Soil organic carbon stability and exogenous nitrogen fertilizer influence the priming effect of paddy soil under long-term exposure to elevated atmospheric CO2.

Soil organic carbon (SOC) stability and dynamics are greatly influenced by long-term elevated atmospheric CO2 [CO2]. The priming effect (PE) is vital in SOC stability and dynamics, but its role in paddy soil under long-term elevated [CO2] remains unclear. To examine how SOC stability changed in paddy soil after long-term elevated atmospheric CO2 enrichment, the PE was quantified through a 13C-glucose-induced experiment with different N levels for topsoil (0-20 cm) from paddy free-air CO2 enrichment (FACE) platform. Compared with the ambient CO2 concentration ([CO2]), 10 years of elevated [CO2] (500 µmol·mol-1) significantly increased SOC and TN content by 18.4% and 19.0%, respectively, while the C/N ratio was not changed. The labile C fractions including dissolved organic carbon (DOC) and readily oxidizable organic carbon (ROC), but excluding microbial biomass C (MBC), accumulated faster than SOC in paddy soil, which implied the reduced SOC stability for long-term elevated [CO2] enrichment. With the decline of SOC stability, the exogenously induced cumulative specific PE (PE per gram of SOC) remarkably increased by 41.1-72.7% for elevated [CO2] fumigation. The cumulative PE, especially the cumulative specific PE, was found significantly linearly correlated with the ROC content or ROC/SOC ratio (labile SOC pool). Furthermore, the application of nitrogen fertilizer slowed down the PE under elevated [CO2] condition. Our results showed that long-term elevated [CO2] enrichment reduced SOC stability and, together with exogenous nitrogen fertilizer, regulated the PE in paddy soil.

Methanogenic and methanotrophic communities determine lower CH4 fluxes in a subtropical paddy field under long-term elevated CO2.

Clarifying the effects of elevated CO2 concentration (e[CO2]) on CH4 emissions from paddy fields and its mechanisms is a crucial part of the research on agricultural systems in response to global climate change. However, the response of CH4 fluxes from rice fields to long-term e[CO2] (e[CO2] duration >10 years) and its microbial mechanism is still lacking. In this study, we used a long-term free-air CO2 enrichment experiment to examine the relationship between CH4 fluxes and the methanogenic and methanotrophic consortia under long- and short-term e[CO2]. We demonstrated that contrary to the effect of short-term e[CO2], long-term e[CO2] decreased CH4 fluxes. This may be associated with the reduction of methanogenic abundance and the increase of methanotrophic abundance under long-term e[CO2]. In addition, long-term e[CO2] also changed the community structure and composition of methanogens and methanotrophs compared with short-term e[CO2]. Partial least squares path modeling analysis showed that long-term e[CO2] also could affect the abundance and composition of methanogens and methanotrophs indirectly by influencing soil physical and chemical properties, thereby ultimately altering CH4 fluxes in paddy soils. These findings suggest that current estimates of short-term e[CO2]-induced CH4 fluxes from paddy fields may be overestimated. Therefore, a comprehensive assessment of climate­carbon cycle feedbacks may need to consider the microbial regulation of CH4 production and oxidation processes in paddy ecosystems under long-term e[CO2].

Elevated CO2 alleviated the dissemination of antibiotic resistance genes in sulfadiazine-contaminated soil: A free-air CO2 enrichment study.

Climate change affects soil microbial communities and their genetic exchange, and subsequently modifies the transfer of antibiotic resistance genes (ARGs) among bacteria. However, how elevated CO2 impacts soil antibiotic resistome remains poorly characterized. Here, a free-air CO2 enrichment system was used in the field to investigate the responses of ARGs profiles and bacterial communities to elevated CO2 (+200 ppm) in soils amended with sulfadiazine (SDZ) at 0, 0.5 and 5 mg kg-1. Results showed that SDZ exposure induced the co-occurrence of beta-lactamase and tetracycline resistance genes, and SDZ at 5 mg kg-1 enhanced the abundance of aminoglycoside, sulfonamide and multidrug resistance genes. However, elevated CO2 weakened the effects of SDZ at 0.5 mg kg-1 following an observed reduction in the total abundance of ARGs and mobile genetic elements. Additionally, elevated CO2 significantly decreased the abundance of vancomycin resistance genes and alleviated the stimulation of SDZ on the dissemination of aminoglycoside resistance genes. Correlation analysis and structural equation models revealed that elevated CO2 could directly influence the spread of ARGs or impose indirect effects on ARGs by affecting soil properties and bacterial communities. Overall, our results furthered the knowledge of the dissemination risks of ARGs under future climate scenarios.

Does elevated CO2 enhance the arsenic uptake by rice? Yes or maybe: Evidences from FACE experiments.

Elevated CO2 (eCO2) strongly affects rice yield and quality in arsenic (As) paddy soils. However, understanding of the As accumulation in rice under coupled stress of eCO2 and soil As is still limited while data are scarce. It greatly limits the prediction for future rice safety. This study investigated the As uptake by rice grown in different As paddy soils under two CO2 conditions (ambient and ambient +200 µmol mol-1) in the free-air CO2 enrichment (FACE) system. Results showed that eCO2 lowered soil Eh at the tillering stage and caused higher concentrations of dissolved As and Fe2+ in soil pore water. Compared with the control, the increased As transfer abilities in rice straws under eCO2 contributed to the higher As accumulation in rice grains, and their total As concentrations were increased by 10.3-31.2%. Besides, the increased amounts of iron plaque (IP) under eCO2 failed to effectively inhibit the As uptake by rice due to the difference in critical stage between As immobilized by IP (mainly in maturing stage) and uptake by rice roots (about 50% contribution before filling stage). Risk assessments suggest that eCO2 enhanced the human health risks of As intake from rice grains produced in low-As paddy soils (<30 mg kg-1). In order to alleviate the As threats to rice under eCO2, we consider that proper soil drainage before filling stage to improve soil Eh can serve as an effective way to reduce As uptake by rice. Pursuing appropriate rice varieties to reduce the As transfer ability may be the other positive strategy.

Elevated CO2 and temperature under future climate change increase severity of rice sheath blight.

Sheath blight (ShB), caused by Rhizoctonia solani, is one of the major threats to rice (Oryza sativa L.) production. However, it is not clear how the risk of rice ShB will respond to elevated CO2 and temperature under future climate change. Here, we conducted, field experiments of inoculated R. solani under combinations of two CO2 levels (ambient and enriched up to 590 µmol mol-1) and two temperature levels (ambient and increased by 2.0°C) in temperature by free-air CO2 enrichment (T-FACE) system for two cultivars (a susceptible cultivar, Lemont and a resistant cultivar, YSBR1). Results indicate that for the inoculation of plants with R. solani, the vertical length of ShB lesions for cv. Lemont was significantly longer than that for cv. YSBR1 under four CO2 and temperature treatments. The vertical length of ShB lesions was significantly increased by elevated temperature, but not by elevated CO2, for both cultivars. The vertical length of ShB lesions under the combination of elevated CO2 and elevated temperature was increased by 21-38% for cv. Lemont and by -1-6% for cv. YSBR1. A significant increase in MDA level was related to a significant increase in the vertical length of ShB lesions under the combination of elevated CO2 and elevated temperature. Elevated CO2 could not compensate for the negative effect of elevated temperature on yield of both cultivars under future climate change. Rice yield and biomass were further decreased by 2.0-2.5% and 2.9-4.2% by an increase in the severity of ShB under the combination of elevated CO2 and elevated temperature. Thus, reasonable agronomic management practices are required to improve both resistance to ShB disease and grain yield for rice under future climate change.

Investigation on the driver-victim pairs in pedestrian and bicyclist crashes by latent class clustering and random forest algorithm.

Pedestrians and bicyclists from marginalized and underserved populations experienced disproportionate fatalities and injury rates due to traffic crashes in the US. This disparity among road users of different races and the increasing trend of traffic risk for underserved racial groups called for an urgent agenda for transportation policy making and research to ensure equity in roadway safety. Pedestrian and bicyclist crashes involved drivers and pedestrians/bicyclists; the latter were usually victims. Traditional safety studies did not account for the interaction between the two parties and assumed that they were independent from each other. In this study we paired the driver and pedestrian/bicyclist involved in the same crash to understand the socioeconomic and demographic make-up of the two parties involved in crashes and assessed the geographic distribution of these crashes and crash-contributing factors. For this purpose, we applied thelatent class clustering analysis (LCA) to classify different crash types and analyze the patterns of the crashes based on the income and ethnicity of both drivers and victims involved in pedestrian and bicyclist crashes. We then used random forest algorithms and partial dependence plots (PDPs) to model and interpreted the contributing factors of the clusters in both pedestrian and bicyclist models. The clustering results showed a pattern of social segregation in pedestrian and bicyclist crashes that drivers and victims with similar socioeconomic characteristics tend to be involved in one crash. Pedestrian/bicyclist exposure, driver's age, victim's age, year of the car in use, annual average daily traffic (AADT), speed limit, roadbed width, and lane width were the most influential factors contributing to this pattern. Crashes that involved drivers and victims with lower income and non-white ethnicity tended to happen in the location with higher pedestrian/bicyclist exposure, higher speed limit, and wider road. The findings of this research can help to inform the decision-making process for improving safety to ensure equitable and sustainable safety for all road users and communities.

Differential responses of soil bacterial communities to elevated CO2 between strongly CO2-responsive and weakly CO2-responsive rice cultivars.

Effects of elevated CO2 (eCO2) on paddy soil microbial communities remain unclear, particularly when different rice cultivars exposed to eCO2. We thus compared responses of soil bacterial communities to ambient CO2 (aCO2) and eCO2 (aCO2 + 200 µmol CO2 mol-1) between two weakly CO2-responsive (Wuyunjing27, W27; Huaidao5, H5) and two strongly CO2-responsive rice cultivars (Yongyou1540, Y1540; LongIIyou1988, L1988) throughout six growth stages (early tillering, late tillering, jointing, heading, grain filling and ripening) in a paddy field in Jiangdu, China in 2018. No significant changes in soil bacterial diversities were observed between eCO2 and aCO2 or between cultivars for any single growth stage at the OTU level, but α diversity significantly changed at the phylum level except for the ripening stage. For a single cultivar, particularly two strongly CO2-responsive cultivars, over their whole growth stage, eCO2 enhanced differences in bacterial ß diversity at both OTU and phylum levels under H5, Y1540 and L1988. Soil bacterial community complexity at the phylum level under eCO2 was weakened under W27, H5 and Y1540, but enhanced under L1988. eCO2-induced changes in soil DOC under these four cultivars had significantly positive impact on bacterial abundances. However, structural equation modeling illustrated that changes in soil DOC induced by eCO2 significantly decreased soil bacterial community richness. eCO2 did not significantly affect relationships between soil bacterial community diversities and rice yields, but displayed significantly negative correlations between strongly CO2-responsive rice cultivars' yields and relative abundances of Proteobacteria at the ripening stage. Dynamics that how soil microbial communities can differentiate their eCO2 responses between strongly- and weakly-responsive rice cultivars will provide a new insight into promoting rice productivity and soil health.

Effects of elevated CO2 on the Cd uptake by rice in Cd-contaminated paddy soils.

The rising atmospheric CO2 is a major driver for climate change, directly affects rice production. Cadmium (Cd) in paddy soils also serves as a persistent concern. Currently, few studies consider the rice response to coupled stresses of elevated CO2 (eCO2) and soil Cd. Experimental evidence understanding the effects and mechanisms of eCO2 on Cd uptake by rice is lacking yet. In a free-air CO2 enrichment (FACE) system, a 3-year pot experiment was conducted to explore the Cd uptake by rice under two CO2 conditions (ambient and ambient + 200 µmol·mol-1) using combinations of in-situ Cd-contaminated soils and associated rice varieties. Results showed that more low-crystalline Fe oxides (Feh) in iron plaque (IP) were deposited on root surface with the increased dissolved Fe2+ due to lower soil redox status under eCO2. The Cd accumulation in rice was hindered due to more Cd associated with Feh (Feh-Cd) rather than uptake by roots. Taken together, the relative effects of eCO2 on Cd uptake by rice were consistent across years under different Cd-contaminated soils. Our findings will help to better understand the Cd uptake by rice under future climate conditions, and thus push the development of climate-crop-soil models and accurate prediction for food security.

Accelerated sea-level rise is suppressing CO2 stimulation of tidal marsh productivity: A 33-year study.

Accelerating relative sea-level rise (RSLR) is threatening coastal wetlands. However, rising CO2 concentrations may also stimulate carbon sequestration and vertical accretion, counterbalancing RSLR. A coastal wetland dominated by a C3 plant species was exposed to ambient and elevated levels of CO2 in situ from 1987 to 2019 during which time ambient CO2 concentration increased 18% and sea level rose 23 cm. Plant production did not increase in response to gradually rising ambient CO2 concentration during this period. Elevated CO2 increased shoot production relative to ambient CO2 for the first two decades, but from 2005 to 2019, elevated CO2 stimulation of production was diminished. The decline coincided with increases in relative sea level above a threshold that hindered root productivity. While elevated CO2 stimulation of elevation gain has the potential to moderate the negative impacts of RSLR on tidal wetland productivity, benefits for coastal wetland resilience will diminish in the long term as rates of RSLR accelerate.

Elevated CO2 does not necessarily enhance greenhouse gas emissions from rice paddies.

Elevated atmospheric carbon dioxide (eCO2) greatly impacts greenhouse gas (GHG) emissions of CH4 and N2O from rice fields. Although eCO2 generally stimulates GHG emissions in the short term (<5 years) experiments, the responses to long-term (≥10 years) eCO2 remain poorly known. Here we show, through a series of experiments and meta-analysis, that the eCO2 does not necessarily increase CH4 and N2O emissions from rice paddies. In an experiment of free-air CO2 enrichment for 13-15 years, CH4 and N2O emissions were decreased by 11-54% and 33-54%, respectively. The decline of CH4 emissions was related to the reduction of CH4 production and enhancement of CH4 oxidation via raising soil Eh and soil-water interface [O2] under eCO2. Moreover, the eCO2 significantly decreased NH4+-N content, suggesting a reduction of soil nitrification and thereby N2O emissions. A meta-analysis showed that CH4 and N2O emissions were stimulated under short-term eCO2 while reduced under long-term eCO2. The eCO2-induced increase in yield and biomass and the ratio of mcrA genes/pmoA genes declined with eCO2 duration, indicating an eCO2-stimulation of methanogenesis lower than that of methanotrophy over time by fewer increased substrates. Upscaling the results of meta-analysis, the eCO2-induced global paddy CH4 and N2O emissions shifted from an increase (+0.17 Pg CO2-eq year-1) in the short term into a decrease (-0.11 Pg CO2-eq year-1) in the long term. Our findings suggest that the effect of eCO2 on GHG emissions changes over time, and this should be considered in future climate change research.

[Response of Yield, CH4, and N2O Emissions from Paddy Fields to Long-term Elevated CO2 Concentrations].

Elevated atmospheric CO2 concentrations([CO2]e) are the main driving force of global climate change, which directly and indirectly affect carbon and nitrogen cycling in the paddy ecosystems. Therefore, understanding the response of rice yield and greenhouse gas emissions to long-term(more than 10 years)[CO2]e from paddy fields is of great significance for food security and future climate change assessment. In this study, strongly and weakly responsive cultivars were used as the experimental materials. Based on a free-air CO2 enrichment(FACE) platform continuously run for 14 years, two treatments of different[CO2] were set:a control(i.e., normal[CO2] and[CO2]a) and a 200 µmol·mol-1 higher than[CO2]a condition, ([CO2]e). CH4 and N2O emissions from the rice paddy fields were monitored in situ by static transparent chamber-gas chromatography, and grain yields were also obtained. The results showed that compared with the[CO2]a treatment, long-term[CO2]e increased grain yields of the strongly and weakly responsive cultivars by 29%-31%(P<0.05) and 12%-14%(P>0.05), and CH4 emissions of the strongly and weakly responsive cultivars were reduced by 21%-59% and 11%-54%, respectively. Furthermore, N2O emissions from the strongly and weakly responsive cultivars were significantly reduced by 70%(P<0.05) and 40%(P<0.05), respectively. The short- and long-term responses of grain yields and CH4 emissions from rice paddy fields to[CO2]e were significantly different. Specifically, with the increase in the duration of[CO2]e, the increases in rice yields and CH4 emissions significantly decreased, while the N2O emissions showed no significant changes. Therefore, under long-term[CO2]e conditions, the strongly responsive cultivar has a high potential to reduce greenhouse gas emission and increase grain yields.

[Effect of Elevated CO2 on N2O Emissions from Different Rice Cultivars in Rice Fields].

Using the free air CO2 enrichment (FACE) platform, an in-situ field experiment was conducted to explore the impacts of elevated CO2 mole fraction (x[CO2]) on N2O emissions from strongly and weakly responsive rice cultivars. Under elevated x[CO2], grain yield of the strongly responsive rice cultivars increased significantly, by more than 30%, whereas the weakly responsive cultivars showed a growth rate of 10%-15%. The four treatments comprised A-W (normal x[CO2]+weakly responsive cultivar), F-W (elevated x[CO2]+weakly responsive cultivar), A-S (normal x[CO2]+strongly responsive cultivar), and F-S (elevated x[CO2]+strongly responsive cultivar). Compared to the normal x[CO2] treatments (A-S and A-W), when the strongly and weakly responsive cultivars were exposed to elevated x[CO2](F-S and F-W), N2O emissions decreased by 52.54% (P<0.05) and 38.40% (P<0.05), rice yield increased by 22.96% (P<0.05) and 12.11% (P>0.05), and N2O emission intensity decreased by 61.68% (P<0.05) and 45.13% (P<0.05), respectively. Moreover, N2O emissions of all treatments were significantly positively correlated with NH4+-N content (P<0.05), whereas not correlated with NO2--N content. Soil temperature is an important factor affecting the N2O emissions of the strongly responsive cultivar in rice fields under elevated x[CO2] conditions. Through comprehensive consideration of climate conditions, in the future, priority should be given to planting the strongly responsive cultivar, ensuring high rice yield and significant reduction in N2O emissions.

Elevated Atmospheric CO2 and Nitrogen Fertilization Affect the Abundance and Community Structure of Rice Root-Associated Nitrogen-Fixing Bacteria.

Elevated atmospheric CO2 (eCO2) results in plant growth and N limitation, yet how root-associated nitrogen-fixing bacterial communities respond to increasing atmospheric CO2 and nitrogen fertilization (eN) during the growth stages of rice is unclear. Using the nifH gene as a molecular marker, we studied the combined effect of eCO2 and eN on the diazotrophic community and abundance at two growth stages in rice (tillering, TI and heading, HI). Quantitative polymerase chain reaction (qPCR) showed that eN had no obvious effect on nifH abundance in rice roots under either ambient CO2 (aCO2) or eCO2 treatment at the TI stage; in contrast, at the HI, nifH copy numbers were increased under eCO2 and decreased under aCO2. For rhizosphere soils, eN significantly reduced the abundance of nifH under both aCO2 and eCO2 treatment at the HI stage. Elevated CO2 significantly increased the nifH abundance in rice roots and rhizosphere soils with nitrogen fertilization, but had no obvious effect without N addition at the HI stage. There was a significant interaction [CO2 × N fertilization] effect on nifH abundance in root zone at the HI stage. In addition, the nifH copy numbers in rice roots were significantly higher at the HI stage than at the TI stage. Sequencing analysis indicated that the root-associated diazotrophic community structure tended to cluster according to the nitrogen fertilization treatment and that Rhizobiales were the dominant diazotrophs in all root samples at the HI stage. Additionally, nitrogen fertilization significantly increased the relative abundance of Methylosinus (Methylocystaceae) under eCO2 treatment, but significantly decreased the relative abundance of Rhizobium (Rhizobiaceae) under aCO2 treatment. Overall, the combined effect of eN and eCO2 stimulates root-associated diazotrophic methane-oxidizing bacteria while inhibits heterotrophic diazotrophs.

Elevated CO2 concentration modifies the effects of organic fertilizer substitution on rice yield and soil ARGs.

Antibiotic resistance and rising CO2 levels are considered among the most significant challenges we will face in terms of global development over the following decades. However, the impact of elevated CO2 on soil antibiotic resistance has rarely been investigated. We used a free-air CO2 enrichment system to investigate the potential risks posed by applying mineral and organic fertilizers to paddy soil at current CO2 concentration (370 ppm) and future elevated CO2 (eCO2, 570 ppm predicted for 2100). Organic fertilizer substitution (substituting the mineral fertilizer by 50% N) alone increased the plant uptake and soil residue of sulfamethazine, and enriched sulfonamide resistance genes (sul1, sul2), tetracycline resistance genes (tetG, tetM) and class 1 integron (intl1). But it decreased the rice grain yield (by 7.6%). Comparatively, eCO2 decreased the sul2, tetG and intl1 gene abundances by organic fertilizer substitution, and meanwhile increased grain yield (by 8.4%). Proteobacteria and Nitrospirae were potential hosts of antibiotic resistance genes (ARGs). Horizontal gene transfer via intl1 may play an important role in ARGs spread under eCO2. Results indicated that future elevated CO2 concentration could modify the effects of organic fertilizer substitution on rice yield and soil ARGs, with unknown implications for future medicine and human health.

Large losses of ammonium-nitrogen from a rice ecosystem under elevated CO2.

Inputs of nitrogen into terrestrial ecosystems, mainly via the use of ammonium-based fertilizers in agroecosystems, are enormous, but the fate of this nitrogen under elevated atmospheric carbon dioxide (CO2) is not well understood. We have taken advantage of a 15-year free-air CO2 enrichment study to investigate the influence of elevated CO2 on the transformation of ammonium-nitrogen in a rice ecosystem in which ammonium is usually assumed to be stable under anaerobic conditions. We demonstrate that elevated CO2 causes substantial losses of ammonium-nitrogen that result from anaerobic oxidation of ammonium coupled to reduction of iron. We identify a new autotrophic member of the bacterial order Burkholderiales that may use soil CO2 as a carbon source to couple anaerobic ammonium oxidation and iron reduction. These findings offer insight into the coupled cycles of nitrogen and iron in terrestrial ecosystems and raise questions about the loss of ammonium-nitrogen from arable soils under future climate-change scenarios.

Effects of Decabromodiphenyl Ether and Elevated Carbon Dioxide on Rice (Oryza sativa L.).

We assessed the effects of carbon dioxide (CO2) and decabromodiphenyl ether (BDE-209, 0, 3 and 30 mg/kg) on rice (Oryza sativa L. cv. Wuyunjing) in field free-air CO2 enrichment system. Rice at elevated (580 ppm) CO2 had increased net photosynthetic rate, intercellular CO2 concentration, shoot biomass, yield and phosphorus content in grains. However, there were no significant changes in such parameters observed on rice at elevated CO2 combined with BDE-209 (3 and 30 mg/kg). Elevated CO2 alone had no significant effects on sugar or starch content in rice grains, whereas its combination with BDE-209 (3 mg/kg) significantly decreased grain sugar and starch content. In conclusion, rice reared in soil polluted by BDE-209 under elevated CO2 modulates the effects in grain feature.

Elevated CO2 improved soil nitrogen mineralization capacity of rice paddy.

Elevated CO2 would increase rice yields and may lead to nitrogen limitation and potentially influence the sustainability of agricultural production. Blindly increasing the amount of chemical fertilizer will damage the environment and is very unwise. Therefore, clarifying the response of soil nitrogen mineralization capacity to elevated CO2 is critical for both sustainable agriculture production and environmental protection. Here, we relied on Free-Air CO2 Enrichment (FACE) platform and used a waterlogged incubation method to investigate the effects of elevated CO2 on soil nitrogen mineralization capacity under different fertilization levels when planted different rice cultivars (strong and weak-CO2 response rice). According to the first-order kinetic equation fitting, compared with Ambient, elevated CO2 increased soil potential mineralized nitrogen (Np) by 16.18%. Path analysis indicated that fertilization status, rice cultivar, soil organic carbon and soil C: N ratio might affect Np. There was a significant positive correlation between soil nitrogen mineralization rate and Np. Under different fertilization conditions and rice cultivars, the improvement degree of soil nitrogen mineralization capacity (Np and soil nitrogen mineralization rate) by elevated CO2 was different. These findings suggest that more parameters and influencing factors should be taken into account when studying soil nitrogen cycle models under the condition of global change.

Plant-mediated effects of elevated CO2 and rice cultivars on soil carbon dynamics in a paddy soil.

Soil organic carbon (SOC) sequestration under elevated CO2 concentration (eCO2 ) is a function of carbon (C) input and C retention. Nitrogen (N) limitation in natural ecosystems can constrain plant responses to eCO2 and their subsequent effects on SOC, but the effect of eCO2 on SOC in N-enriched agroecosystems with cultivars highly responsive to eCO2 is largely unknown. We reported results of SOC dynamics from a field free-air CO2 enrichment experiment with two rice cultivars having distinct photosynthetic capacities under eCO2 . A reciprocal incubation experiment was further conducted to disentangle the effect of changes in litter quality and soil microbial community on litter-derived C dynamics. eCO2 significantly increased total SOC content, dissolved organic C and particulate organic C under the strongly responsive cultivar, likely due to enhanced organic C inputs originated from CO2 stimulation of shoot and root biomass. Increases in the residue C : N ratio and fungal abundance induced by eCO2 under the strongly responsive cultivar reduced C losses from decomposition, possibly through increasing microbial C use efficiency. Our findings suggest that applications of high-yielding cultivars may substantially enhance soil C sequestration in rice paddies under future CO2 scenarios.

The potential role of sucrose transport gene expression in the photosynthetic and yield response of rice cultivars to future CO2 concentration.

The metabolic basis for observed differences in the yield response of rice to projected carbon dioxide concentrations (CO2 ) is unclear. In this study, three rice cultivars, differing in their yield response to elevated CO2 , were grown under ambient and elevated CO2 conditions, using the free-air CO2 enrichment technology. Flag leaves of rice were used to determine (1) if manipulative increases in sink strength decreased the soluble sucrose concentration for the 'weak' responders and (2), whether the genetic expression of sucrose transporters OsSUT1 and OsSUT2 was associated with an accumulation of soluble sugars and the maintenance of photosynthetic capacity. For the cultivars that showed a weak response to additional CO2 , photosynthetic capacity declined under elevated CO2 and was associated with an accumulation of soluble sugars. For these cultivars, increasing sink relative to source strength did not increase photosynthesis and no change in OsSUT1 or OsSUT2 expression was observed. In contrast, the 'strong' response cultivar did not show an increase in soluble sugars or a decline in photosynthesis but demonstrated significant increases in OsSUT1 and OsSUT2 expression at elevated CO2 . Overall, these data suggest that the expression of the sucrose transport genes OsSUT1 and OsSUT2 may be associated with the maintenance of photosynthetic capacity of the flag leaf during grain fill; and, potentially, greater yield response of rice as atmospheric CO2 increases.