Entire Rice-growing Season Research Articles

Assessment of mercury accumulation in various tissues of rice plant (Oryza sativa L.) from a Ramsar site in India

Dietary exposure to mercury through rice consumption is an issue of solemn global concern. The favourable bio-geochemical environment of paddy fields often makes them potential mercury cycling hotspots. India is one of the largest producers and consumers of rice globally. To date, studies focusing on mercury bioaccumulation in rice during the different stages of its growth cycle have not been studied in Indian agro-ecosystems.This inturn resulted in a knowledge gap in the global mercury assessment. Being a party to the Minamata Convention, there is an urgent need to unravel the scenario of mercury contamination in rice, one of the staple food crops across the world. In the present study, total mercury (THg) in the rhizosphere soil as well as its accumulation in different tissues of the rice plant were examined by a field investigation in Kuttanad – a unique Ramsar site in Kerala, India. For the purpose of the study five permanent plots were selected and monitored for the entire rice-growing season (from the 30th to the 120th day). Direct Mercury Analyzer (DMA – 80, Milestone, USA) was used for analyzing THg in rhizosphere soil and plant tissues. Mercury concentration in soils varied from 0.037 mg/kg to 0.117 mg/kg with a mean concentration of 0.088 mg/kg. The THg accumulation pattern in plant tissues followed the order: root > leaf > grain > husk > stem. The mean THg concentration in rice grains was 0.22 mg/kg. The low values for bioaccumulation factor (0.616) and translocation factor (0.425) revealed that a major portion of mercury accumulated by the plant is stored in the roots.

Nutrient runoff loss from saline-alkali paddy fields in Songnen Plain of Northeast China via different runoff pathways: effects of nitrogen fertilizer types.

The application of nitrogen (N) fertilizer aggravates the nutrient runoff loss from paddy, causing serious agricultural non-point source pollution, and leading to a serious decline in water quality. The global area of saline-alkali paddy has expanded, but the response of nutrient loss via runoff to N-fertilizer applications in saline-alkali paddy is still unclear. This study conducted a 147-day field experiment to evaluate the nutrient runoff loss from saline-alkali paddy with different N-fertilizer application strategies in Songnen Plain of Northeast China. Regardless of N-fertilizer types, the nutrient loss via rainfall runoff in the entire rice-growing season was significantly (p < 0.05) higher than that via artificial drainage. The N and phosphorus (P) concentrations in runoff water were correlated with salinity and alkalinity. Especially, pH had a significant positive correlation with total-P (TP) (r = 0.658, p < 0.01). In the entire rice-growing season, the TN runoff losses in urea (U), microbial fertilizer (MF), and inorganic compound fertilizer (ICF) treatments were significantly (p < 0.05) lower compared with carbon-based slow-release fertilizer (CSF) and organic-inorganic compound fertilizer (OCF), respectively. Meanwhile, the TP runoff losses in OCF and ICF treatments were significantly (p < 0.05) lower than U and MF, respectively. Overall, the application of ICF is a better choice to avoid N and P losses via runoff from saline-alkali paddy fields.

Ammonia volatilization, greenhouse gas emissions and microbiological mechanisms following the application of nitrogen fertilizers in a saline-alkali paddy ecosystem

The application of nitrogen (N) fertilizer can promote rice yield, but is accompanied by considerable quantities of ammonia (NH3) volatilization and greenhouse gas (GHG) emissions. The effect of N-fertilization to these gas emissions in saline-alkali paddy ecosystems is unclear. A 137-day mesocosm-scale experiment was carried out to clarify the effect of different N-fertilizer application strategies on NH3 volatilization and GHG emissions in saline-alkali paddy ecosystems and to determine the microbiological mechanisms. Five N-fertilizer treatments consisting of control without N-fertilizer (CK), urea (U), urea with urease-nitrification inhibitors (UI), organic–inorganic compound fertilizer (OCF) and carbon-based slow-release fertilizer (CSF) were established. During the entire rice-growing season, the cumulative NH3 emissions of UI, OCF and CSF treatments were 22.60–25.55 % significantly (p < 0.05) higher than U, respectively. The cumulative methane emissions in all N-fertilizer treatments were decreased by 9.23–28.24 % compared with CK. The cumulative carbon dioxide emissions were U > CSF > OCF > CK > UI, and the cumulative nitrous oxide (N2O) emissions were U > CSF > UI > CK > OCF. The global warming potential of UI was 5.39–32.35 % lower than all the other treatments. Compared with the other N-fertilizer treatments, the relative abundance of N-related functional bacteria (e.g., Sphingomonas and Alphaproteobacteria) in OCF treatment was increased, and the (nirS + nirK)/nosZ ratio was reduced, thereby decreasing the N2O emission. Overall, U is a better choice for controlling NH3 volatilization in saline-alkali paddy ecosystems, whereas UI and OCF are more beneficial for reducing GHG emissions.

Nitrogen migration and transformation in a saline-alkali paddy ecosystem with application of different nitrogen fertilizers.

With the increasing transformation of saline-alkali land into paddy, the nitrogen (N) loss in saline-alkali paddy fields becomes an urgent agricultural-environmental problem. However, N migration and transformation following the application of different N fertilizers in saline-alkali paddy fields remains unclear. In this study, four types of N fertilizers were tested to explore the N migration and transformation among water-soil-gas-plant media in saline-alkali paddy ecosystems. Based on the structural equation models, N fertilizer types can change the effects of electrical conductivity (EC), pH, and ammonia-N (NH4+-N) of surface water and/or soil on ammonia (NH3) volatilization and nitrous oxide (N2O) emission. Compared with urea (U), the application of urea with urease-nitrification inhibitors (UI) can reduce the potential risk of NH4+-N and nitrate-N (NO3--N) loss via runoff, and significantly (p < 0.05) reduce the N2O emission. However, the expected effectiveness of UI on NH3 volatilization control and total N (TN) uptake capacity of rice was not achieved. For organic-inorganic compound fertilizer (OCF) and carbon-based slow-release fertilizer (CSF), the average TN concentrations in surface water at panicle initiation fertilizer (PIF) stage were reduced by 45.97% and 38.63%, respectively, and the TN contents in aboveground crops were increased by 15.62% and 23.91%. The cumulative N2O emissions by the end of the entire rice-growing season were also decreased by 103.62% and 36.69%, respectively. Overall, both OCF and CSF are beneficial for controlling N2O emission and the potential risks of N loss via runoff caused by surface water discharge, and improving the TN uptake capacity of rice in saline-alkali paddy fields.

Incorporating DNA-level microbial constraints helps decipher methane emissions from Chinese water-saving ground cover rice production systems

• Relating CH 4 fluxes to methanogens and methanotrophs under GCRPS remains unclear. • GCRPS reduced CH 4 emissions and the yield-scaled CH 4 emissions. • GCRPS decreased the abundance of mcrA gene while stimulated that of pmoA gene. • Microbial parameters are useful to better predict CH 4 emissions from rice paddies. While water management has been documented to regulate CH 4 emissions from rice paddies, the mechanisms behind the association between CH 4 emissions and functional microbes remain unclear under water-saving ground cover rice production systems (GCRPS). In GCRPS, the soil is kept aerobic by maintaining 80 %–90 % of the water holding capacity but without standing water over the entire rice-growing season, and the soil surface is covered with rice straw or plastic film to reduce water evaporation. A split-plot field experiment was conducted to examine the effects of various GCRPS-related water-saving regimes on CH 4 emissions by linking to CH 4 -related microbes over the 2015 and 2016 rice growing seasons in a typical Chinese rice field. Methane fluxes and related functional genes [methanogens ( mcrA and methanogenic archaeal 16S rRNA ) and methanotrophs ( pmoA )] abundances were simultaneously measured using the closed chamber method and molecular techniques, respectively. The results showed that relative to the conventional waterlogged control (WRPS), GCRPS-related water-saving practices consistently reduced CH 4 emissions, which was largely attributed to the decreased relative abundance of the methanogenic functional gene mcrA while the methanotrophic functional gene pmoA increased. When averaged over the two rice seasons, the grain yield was 17.38 % and 12.22 % greater under the GCRPS -straw and GCRPS -film water-saving systems relative to WRPS, respectively. Water-saving regimes decreased the yield-scaled CH 4 emissions as compared with the WRPS, especially under GCRPS -film . The CH 4 fluxes showed a positive correlation with the relative abundance of mcrA and its ratio to pmoA , but a negative correlation with the relative abundance of pmoA . The performance of CH 4 -simulated models could be improved by incorporating microbial parameters to predict CH 4 emissions from rice fields. Overall, this study updates our understanding of the microbial mechanisms underlying CH 4 emissions from water-saving GCRPSs. We proposed the GCRPS regime with biodegradable film mulching as a desirable water-saving strategy to reconcile high yields and low CH 4 emissions in rice production.

Simultaneous quantification of N2 , NH3 and N2 O emissions from a flooded paddy field under different N fertilization regimes.

Gaseous nitrogen (N) emissions, especially emissions of dinitrogen (N2 ) and ammonia (NH3 ), have long been considered as the major pathways of N loss from flooded rice paddies. However, no studies have simultaneously evaluated the overall response of gaseous N losses to improved N fertilization practices due to the difficulties to directly measure N2 emissions from paddy soils. We simultaneously quantified emissions of N2 (using membrane inlet mass spectrometry), NH3 and nitrous oxide (N2 O) from a flooded paddy field in southern China over an entire rice-growing season. Our field experiment included three treatments: a control treatment (no N addition) and two N fertilizer (220kgN/ha) application methods, the traditional surface application of N fertilizer and the incorporation of N fertilizer into the soil. Our results show that over the rice-growing season, the cumulative gaseous N losses from the surface application treatment accounted for 13.5% (N2 ), 19.1% (NH3 ), 0.2% (N2 O) and 32.8% (total gaseous N loss) of the applied N fertilizer. Compared with the surface application treatment, the incorporation of N fertilizer into the soil decreased the emissions of NH3 , N2 and N2 O by 14.2%, 13.3% and 42.5%, respectively. Overall, the incorporation of N fertilizer into the soil significantly reduced the total gaseous N loss by 13.8%, improved the fertilizer N use efficiency by 14.4%, increased the rice yield by 13.9% and reduced the gaseous N loss intensity (gaseous N loss/rice yield) by 24.3%. Our results indicate that the incorporation of N fertilizer into the soil is an effective agricultural management practice in ensuring food security and environmental sustainability in flooded paddy ecosystems.

Movement and Retention of NH4-N in Wetland Rice Soils as Affected by Urea Application Methods

A field experiment was conducted to investigate the dynamics of ammonium nitrogen (NH4+-N) in the soil during an entire rice-growing season. The NH4+-N dynamics were measured in paddy soils from two N application methods, namely, urea deep placement (UDP) and broadcast prilled urea (PU). The pore water samples from a 10 cm soil depth were collected using a “rhizon sampler.” The samples were collected at 0, 7, 10, 14, 20, and 22 cm from the urea briquette (UB) placement point at 7, 14, 21, 35, 64, and 83 days after transplanting (DAT) of rice. The NH4+-N in the floodwater sample was measured for a week after each split application of PU. UDP retained NH4+-N at the placement site (7–10 cm depth) until 64 DAT. A small amount of NH4+-N moved horizontally up to 14 cm from the placement site. It’s movement to the soil surface and floodwater was very low to negligible. In contrast, PU produced more NH4+-N in both the floodwater and in the soil surface. Therefore, broadcast urea had significantly (p < 0.05) higher ammonia volatilization (~15% of applied N) compared to UDP (<1%). UDP significantly (p < 0.05) increased grain yields, N uptake, and N recovery compared to broadcast urea. These results confirm that a single application of UDP could meet plants’ N demand throughout the rice-growing period, particularly for short- and medium-duration rice varieties.

Microbial explanations for field-aged biochar mitigating greenhouse gas emissions during a rice-growing season.

Knowledge about the impacts of fresh and field-aged biochar amendments on greenhouse gas (CH4, N2O) emissions is limited. A field experiment was initiated in 2012 to study the effects of fresh and field-aged biochar additions on CH4 and N2O emissions and the associated microbial activity during the entire rice-growing season in typical rice-wheat rotation system in Southeast China. CH4 and N2O fluxes were monitored, and the abundance of methanogen (mcrA), methanotrophy (pmoA), ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nitrite reductase (nirS, nirK), N2O reductase (nosZ), and potential soil enzyme activities related to CH4 and N2O were simultaneously measured throughout different rice developmental stages. There were three treatments: control (urea without biochar), fresh BC (urea with fresh biochar added in 2015), and aged BC (urea with 3-year field-aged biochar added in 2012). Results showed that field-aged biochar significantly decreased seasonal CH4 emissions by 16.8% in relation to the fresh biochar, though no significant differences were detected between biochars and control treatment. The structural equation model indicated that soil pH, microbial biomass carbon (MBC), pmoA, and mcrA were the main factors directly influenced by fresh and aged biochar amendments; aged biochar showed a negative effect while fresh biochar showed positive effects on CH4 fluxes. Both fresh and field-aged biochar obviously increased AOA and AOB abundances and reduced the (nirS+nirK)/nosZ ratio during the entire rice-growing season, although no significant effects were observed on seasonal N2O emissions. Therefore, biochar amendment produced long-term effects on total CH4 and N2O emissions through observed influences of soil pH and functional gene abundance. The figure shows how fresh and field-aged biochar differentially affected CH4 production and oxidation and N2O production and reduction through related functional gene abundances. Blue arrows indicate suppressing while pink arrows indicate promoting effect.

Different effects of biochar and a nitrification inhibitor application on paddy soil denitrification: A field experiment over two consecutive rice-growing seasons

Biochar and nitrification inhibitors are increasingly being proposed as amendments to improve nitrogen use efficiency (NUE). However, their effects on soil denitrification and the major N loss in rice paddies over an entire rice-growing season are not well understood. In this study, using intact soil core incubation combined with N2/Ar technique, the impacts of biochar and a nitrification inhibitor (Ni), 2-chloro-6-(trichloromethyl)-pyridine, on rice yield and soil denitrification, as well as ammonia (NH3) volatilization, were investigated over two rice-growing seasons in the Taihu Lake region of China. Field experiments were designed with four treatments: N0 (no N applied), N270 (270kg N ha−1 applied), N270+C (25tha−1 biochar applied) and N270+Ni (2-chloro-6- [trichloromethyl] -pyridine, 1.35kgha−1N applied). Compared with single application of N fertilizer alone (N270), biochar (N270+C) and Ni (N270+Ni) applications increased rice yields by 4.2–5.2% and 6.2–7.3%, respectively. The cumulative N2-N and NH3-N losses in different treatments varied from 11.9 to 21.8% and from 11.5 to 22.0% of the applied N, respectively. Compared with the single application of N fertilizer, the Ni application increased total NH3 emission by 4.0–20.6% and significantly decreased total N2-N emission by 9.7–19.4% (p<0.05), while the biochar application increased total NH3 and N2-N emissions by 8.6–17.9% and 3.3–9.7%, respectively. Overall, the biochar application resulted in an 11–15% higher net gaseous N than the Ni application. Although the biochar application may increase the rice yield and consequently the plant N uptake, it also promoted N loss more than Ni. Therefore biochar may not be good for maintaining soil fertility over a long period. Instead, applying Ni may be an optimal practice to ensure food security, while decreasing gaseous N loss, for rice production in the Taihu Lake region of China.

Greenhouse gas emissions during the seedling stage of rice agriculture as affected by cultivar type and crop density

Methane (CH4) and nitrous oxide (N2O) emissions from a paddy nursery at the rice seedling stage were measured on a daily basis by using the conventional rice cultivar Nangeng 56 under both conventional (NG-C) and reduced (NG-R) sowing density, and the hybrid rice Changyou 3 under both conventional (CY-C) and reduced (CY-R) sowing density. High N2O and CH4 emissions were observed during the first and last 2 weeks, respectively. Cumulative CH4 emissions were significantly (P < 0.001) affected by sowing density rather than by the rice cultivar. Cumulative CH4 emissions reached 68.2 kg C ha−1 in the CY-C treatment and 121.6 kg C ha−1 in the NG-C treatment, which were significantly (P < 0.001) higher than the emissions at reduced sowing densities (15.9 kg C ha−1 in the CY-R treatment and 20.9 kg C ha−1 in the NG-R treatment). Under the conventional sowing density, cumulative CH4 emissions during the seedling stage were comparable to data of rice-growing season. Both the rice cultivar and the sowing density significantly (P < 0.05–0.01) affected cumulative N2O emissions. Relative to the CY cultivar, the NG cultivar increased global warming potential (GWP) over a 100-year horizon by 62.1% and 70.7% under the reduced and conventional sowing densities, respectively. The GWP of N2O and CH4 during the seedling stage was equivalent to the GWP of the entire rice-growing season in this region, indicating that the seedling stage is an important greenhouse gas emission source of rice agriculture.

The influence of free-air CO 2 enrichment on microorganisms of a paddy soil in the rice-growing season

The atmospheric CO 2 concentration is dramatically rising, and this rise may affect soil methanogens, methanotrophs, nitrifiers, and denitrifiers, which are important microorganisms for the processes of carbon and nitrogen turnover. An experimental platform of free-air CO 2 enrichment (FACE) was established in mid-June of 2001 over a rice–wheat rotation ecosystem located in a suburb of Wuxi, China, and its CO 2 fumigation was continued until mid-February of 2004. Using the most probable number (MPN) method, we measured the numbers of methanogens, methanotrophs, nitrifiers, and denitrifiers by sampling fresh soils from the fields exposed to the elevated and ambient CO 2 during the rice-growing season in 2002. Our results show that the elevated CO 2 significantly increased methanogen populations of the cultivated soil layers during the entire rice-growing season. This positive effect of elevated CO 2 may be attributed to stimulated rice growth, which may provide more substrates for methanogens. The methanotroph population was decreased by elevated CO 2 in the upper soil layer (0–5 cm) but was increased in the lower one (5–10 cm) in most rice-growing stages, and the effect of CO 2 elevation was reversed at rice maturity. Elevated CO 2 increased nitrifier and denitrifier populations in most rice stages, but it occasionally decreased the number of nitrifiers late in the growing season and that of denitrifiers early. The methanogen population gradually increased until the filling stage of rice growth but then declined under either elevated or ambient CO 2. Meanwhile the numbers of methanotrophs and nitrifiers gradually decreased during the entire rice season. The number of denitrifiers in the wet/flooded soil during the growing season was also decreased as compared to the dry soil before rice season.

Comparing field and microcosm experiments: a case study on methano- and methylo-trophic bacteria in paddy soil

Methane-oxidising bacteria (MOB) play an important role in the reduction of methane emissions from rice agriculture. In rice fields, they are subjected to many environmental and field management parameters, which may have a significant impact on their community composition. To study this in greater detail, the community structure of methano- and methylo-trophic bacteria was investigated in a rice field in northern Italy during the summer 1999 and compared to a microcosm study described previously. We used PCR-based denaturing gradient gel electrophoresis applying 16S rDNA (9α and 10γ) and mxaF (methanol-dehydrogenase) primer sets. In parallel, population size and activity of MOB were determined. This study provides the first comprehensive investigation of different compartments (bulk soil, rhizosphere, rhizoplane, and homogenate) throughout an entire rice-growing season in the field. Lower cell numbers of MOB were detected in the field compared to the microcosms, possibly due to lower CH 4 concentrations in the soil pore water. In both studies, growth of MOB occurred predominantly at the root surface (rhizoplane) and in the root (homogenate), whereas cell numbers in bulk soil showed only minor changes throughout the season. Molecular analysis detected only few changes in α-proteobacterial methylotrophs during the season, whereas a higher variability was detected in γ-proteobacteria. Nevertheless, the sequences of electrophoretic bands showed that the diversity in the field study and in the microcosms was comparable. Activity patterns of MOB and the population structure of methylotrophic bacteria agreed well between both studies, even though the detected quantities differed. Extrapolations of microcosm data to the field scale are thus possible, but should be used carefully when concerning quantitative changes.

Carbon dioxide, methane, and nitrous oxide emissions from a rice-wheat rotation as affected by crop residue incorporation and temperature

Field measurements were made from June 2001 to May 2002 to evaluate the effect of crop residue application and temperature on CO2, CH4, and N2O emissions within an entire rice-wheat rotation season. Rapeseed cake and wheat straw were incorporated into the soil at a rate of 2.25 t hm−2 when the rice crop was transplanted in June 2001. Compared with the control, the incorporation of rapeseed cake enhanced the emissions of CO2, CH4, and N2O in the rice-growing season by 12.3%, 252.3%, and 17.5%, respectively, while no further effect was held on the emissions of CO2 and N2O in the following wheatgrowing season. The incorporation of wheat straw enhanced the emissions of CO2 and CH4 by 7.1% and 249.6%, respectively, but reduced the N2O emission by 18.8% in the rice-growing season. Significant reductions of 17.8% for the CO2 and of 12.9% for the N2O emission were observed in the following wheatgrowing season. A positive correlation existed between the emissions of N2O and CO2 (R 2 = 0.445,n = 73,p < 0.001) from the rice-growing season when N2O was emitted. A trade-off relationship between the emissions of CH4 and N2O was found in the rice-growing season. The CH4 emission was significantly correlated with the CO2 emission for the period from rice transplantation to field drainage, but not for the entire rice-growing season. In addition, air temperature was found to regulate the CO2 emissions from the non-waterlogged period over the entire rice-wheat rotation season and the N2O emissions from the nonwaterlogged period of the rice-growing season, which can be quantitatively described by an exponential function. The temperature coefficient (Q 10) was then evaluated to be 2.3±0.2 for the CO2 emission and 3.9±0.4 for the N2O emission, respectively.

Effects of rice cultivars on methane fluxes in a paddy soil

CH4 emission and its relevant processes involved (i.e. CH4 production, rhizospheric CH4 oxidation and plant-mediated CH4 transport) were studied simultaneously to comprehensively understand how rice cultivars (Yanxuan, 72031, and 9516) at growth stages (early and late tillering, panicle initiation, ripening, and harvest stage) affect CH4 emission in a paddy soil. Over the entire rice-growing season, Yanxuan had the highest CH4 emission flux with 5.98 μg CH4 m−2 h−1 followed by 72031 (4.48 μg CH4 m−2 h−1) and 9516 (3.41 μg CH4 m−2 h−1). The highest CH4 production rate of paddy soils planted to Yanxuan was observed with 18.0 μg CH4 kg{ (d.w.soil)} h−1 followed by the soil planted to 9516 (17.5 μg CH4 kg{ (d.w.soil)} h−1). For each cultivar, both rhizospheric CH4 oxidation ability and plant-mediated CH4 transport efficiency varied widely with a range of 9.81–76.8% and 15.5–80.5% over the duration of crop growth, respectively. Multiple regression analyses showed that CH4 emission flux was positively related with CH4 production rate and rice plant-mediated CH4 transport efficiency, but negatively with rhizospheric CH4 oxidation (R 2=0.425 for Yanxuan, P<0.01; R 2=0.426 for 72031, P<0.01; R 2=0.564 for 9516, P<0.01). The contribution of rice plants to CH4 production seems to be more important than to rhizospheric CH4 oxidation and plant-mediated transport in impact of rice plants on CH4 emission.