Effects Of Rice Plant Productivity And Photosynthate Allocation On Greenhous Gas Emissions From Paddies

Rice(Oryza sativa L.)is the most important staple food for feeding more than half the world’s population.Because of population growth and economic development,global rice demand is projected to rise by about 30%in 2050 on existing limited arable land.Meanwhile,rice paddies are a major source of anthropogenic methane(CH4)emissions,and are responsible for about 11%of anthropogenic CH4 emissions.Methane is the second most important greenhouse gas after carbon dioxide(CO2),and contributes to about 20%of the global warming effect.Also,paddy fields are a major source of anthropogenic nitrous oxide(N2O),accounting for approximately 11%of global agricultural N2O emissions.Nitrous oxide(N2O)is the third most important greenhouse gas after carbon dioxide(CO2)and methane(CH4),and is responsible for approximately 6-8%of the current global warming.Additionally,N2O enhances atmospheric PM2.5 accumulation and aggravates stratospheric O3 depletion.Therefore,a challenge for rice agriculture is to increase total rice yield while minimizing CH4 and N2O emissions.A further increase in rice yield is the key approach to ensuring food security because of the limited paddy area and water resource availability worldwide.Increasing rice yield through breeding can be achieved by(i)increasing biomass and(ii)improving the HI.Previous studies showed that increasing photosynthate allocation to the rice grains reduces CH4 emission during grain filling stage.Yet,the effect of photosynthate allocation on CH4 emission during panicle formation stage is still poorly understood.Additionally,water management practice of alternate wetting and drying is widely adopted,which may stimulate N2O emissions.However,to our knowledge,no experiments have so far assessed its effects on N2O emissions.The HI value for rice has increased from approximately 0.3 to 0.55 since the 1950 due to the rice breeding and cultivation imporvement,is approaching the theoretical upper limit of 0.65.Therefore,the modern strategy for high-yielding is to enhance biomass while maintaining the current HI.However,the trade-off or synergy between yield and CH4 emissions involved in high biomass are still not been well understood.Therefore,we combined experiments and meta-analysis to determine the effects of photosynthate and its allocation on greenhouse gas emissions.Main results are as follows:1.Increasing photosynthate allocation to panicles can reuce CH4 emissions from rice paddies while increasing rice yield under flooded systems.The variety experiment under field conditions at two sites showed that mean CH4 emissions during the reproductive stage were significantly and negatively correlated with spikelet number under flooding.There was no significant correlation between spikelet number and CH4 emission during the grain filling stage under alternate wetting and drying irrigation.Rice yield was significantly and positively correlated with the spikelet number.The rice mutant experiment under pot and field conditions using a wild type rice variety Yangdao 6 and its mutant demonstrated that CH4 emissions were significantly lower in the former with large panicles than in the later with small panicles,during the reproductive and grain filling stages,whereas the rice yields showed an opposite.Root exudates and soil dissolved organic C concentration were significantly lower under the Yangdao 6 than under the mutant.Soil CH4 production potential and methanogenic populations of the soil under mutant were significantly higher than those of the soil under Yangdao 6.There was no significant difference in soil CH4 oxidation potential and methanotrophic populations between soils of these two rice varieties.2.Increasing harvest index through enhancing photosynate allocation to panicles can reduces N2O emissions under water-saving systems.In the variety experiment under field conditions at two sites showed N2O fluxes were significantly and negatively correlated with HI under alternate wetting and drying irrigation.In the mutant experiment,N2O fluxes of the mutant were significantly higher than that of the Yangdao 6 under the field and pot conditions.In the spikelet clipping experiment,spikelet clipping significantly reduced harvest index but increased N2O fluxes in Yangdao 6 and Ningjing 1,under the field and pot conditions.In the mutant experiment,the biomass and plant N uptake of the Yangdao 6 was significantly larger than that of the Mutant.In the spikelet clipping experiment,spikelet clipping decreased biomass and plant N uptake,especially for the Ningjing 1.Reducing photosynthate allocation to the grain by spikelet clipping significantly increased white root biomass and soil dissolved organic C and reduced plant N uptake,resulting in high soil denitrification potential.Our findings demonstrate that optimizing photosynthate allocation to the grain can reduce paddy N2O emissions through decreasing belowground C input and increasing plant N uptake.3.The potential of increasing harvest index for CH4 mitigation is limited.The meta-analysis showed that rice cultivars with higher HI significantly reduced CH4 emission under flooded systems,but did not affect CH4 emission under water-saving systems.The spikelet removal significantly reduced the HI,but did not significantly affect CH4 emissions under either Yangdao 6 or Ningjing 1.The results from the meta-analysis also showed that the effects of HI improvement on CH4 mitigation mainly occurred in rice post-booting period,while no significant effect was found in the pre-booting period under the flooded conditions.The CH4 emissions after heading only constituted a small fraction of the total emissions.In flooded systems,post-booting CH4 emissions constituted 39.0%of the total CH4 emission across the globe.In the systems with alternative water management,however,the contribution of post-booting CH4 emissions is only 22.8%.According to the contribution of post-booting emissions and the effects of increasing HI on post-booting CH4 emission,the potential of increasing HI from 0.55 to 0.65 for CH4 mitigation only 7.2%under flooded systems.4.The effects of rice cultivars on CH4 emissions depended on paddy soil organic carbon(SOC)contents,high-yield cultivar can reduce CH4 emission from a typical paddy field with high SOC availability.The results from three independent but complementary experiments showed that cultivars with high biomass significantly increased CH4 emission from low labile organic carbon(i.e.1.0 g kg-1 and 1.4 g kg-1)soils,but significantly reduced it from high labile organic carbon(i.e.3.6 g kg-1 and 6.5 g kg-1)soils.The results from meta-analysis also indicated that high biomass cultivars significantly enhanced CH4 emission from low organic carbon(<8 g kg-1),but significantly reduced it from high organic carbon(>12 g kg-1)soils.The emission reduction caused by high-yielding cultivars in high organic carbon soils was tremendously greater than the emission increment occurring in low organic carbon soils.Also,cultivars with high productivity significantly increased soil methanogenic populations only in the low labile organic carbon soils.In contrast,the high-yielding cultivars significantly enhanced soil methanotrophic populations in the high labile organic carbon soils.Given the high soil organic carbon(16.5 g kg-1)and labile organic carbon(5.0 g kg-1)contents and widely crop straw application in major Chinese paddy fields,our findings indicate that modern rice breeding strategy for high-yielding cultivars can help mitigate paddy CH4 emission in China and other similar regions.Taken together,our results suggest that the modern rice breeding strategies not only increase rice yield but also reduce greenhouse gas emissions from rice paddies.Alao,it can provices theoretical basis for rice cultivar breeding and rice cropping innovation for less GHG emissions.