| Agroecosystems play a vital role in the global balance of atmospheric greenhouse gases(GHGs)and act as a main source of GHGs emissions.As typical rotation agroecosystem in China,rice-wheat rotation cropping system has a strong impact on GHGs emissions,and in turn to comprehensively evaluate the global warming potential(GWP),greenhouse gas intensity(GHGI)and its relevant ecological benefits derived from agricultural production would be highly needed.This study has two research sections,including field experiment and meta-analysis.In this study,we will combine experimental results with analytical results to approach the effects of elevated CO2 and temperature on rice-wheat rotation cropping system GHGs emissions.A rice-wheat rotation cropping system,as one of the major cropping systems in Southeast China,was selected in the present study to simultaneously measure CH4 and N2O fluxes with T-FACE(Temrature+Free-air CO2 enrichment)field experimental facility using static opaque chamber-gas chromatograph method.The primary objective of this study was to gain an insight into the effect of elevated CO2 concentration and atmospheric temperature on GHGs emissions,and in turn to explore the related processes and mechanisms involved.The results of this study are as follows:(1)Field experiments showed that the total biomass and yieldsof rice and wheat were significantly increased under elevated CO2 concentration.The biomass of rice and wheat increased by 9.66%and 11.26%,respectively.In addition,the yields of rice and wheat increased by 5.64%and 5.7%,respectively.We also noticed that was a difference between the effects of elevated CO2 concentration on the above-ground biomass and below-ground biomass of rice and wheat.Under the condition of high CO2 concentration,the below-ground biomass of rice and wheat increased by 41.07%and 39.59%,respectively.Meanwhile,the above-ground biomass of rice and wheat increased by just 8.57%and 8.89%,respectively.This difference between the above-ground biomass and below-ground biomass of rice and wheat has resulted in increasing of root-shoot ratio of both of rice and wheat.Higher temperature significantly decreased total biomass and yield of rice and wheat.The total biomass and yield of rice decreased by 21.09%and 31.6%,respectively under the condition of elevated temperature.The total biomass and yield of wheat decreased by 17.97%and 17.68%,respectively.The results of field experiment showed that there was no significant interaction effects between elevated CO2 concentration and temperature on total biomass or yield of rice and wheat.(2)We conducted a meta-analysis to synthesize 225 studies assessing the responses of rice and wheat production to elevated atmospheric CO2,and the interaction of elevated CO2 with rising temperature or N supply.Elevated CO2 significantly enhanced rice and wheat yield by 19.8%and 17.6%,respectively.Significant increases not only in their determinants but also in physiological parameters of the both crops were responsible for this response,where grain mass consistently showed the least but significant response amongst all the variables.Elevated CO2 significantly increased rice and wheat biomass by 24.0%and 15.4%,respectively.Elevated CO2 concentration increases wheat aboveground biomass and belowground biomass by 18.4%and 17.3%,respectively.Belowground biomass(+41.9%)of rice increased at elevated CO2 to a larger extent than aboveground biomass(+27.9%).This differences in rice biomass response to elevated CO2 concentration lead to an increase in root-shoot ratio.Among the Japonica,Indica and Hybrid rice cultivars,Hybrid cultivars generally showed the greatest growth response to elevated CO2.The responses of rice and wheat yields to elevated CO2 significantly varied with CO2 funigation methods,with the less response observed in e condition compared with the others.(3)Elevated CO2 concentration significantly increased the concentrationo of soil organic matter(SOM)of rice-wheat rotation cropping system.The concentration of SOM of rice paddy significantly increased under the elevated temperature treatment,while similar results were not observed in wheat seasons.Elevated temperature significantly increased the concentration of nitrate in rice paddy,but it didn't regulate the concentration of ammonium in rice paddy.(4)The cumulative CH4 emissions from rice paddies were increased under the conditions of higher CO2 concentrations and temperature,while CH4 sink of wheat field slightly decreased by elevated CO2 concentration.As compared to the plots without rice plantation,CH4 emissions were enhanced under the plots with rice plantation.(5)In comparison with Ambient treatment,N2O emission of C treatment increased by 15.25%and 39.92%for rice and wheat season,respectively;Elevated temperature significantly increased N2O emissions in wheat season by an average of 20.47%,while no significant effects were observed in rice season;The seasonal pattern of N2O emission from rice-wheat rotation field were not affected by elevated CO2 concentration and temperature;Although with a considerable interannual variability,the interaction of elevated CO2 concentration and temperature tended to increase N2O emission in rice season:The emissions of N2O in wheat season were significantly increased by an average of 45.99%under the condition of C+T treatment;The responses of N2O emission to elevated CO2 concentration and temperature in rice season were regulated by irrigation stages;There was a positive correlation between N2O emission and wheat below-ground biomass.This result strongly suggests that the emission of N2O from wheat fields were closely related to wheat growth.(6)For comparing total annual CH4 and N2O emissions under different CO2 concentration and temperature treatments,GWPs expressed in CO2 equivalents were calculated.The results of experiment in the field showed that the elevated CO2 concentration,temperature and their interaction all contribute to the increase of GWP derived from CH4 and N2O emission from rice-whest rotation agroecosystem.The GWP was highest for the treatment of elevated CO2 combined with elevated temperature(+68.85%),followed by elevated CO2(+52.14%)and elevated temperature(+33.29%).Overall,GWP of rice cultivation was dominated by high CH4 emissions while GWP of wheat field was dominated by N2O emissions.Since there was no significant changes for N2O emissions from neither rice paddies nor wheat fields under elevated temperature treatment,the increase of CH4 emissions contributed the majority changes to annual GWP of rice-wheat rotation system.Compare to the ? GWP of elevated CO2 treatment,? GWP derived from the variation of CH4 emissions significantly increased under the condition of elevated CO2 combined with elevated temperature.Elevated CO2 concentration,temperature and their interaction significantly(P=0.05)increased the greenhouse gas intensity(GHGI)of rice by 46.43%,98.21%and 137.47%,respectively.And for wheat,the increment of GHGI were 43.59%,66.67%and 87.18%,respectively.(7)We synthesized 1772 paired measurements derived from 171 peer-reviewed publications using meta-analysis.Here we show that elevated CO2 significantly stimulates soil CO2 fluxes by 24%,and methane(CH4)fluxes by 34%from rice paddies and by 12%from natural wetlands,while it decreases CH4 uptake of upland soils by 3.8%.Elevated CO2 causes an insignificant increase in soil nitrous oxide(N2O)fluxes(4.6%),soil organic C(4.3%)and N(2.5%)pools.An elevated CO2-induced rise in soil CH4 and N2O emissions corresponds to as much as 78.5%of soil C storage enhancement(1.90/2,42 Pg CO2-equivalent yr-1)and nearly 47.6%of C mitigation potential contributed by terrestrial net ecosystem production(NEP,3.99 Pg CO2-eq yr-1).In conclusion,elevated CO2 concentration significantly increased the total biomass and yield of rice and wheat,while elevated temperature significantly decreased both total biomass and yield of rice and wheat.The positive effects of high CO2 concentration on the biomass and yield of rice and wheat were not enough to offset the negative effects of elevated temperature.Elevated CO2 concentration significantly increased the concentration of soil organic matter(SOM)of rice-wheat rotation cropping system by increasing soil C input.A significant positive correlation was found between crop below-ground biomass and SOM concentration.Elevated CO2 and rising temperature significantly increased the emission of CH4 from rice paddy.Elevated CO2 and rising temperature consistently and significantly increased seasonal CH4 emissions over all expermental seasons,while their interactions were not pronounced,representing an additive effect of elevated CO2 and rising canopy temperature on CH4 emissions from rice paddies.Elevated CO2 concentration significantly increased the emission of N2O in both rice and wheat seasons.Elevated temperature significantly increased N2O emission in wheat season,but no significant change was observed in rice season.The emission of N2O from rice-wheat rotation field significantly increased under the condition of C+T treatment,but there was no significant difference between C+T treatment and C treatment.Our meta-analysis result adds an updated perspective that the elevated CO2-stimulated sink capacity of terrestrial ecosystems to slow climate warming might have been largely offset by its induced increases in soil GHG emissions. |
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