| The increasing concentrations of greenhouse gases in the atmosphere, such as CO2, CH4and N2O, have been paid a lot of attentions due to their substantial contributions to climatic warming and ozone depletion. Agroecosystem plays an important role in the budget of atmospheric greenhouse gases. China is a large rice production country, and Chinese rice area accounts for about18.8%of the world’s rice area. CH4emissions from the rice fields accounts for about17.93%of Chinese total CH4emissions. Nitrous oxide emissions from paddy fields in China accounts for about7%to11%of the total N2O emission of nitrogen fertilizer directly. The application of nitrogen fertilizers is the main technical measures to raise rice yield and has direct relation with the emissions of paddy nitrous oxide, futher more, it also has an influence on CH4emission by stimulate crop growth. In short, it is of great significance for China’s food security and energy savings to achieve high-yielding and low GHG emission, by the way of altering crop group and fertilizer management.This study was carried out in a paddy ecosystem in Danyang City, Jiangsu Province with static opaque Ehmaber-gas chromatograph method to synchronously measure CH4and N2O fluxes. The impacts of different treatments of N application rates and rice planting densities on greenhouse gas emissions, as well as rice yield, were performed in this experiment. Using urea as a nitrogen source and using conventional rice Ninggeng3and Indica Hybrid Rice Y Liangyou302as test varieties, according to the planting density and amount of fertilizer in conventional rice cropping for high yield, four different patterns were designed in this study. The objectives were to maintain rice yield with lower greenhouse gas emissions through adjusting rice planting density and N application amount. The main results are as follows:The trend of methane emission flux is consistent under different treatments. Ninggeng 3has bimodal peaks appearing respectively in the tillering stage and heading stage, the flux distribution of methane is between-0.26to68.79mg m-2h-1.Y Liangyou302after transplanting maintained high methane emissions, and show a downward trend in the growth period, and the distribution of methane emission ranged from-1.60to46.62mg m-2h-1. Accompanied by an increase in density, reduction in N application, total emissions of Ningjing3tended to decrease.The largest is the conventional mode, and methane emissions is495.3kg ha-1.On the contrary, the Y Liangyou302showed an opposite trend, M2was the largest valued as489.7kg ha-1.The intensity of methane emission varied from different treatments. The highest of yield-scaled methane emissions was conventional model of Ninggeng3(43.51g kg-1), which was38,34,39and55%higher than those of the M4, M3, M2and M1patterns, respectively. For Y Liangyou302, M2was the largest (42.52g kg-1), which was higher than that of the conventional model by42%. Biomass-scaled methane emissions of Ninggeng3under conventional model were always higher than the planting density increase and N application decrease treatments, but the trend of Y Lingyou302is contract. This indicates that the difference between Ninggeng3and Y Lingyou302in emission intensity may be related to the response to different levels of nitrogen fertilizer and plant growth characteristics.Accompanied by an increase in density, N application reduction graduately decreased the emission of nitrous oxide. Under conventional mode, the total emission of nitrous oxide of Y Lingyou302and Ninggeng3were1.98kg ha-4and1.35kg ha-1, respectively. During the entire growth period, there were large changes in the fluxes of nitrous oxide emissions from the fields both with Y Lingyou302and Ninggeng3. After nitrogen fertilizer application, the flux increased significantly. Under conventional mode and M1, the fluxes of Y Lingyou302and Ninggeng3changed by a big margin, which were higher than those in the other treatments.The total emissions of greenhouse gases were different among the treatments. Methane emissions accounted for more than93%of the total emissions. Accompanied by an increase in planting density, reduction in N application, the ratio of nitrous oxide decreased and the flux of nitrous oxide showed a downtrend, but Y Lingyou302showed an upward trend and Ninggeng3showed a downward trend in the total emission of greenhouse gases. The total emission of Y Liangyou302in M2was highest, up to12295.50kg ha-1CO2eq, which was higher than conventional mode by50%. The total emission of Ninggeng3in the conventional mode was12461.17kg ha-1CO2eq, and the total emission of other modes ranged from7000to9000kg ha-1CO2eq.The effects of different planting density increase and N application decrease treatments on rice yield were different. The yield of Y Lingyou302ranked as follow:M1> M3> C> M4> M2, the highest yield per acre was804.85kg. The yield of Ninggeng3in the conventional mode was the highest, up to770.9kg. Accompanied by an increase in the density and a reduction in N application, the effective panicles of Y Lingyou302gradually increased and those of Ninggeng3gradually reduced, which was in line with the rice yield. Among the treatments, there were no significant differences in grain weight and harvest index. This may be due to the different endurance for fertilizer and the response of planting density between Lingyou302and Ninggeng3, Planting density increase made up for the decrease of effective tiller as aresult of less fertilizer, so as to maintain a high level of rice yield. |
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