global warming potentials (GWPs) of CH4 and N2O emissions from organic rice paddies relative to conventional rice paddies were significantly higher or comparable under various water regimes. The greenhouse gas intensities were greater, while carbon efficiency ratios were lower in organic relative to conventional rice paddies. The results of this study suggest that organic cropping system might not be an effective option for mitigating the combined climatic impacts from CH4 and N2O in paddy rice production.

Modeling methane emissions from rice agriculture in China during 1961–2007

We assessed decadal changes in CH4 fluxes from rice fields in China during 1961–2007 using an empirical model that was modified to include the effects of changing patterns of fertilizer use and water management. We reviewed studies of the effects of organic amendments and found that an application rate of 6 tons/ha increased emissions by 115 ± 42% based on experimental manipulations from 10 studies. We also reviewed studies of mid-season drainage in rice fields and found that drainage reduced CH4 emissions by 35 ± 12% based on experiments reported from nine studies. Our simulations showed that the CH4 flux was about 8 Tg/year in 1961, gradually increased to a maximum of approximately 17 Tg/year in 1982, and then gradually declined to 7.5 Tg/year in 2007. The reduction in the total rice emissions after 1982 was caused primarily by changing agricultural practices, including mid-season drainage, increases in inorganic fertilizer use, improved crop yields, and decreases in the area used for rice production.

Evaluation of yield and physiological attributes of high-yielding rice varieties under aerobic and flood-irrigated management practices in mid-hills ecosystem

Abstract In the valley land of North-East Hill (NEH) ecosystems of India, about 70% area under rice ( Oryza sativa L.) is transplanted. Physiological attributes and yield performance of aerobic rice over conventional flood-irrigated rice need to be assessed while promoting water saving technology. A field experiment was conducted at the experimental farm, ICAR Research Complex for NEH Region, Umiam (950 m msl), Meghalaya during rainy seasons of 2006 and 2007 under aerobic and flooded conditions with aerobic rice variety collected from IRRI, Philippines. Some important high-yielding varieties (HYVs) recommended for the region were also included in the study. The objectives of this study were (i) to evaluate the influence of frequent mid-season drainage as a measure of water saving technique besides inducing the pre-conditioning effect on genotypes to withstand water stress during the subsequent growth period of crop ontogeny, (ii) to compare crop performance between aerobic and flooded rice management practices, and (iii) to identify attributes responsible for the yield gap between aerobic and flooded rice. The results revealed that the yield difference between aerobic (average yield, 1.67 t/ha) and flooded rice (average yield, 2.31 t/ha) ranged from 18.4 to 37.8% ( P 0.05) depending on varieties, highest difference being observed with rice hybrid DRRH 1. Cultivation of rice under aerobic condition resulted in 27.5% yield reduction over flooded rice. Among the yield components assessed, sink size (spikelets per panicle) contributed more to the yield and is considered to be most important factor responsible for yield gap between aerobic and flooded rice. The study suggests that, variety Sahsarang 1 with its moderate values of photosynthesis rate, transpiration rate and water use efficiency (WUE) along with higher grain yield seems to be better choice for both stress (aerobic) as well as normal (flooded) condition. Aerobic rice varieties with minimum yield gap compared to flooded rice is the key for success of aerobic rice cultivation.

Mitigating greenhouse gas and nitrogen loss with improved fertilizer management in rice: quantification and economic assessment

Several technologies have been developed to improve the recovery efficiency of N (REN) but their impacts on greenhouse gas (GHG) emission, N loss and economic implication are rarely analysed. A decision support system (DSS) has been developed to quantify inputs, outputs and balance of N in soil; GHG emission and REN with the prominent N management technologies in rice. This DSS, named InfoNitro (Information on Nitrogen Management Technologies in Rice), integrated analytical and expert knowledge with databases on bio-physical, agronomic and socio-economical features to establish input–output relationships related to N management in rice. Sixteen technologies, which differed in terms of water regime, method of N application, forms of N and tools of fertilizer recommendation were analysed for their REN, N losses, GHG emission and economic return in Haryana, a rice growing region in India. In the current farmers’ practice, REN was 32.7%, which increased up to 40.8% with various technologies except in mid-season drainage and alternate flooding technologies where it decreased up to 29.3%. Loss of N through leaching, volatilization and denitrification in the farmers’ practice (67.5 kg N ha−1) decreased up to 40.5 kg N ha−1 except in mid-season drainage and alternate flooding technologies where it increased. The technologies also reduced global warming potential (GWP) by 1 to 9%. However, the technologies except no tillage, mid-season drying and alternate flooding reduced the net income of the farmers. When the environmental cost (cost of N loss and GWP) was included net income with various technologies was either at par or more than the farmers’ practice. The marginal abatement cost of N loss was Rs. 8 to 134 kg−1 N and for GWP was Rs. 766 to 4854 Mg−1 CO2 eq. Resource conserving technology was the most cost effective strategy to reduce N loss and GHG emission whereas integrated N management cost high for mitigating GHG emission.

Effect of Shallow Water Management on Growth and Yield of Rice

Rice, the most staple food crop of the world has been cultivated about 157 million hectares of land (FAO, 2008). Irrigated rice ecosystem contributes 75% of global rice production. Growing rice using conventional irrigation (continuous flooding) requires a tremendous amount of water. The high water demand of irrigated lowland rice mainly arises from keeping the field continuously submerged. There is decreasing trend in water resource availability year by year due to various reasons. The shortage of water resources for rice production has now become an important issue worldwide. It indicates that a reduction in water input without compromising yield and optimization of scarce water in rice production are required. Shallow water management can be one of the alternatives to use water efficiently, achieve good yield and minimize the methane emission from the rice fields. However, information about growth and yield of rice grown under shallow water management are limited. Therefore, this study was conducted to investigate whether shallow water could influence plant growth and yield of irrigated rice under field conditions. Materials and Methods A field experiment was conducted at Yamagata University Experimental Farm, Takasaka, Tsuruoka, Japan in 2009. Two plant spacings – [30*30cm (wider) and 30*15cm (narrow)] and water managements shallow and conventional were practiced .The four treatments were set up as conventional wider (CW), conventional narrow (CN), shallow wider (SW) and shallow narrow (SN). In conventional method, five to six cm water ponding depth was maintained throughout the growing period except for mid season drainage and totally drained out 15 days before the harvest. For shallow water ,ponding water depth of 1-2cm with wetting and drying was done till panicle initiation stage and then 1-2 cm was maintained and totally drained out 30 days before harvesting. Transplanting of 28 days old (3.5leaf age) seedlings of Sasanishiki variety was done on12th May and harvested on 1 st October 2009. The total dry matter & nitrogen (N) uptake by plant was measured at 37, 44, 51, 58, 65, 77, 91 and 122 days after transplanting (DAT). The yield was estimated by both yield components (10hills) and yield examination (50hills).This preliminary experiment was carried without replication. Results and Discussion Comparing with water management practices, aboveground biomass was higher in shallow than conventional. Interestingly, wider spacing of shallow water management showed almost same as to conventional narrow spacing (fig.1).

Water management — A tool for methane mitigation from irrigated paddy fields

Water drainage is considered to be one of the important practices that reduce the CH 4 efflux from paddy fields. In this study, four different drainage systems (continuous flooding, tillering stage drainage, mid-season drainage and multiple drainage) were compared to find out the best one, for attenuation of CH 4 emission from rice fields. Except for continuous flooding, from all the other three drainage systems, irrigation water from the paddy fields was drained out at the different stages of the crop cycle. Highest efflux of the methane was recorded from continuously flooded plots (346.6 mg/m 2/day), followed by 9% less CH 4 efflux from tillering stage drainage (315.1 mg/m 2/day), 36.7% less efflux from mid-season drainage (219.3 mg/m 2/day) and the least 41% CH 4 efflux from multiple drainage plots (204.7 mg/m 2/day). Among all the four different drainage systems applied, mid-season drainage and multiple drainage were found to be highly effective in mitigating methane efflux. Redox potential of the soil of the drainage system was found to be inversely proportional to the methane efflux from all the treatments.

Effect of timing of joint application of hydroquinone and dicyandiamide on nitrous oxide emission from irrigated lowland rice paddy field

A field experiment was conducted to study the effect of timing of joint application of urease inhibitor hydroquinone (HQ) and nitrification inhibitor dicyandiamide (DCD) on N 2O emission from irrigated lowland rice paddy field. Four treatments including Treatment CK (the control with urea alone), HQ/DCD-1 (application of HQ and DCD together with fertilizer before transplanting), HQ/DCD-2 (HQ and DCD with fertilizer at tillering stage) and HQ/DCD-3 (HQ and DCD with fertilizer at panicle initiation stage) were designed and implemented separately during rice growth period. Seasonal peaks of N 2O flux occurred during midseason drainage and significant negative correlation between N 2O flux and water layer depth was observed ( r = −0.69 to −0.75, P < 0.01). Mean N 2O flux was the highest in the control with urea alone, while joint addition of HQ and DCD with urea lowered mean N 2O flux considerably ( P < 0.05). Total N 2O emission during rice growth season in Treatment CK, HQ/DCD-1, HQ/DCD-2 and HQ/DCD-3 was 3.90, 2.98, 1.73 and 3.23 kg N 2O–N ha −1, respectively. Application of HQ and DCD together with basal fertilizer, tillering fertilizer and panicle initiation fertilizer decreased the total N 2O emission by 24%, 56% and 17%, respectively, while increased grain yield by 10%, 18% and 6%, respectively. Effect of application of inhibitors on N 2O emission during the continuous period from incorporation of HQ and DCD to rice harvest was also studied, where results indicating that the highest inhibiting efficiency of inhibitors on N 2O emission was recorded when HQ and DCD applied with fertilizer at tillering stage.

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Abstracts of Nippon Dojo-Hiryogaku Zasshi

Changes in fertilizer‐induced direct N2O emissions from paddy fields during rice‐growing season in China between 1950s and 1990s

AbstractNitrogen fertilizer‐induced direct nitrous oxide (N2O) emissions depend on water regimes in paddy fields, such as seasonal continuous flooding (F), flooding–midseason drainage–reflooding (F‐D‐F), and flooding–midseason drainage–reflooding–moist intermittent irrigation but without water logging (F‐D‐F‐M). In order to estimate the changes in direct N2O emission from paddy fields during the rice‐growing season in Mainland of China between the 1950s and the 1990s, the country‐specific emission factors of N2O‐N under different water regimes combined with rice production data were adopted in the present study. Census statistics on rice production showed that water management and nitrogen input regimes have changed in rice paddies since the 1950s. During the 1950s–1970s, about 20–25% of the rice paddy was continuously waterlogged, and 75–80% under the water regime of F‐D‐F. Since the 1980s, about 12–16%, 77%, and 7–12% of paddy fields were under the water regimes of F, F‐D‐F, and F‐D‐F‐M, respectively. Total nitrogen input during the rice‐growing season has increased from 87.5 kg N ha−1 in the 1950s to 224.6 kg N ha−1 in the 1990s. The emission factors of N2O‐N were estimated to be 0.02%, 0.42%, and 0.73% for rice paddies under the F, F‐D‐F, and F‐D‐F‐M water regimes, respectively. Seasonal N2O emissions have increased from 9.6 Gg N2O‐N each year in the 1950s to 32.3 Gg N2O‐N in the 1990s, which is accompanied by the increase in rice yield over the period 1950s–1990s. The uncertainties in N2O estimate were estimated to be 59.8% in the 1950s and 37.5% in the 1990s. In the 1990s, N2O emissions during the rice‐growing season accounted for 8–11% of the reported annual total of N2O emissions from croplands in China, suggesting that paddy rice development could have contributed to mitigating agricultural N2O emissions in the past decades. However, seasonal N2O emissions would be increased, given that saving‐water irrigation and nitrogen inputs are increasingly adopted in rice paddies in China.

Sewage irrigation increased methane and nitrous oxide emissions from rice paddies in southeast China

Greenhouse gas emissions from rice paddies under sewage irrigation deserve much attention since domestic sewage effluents are increasingly used for agriculture in developing countries. A field experiment was conducted to simultaneously measure methane (CH 4) and nitrous oxide (N 2O) emissions from rice ( Oryza sativa L.) paddies under sewage and unpolluted river water irrigation in southeast China. The rice paddies were under a local typical water regime, which was characterized by flooding–midseason drainage–reflooding–moist intermittent irrigation but without water logging. Relative to unpolluted river water irrigation, sewage irrigation significantly increased CH 4 and N 2O emissions from rice paddies. Seasonal fluxes of CH 4 averaged 1.51 mg m −2 h −1 for the plots irrigated by river water and 1.92 mg m −2 h −1 for the plots irrigated by sewage. In contrast with river water irrigation, sewage irrigation increased CH 4 by 27% and 33% for paddy plots with and without chemical N addition, respectively. Under sewage irrigation, seasonal fluxes of N 2O-N averaged 26.79 μg m −2 h −1 for the plots without N application and 74.07 μg m −2 h −1 for the plots applied at the rate of 200 kg N ha −1. Relative to river water irrigation, sewage irrigation increased N 2O by 68% and 170% for the plots with and without N application, respectively. The direct emission factor of fertilizer N for N 2O was estimated to be 0.71% for the rice paddies under sewage irrigation and 0.52% for the plots irrigated by river water. Besides direct N 2O emissions, N input by sewage irrigation induced substantial indirect N 2O emission from rice paddies. The results of the net GWPs from CH 4 and N 2O indicate that sewage irrigation would intensify the radiative forcing of rice paddies with midseason drainage and moist irrigation.

Persistence of CaCl2 washing effect for amelioration of a Cd-contaminated paddy field soil

Soil washing is one of the methods used to remediate soil contaminated with heavy metals, and when the contaminated elements have been effectively removed the washed soil can be used for agriculture. Soil washing was conducted using 0.5 mol L−1 CaCl2 solution at pH 4 as an extracting agent to remediate a paddy field soil contaminated with Cd. Dolomite powder was applied to neutralize the soil to the original pH 6.2. After CaCl2 washing, the content of Cd extractable in 0.1 mol L−1 HCl decreased from 2.4 to 0.8 mg kg−1. Subsequently, a pot experiment was carried out to evaluate the effect of soil washing on Cd concentration in polished rice (Cdpr) for three successive years. Using the washed soil, Cdpr was ≤ 0.2 mg kg−1 with and without a treatment that simulates midseason drainage, whereas it was > 0.5 mg kg−1 in the unwashed soil with the midseason drainage treatment. The reasons for low Cdpr growth in the washed soil were the low content of exchangeable Cd in the soil and the resultant high soil pH (> 7). To evaluate the effect of soil pH on Cdpr in the fourth year, we adjusted soil pH to 5 with H2SO4 before transplanting rice seedlings. The Cdpr in the washed soil with the midseason drainage treatment increased to 0.47 mg kg−1, whereas it was less than 0.2 mg kg−1 under continuous flooding. Thus, high pH or whole season flooding are important to keep Cdpr at ≤ 0.2 mg kg−1 even after soil washing. With the application of dolomite and other ordinary fertilizers, soil properties were little affected by the present soil washing procedure because the difference in rice yield between the washed and unwashed plots was not significant within each year.

Eukaryotic communities associated with the decomposition of rice straw compost in a Japanese rice paddy field estimated by DGGE analysis

To estimate the succession and phylogenetic composition of the eukaryotic communities responsible for the decomposition of rice straw compost under flooded conditions during the cultivation period of paddy rice, denaturing gradient gel electrophoresis (DGGE) analysis targeting 18S rDNA followed by sequencing was conducted in a Japanese paddy field. The eukaryotic communities in rice straw compost incorporated into the flooded paddy field were influenced by the mid-season drainage and mainly composed of fungi (Ascomycota, Zygomycota, and Chytridiomycota) and protozoa (Ciliophora, Euglyphida, and Dactylopodida), most of which existed continuously during the cultivation period of paddy rice. The results indicated that these eukaryotic members were associated with the decomposition of rice straw compost in paddy field soil directly or indirectly.

Introducing greenhouse gas mitigation as a development objective in rice-based agriculture: II. Cost–benefit assessment for different technologies, regions and scales

New tools for land use analysis including detailed cost–benefit assessments are needed to integrate resource management for enhancing farmers’ income and mitigating greenhouse gas (GHG) emissions. The paper comprises an assessment of GHG emissions and economic returns under different mitigation technologies in three rice growing regions in Asia, i.e., Ilocos Norte province (Philippines), Zhejiang province (China) and Haryana state (India). Site-specific data on soil, climate and socio-economics were integrated in the previously developed spreadsheet model TechnoGAS (Technical Coefficient Generator for Mitigation Technologies of Greenhouse Gas Emissions from Agricultural Sectors). Three baseline technologies that differed in terms of inorganic/organic N supply have been compared to different mitigation technologies in form of Marginal Abatement Cost Curves (MACCs). For the baseline technology of inorganic N (urea) fertilization, amendment with phosphogypsum and nitrification inhibitors are the most promising mitigation options resulting in shadow prices of less than US$10 per ton of carbon dioxide equivalent (CE). Assuming a mix of urea and farm yard manure for the baseline, we have tested several options including different irrigation patterns and husk used as fossil fuel. Mid-season drainage had a better cost–benefit ratio (ca. US$20 per t CE) than alternate flooding, but was less profitable than husk utilization (ca. US$4 per t CE). Assuming high organic inputs, biogas technology is, in most cases, the preferable option (ca. US$10 per t CE). Finally, we compiled regional abatement cost curves for selected administrative units using the outcome from regional optimization models. Implementing the three most promising technologies required US$6000 for Dingras municipality, Ilocos Norte, in the Philippines (ca. 10 3 ha of rice land potentially providing emission savings of ca. 3000 t CE), US$50,000 for Pujiang county in China (ca. 10 4 ha providing ca. 27,000 t CE), and US$1.2 million for Karnal district in India (ca. 10 5 ha providing ca. 220,000 t CE).

Bacterial community in plant residues in a Japanese paddy field estimated by RFLP and DGGE analyses

Plant residues (PRs) are “hot spots” of microbial activities in soil. PRs with the size more than 0.5 mm were collected from a Japanese paddy field during rice cultivation period (from May to September) and fractionated into four categories by size (>4, 2–4, 1–2, and 0.5–1 mm) using sieves. Restriction fragment length polymorphism (RFLP) and denaturing gradient gel electrophoresis (DGGE) patterns were compared among the fractions after DNA extraction from the PRs and PCR amplification. The total amount of PRs with the size over 0.5 mm decreased in the field with the first-order kinetics ( r 2=0.810, p<0.01) with time from rice transplanting to harvest. RFLP analysis showed that the bacterial community structure in PRs with the 0.5–2 mm fraction was different from that in PRs with the >2 mm fraction and the latter community structure changed after the midseason drainage. In contrast, the DGGE patterns of the bacterial community in the PRs indicated the succession from June to September during rice cultivation forming three major groups irrespective of the fraction size. Sequence analysis of DGGE bands showed that Firmicutes (clostridia), α-, γ-, δ-Proteobacteria (myxobacteria), Nitrospira, Acidobacteria, Bacteroidetes, Verrucomicrobia and Spirochaetes were predominant members in the PRs irrespective of fraction size.

Assessing Alternatives for Mitigating Net Greenhouse Gas Emissions and Increasing Yields from Rice Production in China Over the Next Twenty Years

Assessments of the efficacy of mitigation of greenhouse gas (GHG) emissions from paddy rice systems have typically been analyzed based on field studies. Extrapolation of the mitigation potential of alternative management practices from field studies to a national scale may be enhanced by spatially explicit process models, like the DeNitrification and DeComposition (DNDC) model. Our objective was to analyze the impacts of mitigation alternatives, management of water, fertilizer, and rice straw, on net GHG emissions (carbon dioxide, methane, and nitrous oxide fluxes), yields, and water use. After constructing a GIS database of soil, climate, rice cropping area and systems, and management practices, we ran DNDC with 21-yr alternative management schemes for each of the approximately 2500 counties in China. Results indicate that, despite large-scale adoption of midseason drainage, there is still large potential for additional methane reductions from Chinese rice paddies of 20 to 60% over 2000-2020. However, changes in management for reducing CH4 emissions simultaneously affect soil carbon dynamics as well as N2O emissions and can thereby reorder the ranking of technical mitigation effectiveness. The order of net GHG emissions reduction effectiveness found here is upland rice > shallow flooding > ammonium sulfate > midseason drainage > off-season straw > slow-release fertilizer > continuous flooding. Most of the management alternatives produced yields comparable to the baseline; however, continuous flooding and upland rice significantly reduced yields. Water management strategies appear to be the most technically promising GHG mitigation alternatives, with shallow flooding providing additional benefits of both water conservation and increased yields.

Field Validation of DNDC Model for Methane and Nitrous Oxide Emissions from Rice-based Production Systems of India

The DNDC (DeNitrification and DeComposition) model was tested against experimental data on CH4 and N2O emissions from rice fields at different geographical locations in India. There was a good agreement between the simulated and observed values of CH4 and N2O emissions. The difference between observed and simulated CH4 emissions in all sites ranged from −11.6 to 62.5 kg C ha−1 season−1. Most discrepancies between simulated and observed seasonal fluxes were less than 20% of the field estimate of the seasonal flux. The relative deviation between observed and simulated cumulative N2O emissions ranged from −237.8 to 28.6%. However, some discrepancies existed between observed and simulated seasonal patterns of CH4 and N2O emissions. The model simulated zero N2O emissions from continuously flooded rice fields and poorly simulated CH4 emissions from Allahabad site. For all other simulated cases, the model satisfactorily simulated the seasonal variations in greenhouse gas emission from paddy fields with different land management. The model also simulated the C and N balances in all the sites, including other gas fluxes, viz. CO2, NO, NO2, N2 and NH3 emissions. Sensitivity tests for CH4 indicate that soil texture and pH significantly influenced the CH4 emission. Changes in organic C content had a moderate influence on CH4 emission on these sites. Introducing the mid-season drainage reduced CH4 emissions significantly. Process-based biogeochemical modeling, as with DNDC, can help in identifying strategies for optimizing resource use, increasing productivity, closing yield gaps and reducing adverse environmental impacts.

Modeling impacts of farming management alternatives on CO2, CH4, and N2O emissions: A case study for water management of rice agriculture of China

Since the early 1980s, water management of rice paddies in China has changed substantially, with midseason drainage gradually replacing continuous flooding. This has provided an opportunity to estimate how a management alternative impacts greenhouse gas emissions at a large regional scale. We integrated a process‐based model, DNDC, with a GIS database of paddy area, soil properties, and management factors. We simulated soil carbon sequestration (or net CO2 emission) and CH4 and N2O emissions from China's rice paddies (30 million ha), based on 1990 climate and management conditions, with two water management scenarios: continuous flooding and midseason drainage. The results indicated that this change in water management has reduced aggregate CH4 emissions about 40%, or 5 Tg CH4 yr−1, roughly 5–10% of total global methane emissions from rice paddies. The mitigating effect of midseason drainage on CH4 flux was highly uneven across the country; the highest flux reductions (&gt;200 kg CH4‐C ha−1 yr−1) were in Hainan, Sichuan, Hubei, and Guangdong provinces, with warmer weather and multiple‐cropping rice systems. The smallest flux reductions (&lt;25 kg CH4‐C ha−1 yr−1) occurred in Tianjin, Hebei, Ningxia, Liaoning, and Gansu Provinces, with relatively cool weather and single cropping systems. Shifting water management from continuous flooding to midseason drainage increased N2O emissions from Chinese rice paddies by 0.15 Tg N yr−1 (∼50% increase). This offset a large fraction of the greenhouse gas radiative forcing benefit gained by the decrease in CH4 emissions. Midseason drainage‐induced N2O fluxes were high (&gt;8.0 kg N/ha) in Jilin, Liaoning, Heilongjiang, and Xinjiang provinces, where the paddy soils contained relatively high organic matter. Shifting water management from continuous flooding to midseason drainage reduced total net CO2 emissions by 0.65 Tg CO2‐C yr−1, which made a relatively small contribution to the net climate impact due to the low radiative potential of CO2. The change in water management had very different effects on net greenhouse gas mitigation when implemented across climatic zones, soil types, or cropping systems. Maximum CH4 reductions and minimum N2O increases were obtained when the mid‐season draining was applied to rice paddies with warm weather, high soil clay content, and low soil organic matter content, for example, Sichuan, Hubei, Hunan, Guangdong, Guangxi, Anhui, and Jiangsu provinces, which have 60% of China's rice paddies and produce 65% of China's rice harvest.