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Aims: Global climate change is characterized by enhanced atmospheric carbon dioxide concentration ([CO2]) and temperature, with unknown consequences for soil nematode communities. Soil nematode in response to elevated [CO2], warming and their interaction in paddy field remain largely unknown. Here we aimed to understand how factorial combinations of elevated [CO2] and canopy warming affect soil nematode in a rice paddy field. Methods: A rice paddy field was consistently treated with elevated [CO2] (500 ppm), canopy warming (+2 °C) or their combinations. Soil samples after a two-year treatment were collected during the rice growing season and nematode communities were extracted with a modified Baermann funnel extraction to examine the changes in nematode abundance and composition under climate change. Results: Soil nematode communities were altered by elevated [CO2] and warming, but these responses were dependent on rice growing stages. When averaged over the four stages, total nematode abundances were increased by 31.5% under elevated [CO2], and by 25.7% under warming. Elevated [CO2] had no effect on nematode diversity, but slightly altered the composition of different trophic groups. In contrast, warming decreased nematode diversity, but increased plant parasite index, which was negative correlated with crop production. This was attributed to increases in the relative abundance of herbivores under simulated climate change conditions. Conclusions: Elevated [CO2] and warming had a positive effect on nematode abundance, but potentially reduced nematode diversity and soil health. These results suggest that multi-factors interactively affect the responses of soil nematode communities, which is important for food productivity under climate change. [ABSTRACT FROM AUTHOR]

Background and aims: Urea is widely used in rice farming systems to improve the nitrogen (N) availability and crop yield, yet its influence on CH4 emissions remains unclear. The present study aimed to identify the potential microbial mechanisms resulting in the N fertilizer-induced effects on CH4 emissions from paddies. Method: A field experiment was conducted with (N1, 100 kg N ha−1) and without (N0) N fertilization. CH4 emission rate was measured every 3–7 day using the static chamber-gas chromatography method. Quantitative PCR (qPCR) and Illumina MiSeq sequencing approaches were employed to investigate the community composition and structure of methanogenic archaea and methanotrophic bacteria in rice rhizosphere soil at three time points (tillering stage 17d, heading stage 47d and mature stage 67 d). Results: Urea application significantly stimulated the CH4 emissions from paddies, partially due to the decrease of redox potential (Eh) around the root region and the reduction of dissolved oxygen (DO) concentration at the soil-water interface. qPCR indicated a sharp increase in methanogenic archaeal 16S rRNA and methanotrophic bacterial pmoA gene copies with urea fertilization. Next-generation sequencing of these amplicons revealed that the diversity of both methanogenic and methanotrophic communities was decreased by urea input. Additionally, urea fertilization stimulated the genera Methanoregula (methanogen) and Methylococcus (type I methanotrophs) and inhibited the genus Methylocystis (type II methanotrophs). Conclusions: Urea fertilization changed oxygen state of the paddy soil and water and thus changed the community structure of methanogens and methanotrophs, which resulted in increased CH4 emissions. [ABSTRACT FROM AUTHOR]

Background and aims: Elevated CO2 and nitrogen addition can increase rice yield and stimulate rice to absorb more nutrients from the soil. The aim of this study was to investigate how soil organic carbon and aggregates change under such high-output conditions.Methods: Rice cultivars IIY 084 and WYJ 21 (strong and weak response to elevated CO2, respectively) were planted in free-air CO2 enrichment (FACE) platforms. Treatments included: 1) elevated CO2 without nitrogen addition (EC); 2) nitrogen addition and ambient CO2 (EN); 3) combined elevated CO2 and nitrogen addition (ECEN); and 4): ambient CO2 without N addition (CK). Soil organic carbon (SOC), readily-oxidizable organic carbon (ROC) and soil aggregate components were measured.Results: Treatment EC increased SOC and ROC, while EN decreased SOC and ROC compared to CK in soil under either rice cultivar. EC and EN decreased the percentage of large macroaggregates (LM) and the mean weight diameter (MWD) of aggregates compared to CK in 'WYJ 21' soil, while they increased LM and MWD in 'IIY 084' soil. ECEN increased LM and MWD compared to CK in soil under either rice cultivar.Conclusions: High productivity of 'IIY 084' through long-term cultivation under current N fertilization in future climatic conditions of elevated CO2 could be achieved due to increased soil aggregate stability and SOC. [ABSTRACT FROM AUTHOR]

Aims: Two pot experiments in a “walk-in” growth chamber with controlled day and night temperatures were conducted to investigate the influence of elevated temperatures along with rice straw incorporation on methane (CH 4) and nitrous oxide (N 2O) emissions as well as rice yield. Methods: Three temperature regimes–29/25, 32/25, and 35/30 °C (Exp. I) and 29/22, 32/25, and 35/28 °C (Exp. II), representing daily maxima/minima were used in the study. Two amounts of rice straw (0 and 6 t ha −1) were applied with four replications in each temperature regime. CH 4 and N 2O emissions as well as soil redox potential (Eh) were monitored weekly throughout the rice-growing period. Results: Elevated temperatures increased CH 4 emission rates, with a more pronounced effect from flowering to maturity. The increase in emissions was further enhanced by incorporation of rice straw. A decrease in soil Eh to <−100 mV and CH 4 emissions was observed early in rice straw–incorporated pots while the soil without straw did not reach negative Eh levels (Exp. I) or showed a delayed decrease (Exp. II). Moreover, soil with high organic C (Exp. II) had higher CH 4 emissions. In contrast to CH 4 emissions, N 2O emissions were negligible during the rice-growing season. The global warming potential (GWP) was highest at high temperature with rice straw incorporation compared with low temperature without rice straw. On the other hand, the high temperature significantly increased spikelet sterility and reduced grain yield ( p < 0.05). Conclusions: Elevated temperature increased GWP while decreased rice yield. This suggests that global warming may result in a double negative effect: higher emissions and lower yields. [ABSTRACT FROM AUTHOR]

Purpose: We attempted to determine the contribution of entrapped gas bubbles to the soil methane (CH) pool and their role in CH emissions in rice paddies open to the atmosphere. Methods: We buried pots with soil and rice in four treatments comprising two atmospheric CO concentrations (ambient and ambient +200 μmol mol) and two soil temperatures (ambient and ambient +2 °C). Pots were retrieved for destructive measurements of rice growth and the gaseous CH pool in the soil at three stages of crop development: panicle formation, heading, and grain filling. Methane flux was measured before pot retrieval. Results: Bubbles that contained CH accounted for a substantial fraction of the total CH pool in the soil: 26-45 % at panicle formation and 60-68 % at the heading and grain filling stages. At panicle formation, a higher CH mixing ratio in the bubbles was accompanied by a greater volume of bubbles, but at heading and grain filling, the volume of bubbles plateaued and contained ~35 % CH. The bubble-borne CH pool was closely related to the putative rice-mediated CH emissions measured at each stage across the CO concentration and temperature treatments. However, much unexplained variation remained between the different growth stages, presumably because the CH transport capacity of rice plants also affected the emission rate. Conclusions: The gas phase needs to be considered for accurate quantification of the soil CH pool. Not only ebullition but also plant-mediated emission depends on the gaseous-CH pool and the transport capacity of the rice plants. [ABSTRACT FROM AUTHOR]

The diazotrophic communities in a rice paddy field were characterized by a molecular polyphasic approach including DNA/RNA-DGGE fingerprinting, real time RT-PCR analysis of nifH gene and the measurement of nitrogen fixation activities. The investigation was performed on a diurnal cycle and comparisons were made between bulk and rhizosphere / root soil as well as between fertilized / unfertilized soils. Real time RT-PCR showed no significant difference in the total quantity of nifH expression under the conditions investigated. The functional diversity and dynamics of the nifH gene expressing diazotroph community investigated using RT-PCR-DGGE revealed high diurnal variations, as well as variation between different soil types. Most of the sequence types recovered from the DGGE gels and clone libraries clustered within nifH Cluster I and III (65 different nifH sequences in total). Sequence types most similar to Azoarcus spp., Metylococcus spp., Rhizobium spp., Methylocystis spp., Desulfovibrio spp. , Geobacter spp. , Chlorobium spp., were abundant and indicate that these species may be responsible for the observed diurnal variation in the diazotrophic community structure in these rice field samples. Previously described diazotrophic cyanobacterial genera in rice fields, such as Nostoc and Cyanothece, were present in the samples but not detectable in RT-PCR assays. [ABSTRACT FROM AUTHOR]

The effect of water percolation on CH4 emission from ricepaddies was investigated using a lysimeter experimen for 4 years. Water percolation rates of the lysimeters were adjusted to one of threevalues, about 0, 5, or 20 mm day-1, during the cultivation periods of the first 3 years by Periodically opening water stoppingvalves installed at the bottom of the lysimeters. In the 4th year, water was drained by pumps at a rate of 0, 7.7, or 25 mm day- 1, respectively. CH4 emission rates significantly decreased with an increase in the percolation rates. Total emission duringa cultivation period ranged from 5.7 to 13.8, 0.6 to 4.8. and 0.1 to0.3 g m-2 in the no-, moderate-, high-percolation plots, respectively. The decrease in soil Eh during the cultivation periods was markedly retarded by water percolation. The critical initiation soil Eh of CH4 emission observed was around -30 mV at 5 cm depth. CH4 emissions during the fallow periods (from September to May) were equivalent to 14--18% of those during the previous cultivation period if the paddies were continuously flooded all the year round. Low soil temperature during the fallow periods was a majorlimiting factor for CH4 emission during these periods in the case of continuously flooded treatment. [ABSTRACT FROM AUTHOR]

Methane and N2O emissions affected by nitrogen fertilisers were measured simultaneously in rice paddy fields under intermittent irrigation in 1994. Ammonium sulphate and urea were applied at rates of 0 (control), 100 and 300 kg N ha-1. The results showed that CH4 emission, on the average, decreased by 42 and 60% in the ammonium sulphate treatments and 7 and 14% in the urea treatments at rates of 100 and 300 kg N ha-1, respectively, compared to the control. N2O emission increased significantly with the increase in the nitrogen application rate. N2O emission was higher from ammonium sulphate treatments than from the urea treatments at the same application rate. A trade-off effect between CH4 and N2O emission was clearly observed. The N2O flux was very small when the rice paddy plots were flooded, but peaked at the beginning of the disappearance of floodwater. In contrast, the CH4 flux peaked during flooding and was significantly depressed by mid-season aeration (MSA). The results suggest that it is important to evaluate the integrative effects of water management and fertiliser application for mitigating greenhouse gas emissions in order to attenuate the greenhouse effect contributedby rice paddy fields. [ABSTRACT FROM AUTHOR]

Rice paddies emit not only methane but also several volatile sulfur compounds such as dimethyl sulfide (DMS: CH3SCH3). However, little is known about DMS emission from rice paddies. Fluxes of methane and DMS, and the concentrations of methane and several volatile sulfur compounds including hydrogen sulfide (H2S), carbonyl disulfide (CS2), methyl mercaptan (CH3SH) and DMS in soil water and flood water were measured in four lysimeter rice paddies (2.5 W 4 m, depth 2.0 m) once per week throughout the entire cultivation period in 1995 in Tsukuba, Japan. The addition of exogenous organic matter (rice straw) was also examined for its influence on methane or DMS emissions. Methane fluxes greatly differed between treatments in which rice straw had been incorporated intothe paddy soil (rice straw plot) and plots without rice straw (mineral fertilizer plot). The annual methane emission from the rice straw plots (37.7 g m-2) was approximately 8 times higher than that from the mineral fertilizer plots (4.8 g m-2). Application of rice straw had little influence on DMS fluxes. Significant diurnal and seasonal changes in DMS fluxes were observed. Peak DMS fluxes were found around noon. DMS was emitted from the flood water in the early growth stage of rice and began to be emitted from rice plantsduring the middle stage. DMS fluxes increased with the growth of rice plants and the highest flux, 15.1 5g m-2 h-1,was recorded before heading. DMS in the soil water was negligible during the entire cultivation period. These facts indicate that the DMSemitted from rice paddies is produced by metabolic processes in riceplants. The total amount of DMS emitted from rice paddies over the cultivated period was estimated to be approximately 5-6 mg m-2. CH3SH was emitted only from flood water during the first month after flooding. [ABSTRACT FROM AUTHOR]

Diurnal variation in CH4 efflux from continuously floodedfields planted to rice (Oryza sativa L. cv. IR-36) was examined at different crop growth stages using a closed chamber method during the wet season. CH4 emission showed a distinct diurnal patternespecially at tillering, panicle initiation and maturity stages of afield-grown rice crop, with maximum emission in the early afternoon (12.00 to 15.00) followed by a decline to a minimum around midnight. Among several variables (ambient temperature, flood water temperature, redox potential, soil pH, and root oxidase activity), a significantnegative correlation existed between oxidase activity of the root base and diurnal fluctuations in CH4 efflux at tillering stage. Evidence also suggested that redox status in the rhizosphere region and atmospheric, soil, and water temperatures influenced CH4 emission from rice fields probably by their contrasting effectson CH4 production and oxidation. [ABSTRACT FROM AUTHOR]

Ebullition of gas bubbles from the soil surface is, in some cases (e.g., in early growth stage of rice), one of the major pathways for methane transport from rice paddies to the atmosphere. However, the role of the gas phase (entrapped gas) in the paddy soil in plant-mediated methane transport, which is the major pathway for methane emission,has not been clarified. To clarify the effect of the gas phase belowground on the methane emission rate through rice plants, we partly exposed the root and stem base of hydroponically grown rice to a high concentration of methane gas at various gas pressures, and immersed the rest of the roots in a solution with a high methane concentration.The methane emission rate was measured from the top of the rice plant using a flow-through chamber method. The methane emission rate drastically increased with a small increase in gas pressure in the gas phase at the root and stem base zone, with about a 3 times larger emission rate being observed with 10 W 10-3 atm of extra pressure (corresponding to 10 cm of standing water in rice paddy) compared to no extra pressure. However, when alginate was applied to the stem near the base to prevent contact with the gas phase, the methane emission rate did not increase with increasing gas pressure. On the otherhand, from observations in the rice paddy, it was found that the gasis entrapped near the surface (e.g., at a depth of 1 cm) and the gasentrapped in the soil would come into direct contact with a part of the stem near the base of the rice plant. Thus, the gas entrapped in the soil could enter into the rice body directly from the part of thestem near the base which is beneath the soil surface due to gas pressure in the gas phase resulting from the pressure exerted by the standing water. Hence, this mechanism involving the entrapped gas could play an important role in methane emission from rice paddy by affecting the plant-mediated methane transport as well as ebullition of gas bubbles. [ABSTRACT FROM AUTHOR]