Greenhouse Gas Emissions and Management Practices that Affect Emissions in US Rice Systems.

Previous reviews have quantified factors affecting greenhouse gas (GHG) emissions from Asian rice ( L.) systems, but not from rice systems typical for the United States, which often vary considerably particularly in practices (i.e., water and carbon management) that affect emissions. Using meta-analytic and regression approaches, existing data from the United States were examined to quantify GHG emissions and major practices affecting emissions. Due to different production practices, major rice production regions were defined as the mid-South (Arkansas, Texas, Louisiana, Mississippi, and Missouri) and California, with emissions being evaluated separately. Average growing season CH emissions for the mid-South and California were 194 (95% confidence interval [CI] = 129-260) and 218 kg CH ha season (95% CI = 153-284), respectively. Growing season NO emissions were similar between regions (0.14 kg NO ha season). Ratoon cropping (allowing an additional harvestable crop to grow from stubble after the initial harvest), common along the Gulf Coast of the mid-South, had average CH emissions of 540 kg CH ha season (95% CI = 465-614). Water and residue management practices such as alternate wetting and drying, and stand establishment method (water vs. dry seeding), and the amount of residue from the previous crop had the largest effect on growing season CH emissions. However, soil texture, sulfate additions, and cultivar selection also affected growing season CH emissions. This analysis can be used for the development of tools to estimate and mitigate GHG emissions from US rice systems and other similarly mechanized systems in temperate regions.

Reducing greenhouse gas emissions, water use, and grain arsenic levels in rice systems.

Agriculture is faced with the challenge of providing healthy food for a growing population at minimal environmental cost. Rice (Oryza sativa), the staple crop for the largest number of people on earth, is grown under flooded soil conditions and uses more water and has higher greenhouse gas (GHG) emissions than most crops. The objective of this study was to test the hypothesis that alternate wetting and drying (AWD--flooding the soil and then allowing to dry down before being reflooded) water management practices will maintain grain yields and concurrently reduce water use, greenhouse gas emissions and arsenic (As) levels in rice. Various treatments ranging in frequency and duration of AWD practices were evaluated at three locations over 2 years. Relative to the flooded control treatment and depending on the AWD treatment, yields were reduced by <1-13%; water-use efficiency was improved by 18-63%, global warming potential (GWP of CH4 and N2 O emissions) reduced by 45-90%, and grain As concentrations reduced by up to 64%. In general, as the severity of AWD increased by allowing the soil to dry out more between flood events, yields declined while the other benefits increased. The reduction in GWP was mostly attributed to a reduction in CH4 emissions as changes in N2 O emissions were minimal among treatments. When AWD was practiced early in the growing season followed by flooding for remainder of season, similar yields as the flooded control were obtained but reduced water use (18%), GWP (45%) and yield-scaled GWP (45%); although grain As concentrations were similar or higher. This highlights that multiple environmental benefits can be realized without sacrificing yield but there may be trade-offs to consider. Importantly, adoption of these practices will require that they are economically attractive and can be adapted to field scales.

Seasonal methane and nitrous oxide emissions of several rice cultivars in direct-seeded systems.

An understanding of cultivar effects on field greenhouse gas (GHG) emissions in rice ( L.) systems is needed to improve the accuracy of predictive models used for estimating GHG emissions and to evaluate the GHG mitigation potential of different cultivars. We compared CH and NO emissions, global warming potential (GWP = NO + CH), yield-scaled GWP (GWP = GWP Mg grain), and plant growth characteristics of eight cultivars within four study sites in California and Arkansas. Nitrous oxide emissions were negligible (<10% of GWP) and were not different among cultivars. Seasonal CH emissions differed between cultivars by a factor of 2.1 and 1.4 at one California and one Arkansas site, respectively. Plant growth characteristics were generally not correlated with seasonal CH emissions; however, the strongest correlations were observed for shoot and total plant (root + shoot) biomass at heading ( = 0.60) at one California site and for grain at maturity ( = -0.95) at one Arkansas site. Although differences in GWP and GWP were observed, there were inconsistencies across sites, indicating the importance of the genotype × environment interaction. Overall, the cultivars with the lowest CH emissions, GWP, and GWP at the California and Arkansas sites were the lowest and highest yielding, respectively. These findings highlight the potential for breeding high-yielding cultivars with low GWP, the ideal scenario to achieve low GWP, but environmental conditions must also be considered.

Optimal fertilizer nitrogen rates and yield-scaled global warming potential in drill seeded rice.

Drill seeded rice ( L.) is the dominant rice cultivation practice in the United States. Although drill seeded systems can lead to significant CH and NO emissions due to anaerobic and aerobic soil conditions, the relationship between high-yielding management practices, particularly fertilizer N management, and total global warming potential (GWP) remains unclear. We conducted three field experiments in California and Arkansas to test the hypothesis that by optimizing grain yield through N management, the lowest yield-scaled global warming potential (GWP = GWP Mg grain) is achieved. Each growing season, urea was applied at rates ranging from 0 to 224 kg N ha before the permanent flood. Emissions of CH and NO were measured daily to weekly during growing seasons and fallow periods. Annual CH emissions ranged from 9.3 to 193 kg CH-C ha yr across sites, and annual NO emissions averaged 1.3 kg NO-N ha yr. Relative to NO emissions, CH dominated growing season (82%) and annual (68%) GWP. The impacts of fertilizer N rates on GHG fluxes were confined to the growing season, with increasing N rate having little effect on CH emissions but contributing to greater NO emissions during nonflooded periods. The fallow period contributed between 7 and 39% of annual GWP across sites years. This finding illustrates the need to include fallow period measurements in annual emissions estimates. Growing season GWP ranged from 130 to 686 kg CO eq Mg season across sites and years. Fertilizer N rate had no significant effect on GWP; therefore, achieving the highest productivity is not at the cost of higher GWP.

Impact of production practices on physicochemical properties of rice grain quality.

BACKGROUND: Rice growers are interested in new technologies that can reduce input costs while maintaining high field yields and grain quality. The bed-and-furrow (BF) water management system benefits farmers through decreased water usage, labor, and fuel as compared to standard flood management. Fertilizer inputs can be reduced by producing rice in rotation with soybeans, a nitrogen-fixing crop, and with the use of slow-release fertilizers that reduce nitrogen volatilization and run-off. However, the influence of these cultural management practices on rice physicochemical properties is unknown. Our objective was to evaluate the influence of nitrogen fertilizer source, water management system, and crop rotation on rice grain quality. RESULTS: Grain protein concentration was lower in a continuous rice production system than in a rice-soybean rotation. Neither amylose content nor gelatinization temperature was altered by fertilizer source, crop rotation, or water management. BF water management decreased peak and breakdown viscosities relative to a flooded system. Peak and final paste viscosities were decreased by all fertilizer sources, whereas, crop rotation had no influence on the Rapid Visco Analyser profile. CONCLUSION: Sustainable production systems that decrease water use and utilize crop rotations and slow-release fertilizers have no major impact on rice physicochemical properties.

Influences from Long-Term Crop Rotation, Soil Tillage, and Fertility on the Severity of Rice Grain Smuts.

False smut (Ustilaginoidea virens) and kernel smut (Neovossia horrida) are diseases of rice (Oryza sativa) that reduce both grain yield and quality. Susceptible rice cultivars are in widespread use on production acreage in the United States, and the effects from crop management practices on smut control are poorly understood. We studied the long-term effects of crop rotation, soil tillage, and fertility level on rice smut severity. The highest levels of false smut observed in this study were on cultivars grown in rotation with soybean, on traditionally tilled soils, with high fertilizer treatments. The highest levels of kernel smut were observed in a rice-soybean rotation with winter wheat grown between summer crops. These rotations are commonly used in rice-growing regions of the southern United States. Using combinations of crop rotation, soil tillage, and fertility rate, several alternative crop-management practices were identified that provided effective control of smuts in susceptible rice cultivars. The most effective method for controlling both false smut and kernel smut was in 3-year rotations of rice, soybean, and corn. Regardless of rotation order or tillage and fertility treatments within the rotations, rotating out of rice for 2 years was the most effective approach for smut control.

Effect of Furrow Irrigation on the Severity of False Smut in Susceptible Rice Varieties.

False smut (Ustilaginoidea virens) is an important emerging disease of rice (Oryza sativa) in the southern United States, where all major rice cultivars and hybrids are susceptible to the disease. False smut susceptibility was evaluated in traditional paddy-rice fields and under furrow-irrigated conditions to determine the effects of alternative agricultural practices on the severity of this disease. Highly effective false smut suppression was observed in furrow-irrigated rice, where the disease was nearly eliminated in susceptible rice entries. False smut suppression was observed for two hybrids and one conventional rice cultivar, demonstrating that suppression was not limited to specific germplasm sources. Kernel smut severity was also monitored, but no effect on this disease was observed from the irrigation treatments. Therefore, suppression of disease severity in nonflooded rice appears to be a phenomenon unique to the rice-false smut pathosystem, which can be exploited to achieve effective field resistance to this disease.

Effect of Cultural Management Practices on the Severity of False Smut and Kernel Smut of Rice.

False smut (Ustilaginoidea virens) and kernel smut (Neovossia horrida) are diseases of rice (Oryza sativa) that reduce both grain yield and quality. False smut is an emerging disease worldwide that is rapidly gaining in importance, whereas kernel smut has historically been a chronic minor disease with sporadic outbreaks that cause considerable losses. Highly effective disease control was obtained for susceptible cultivars by employing conservation tillage (69% reduction in false smut), continuous rice cropping (88% reduction in false smut), and moderate nitrogen fertility rates (34 and 60% reductions in false smut and kernel smut, respectively). Combining these treatments nearly eliminated smuts from cultivars that were fully susceptible under conventional cultivation practices. Furthermore, using a nursery designed to promote smut diseases, two rice hybrids were identified that possessed kernel smut resistance under the most favorable disease conditions. The genetic basis of the resistance is unknown. However, the utility for disease control is great because hybrids occupy significant portions of production rice acreage.

Water management impacts rice methylmercury and the soil microbiome.

Rice farmers are pressured to grow rice using less water. The impacts of water-saving rice cultivation methods on rice methylmercury concentrations are uncertain. Rice (Oryza sativa L. cv. Nipponbare) was cultivated in fields using four water management treatments, including flooded (no dry-downs), alternating wetting and drying (AWD) (with one or three dry-downs), and furrow-irrigated fields (nine dry-downs) (n=16 fields). Anoxic bulk soil was collected from rice roots during the rice maturation phase, and rice grain was harvested after fields were dried. Total mercury and methylmercury concentrations were determined in soil and polished rice samples, and the soil microbiome was analyzed using 16S (v4) rRNA gene profiling. Soil total mercury did not differ between fields. However, compared to continuously flooded fields, soil and rice methylmercury concentrations averaged 51% and 38% lower in the AWD fields, respectively, and 95% and 96% lower in the furrow-irrigated fields, respectively. Compared to flooded fields, grain yield was reduced on average by <1% in the AWD fields and 34% in the furrow-irrigated fields. Additionally, using 16S (v4) rRNA gene profiling, the relative abundance of genera (i.e., highest resolution via this method) known to contain mercury methylators averaged 2.9-fold higher in flooded and AWD fields compared to furrow-irrigated fields. These results reinforce the benefits of AWD in reducing rice methylmercury concentrations with minimal changes in rice production yields. In the furrow-irrigated fields, a lower relative abundance of genera known to contain mercury methylators suggests an association between lower concentrations of soil and rice methylmercury and specific soil microbiomes.