Effects Of Elevated CO₂ And Fertilization On Nutrition Physiology And Soil Nutrient Of Winter Wheat

Global CO₂ concentrations are increasing due to changes in fossil fuels and land use,and are expected to reach 550 ppm by 2050.Since the response of crops to the elevated CO₂（ECO₂）concentration will be limited by nitrogen（N）and deficiencies of other nutrients,it is urgent to review the strategies for fertilization and yield improvement at higher CO₂ concentrations in the future,to achieve sustainable agricultural development while ensuring food security.In addition,CO₂concentration in the field was higher at nighttime than at daytime due to plant roots and soil respiration.To obtain the accurate estimation of crop yield by interactions between ECO₂ concentration and fertilization,it is necessary to pay attention to the control of CO₂ concentration at nighttime.Wheat is C₃ grass crop,which is the food crop with the largest planting area and the highest total yield in the world,and is also the third largest food crop in China.Its response to ECO₂ has attracted much attention.To test how crops respond to future carbon dioxide（CO₂）concentrations a pot experiment was performed in plant growth chambers,which were controlled by automatic CO₂ generators with intelligent control systems for the projected and required elevated CO₂（ECO₂）concentrations in 2050.The wheat（Triticum aestivum Chuanmai 58）was used as the experimental crop.Two CO₂concentrations and four fertilizer levels were applied in the chambers as fllows.CO₂ concentrations（2 levels）:ACO₂(ambient CO₂,400±30μmol mol^-1 or ppm daytime,450±30μmol mol^-1 nighttime),ECO₂(elevated CO₂ continuously,550±30μmol mol^-1 daytime,600±30μmol mol^-1 nighttime).Fertilizer management（4 levels）:CK（control check,no fertilizer added）;F1（80%conventional dose）;F2（100%conventional dose）;F3（120%conventional dose）.By exploring the photosynthetic characteristics,distribution of dry matter and nutrient accumulation,soil nutrient content and changes in soil carbon（C）and nitrogen（N）key enzyme activity of winter wheat under ECO₂,we tested whether it is possible to reduce conventional fertilization level that could reduce costs,benefit the environment,thereby creating a‘sweet spot’that could significantly maintain high yield,high efficiency and soil nutrient,as well as ecological security exposed to ECO₂ in the future.The main results are as follows:（1）ECO₂ and fertilization promoted the net photosynthetic rate of wheat flag leaves,photosynthetic adaptation appeared in the anthesis stage.ECO₂ resulted in a decrease of net photosynthetic rate and relative chlorophyll at the milk ripe stage,which indicated that senescence of flag leaf was accelerated and photosynthetic rate was inhibited by ECO₂.There were no significant differences in the net photosynthetic rate of flag leaves by increasing or decreasing 20%conventional fertilization in the mature stage.（2）ECO₂ increased the aboveground biomass of winter wheat by 48.2%,35.82%and 32.11%in the heading,anthesis and mature stages,respectively,and decreased the dry matter distribution ratio of leaves,stem sheath and grain in the mature stage by 1.60%,0.70%and 2.10%,respectively;the distribution ratio of glume and spike increased by 4.40%.There were no significant differences in the above-ground dry weight and the proportion of different organs by increasing or decreasing 20%conventional fertilization in the mature stage.Translocation amount of leaf,stem sheath and glume plus rachis increased by 43.48%,64.18%and 40.00%under ECO₂,respectively.ECO₂ increased the dry matter remobilization efficiency and contribution of remobilization rate of winter wheat by 7.42%and 19.75%,respectively.Compared with the non-fertilizing treatment,fertilization significantly increased the dry matter translocation amount of winter wheat,and there were no significant differences between the transfer of leaves,glume plus rachis,and remobilization efficiency of aboveground dry matter.（3）ECO₂ and fertilization positively promoted the yield of winter wheat.The grain yield was reduced by 19.22%under no-fertilization while increased by 42.48%under fertilization.Increased CO₂ concentration increased the spike length,number of panicles and grain number per spike by12.16%,40.61%and 18.18%,respectively.The increase in yield was due to the increase in the number of panicles and grain number per spike.An increase or decrease of 20%conventional fertilization at ECO₂ had no significant effects on yield,ear length,grain number per spike,1000-grain weight and harvest index of winter wheat,and there were no obvious interactions between CO₂ increase and fertilization treatment,and between CO₂ concentration and fertilization on yield and its components of winter wheat.（4）Carbon accumulation under ECO₂ was consistent with biomass changes in winter wheat.Carbon was more likely to accumulate in roots,stems and leaves in the heading and anthesis stage,and distribute to ears and grains in the mature stage.Carbon content of each organ had a different response to ECO₂.ECO₂ increased the C content in leaves,stems and ears,while decreased the C content in roots.There were no significant differences in the aboveground organs under fertilization.ECO₂ reduced the N concentrations in leaves,stems,ears and roots in the heading,anthesis and maturity stage,but increased the aboveground N accumulation by 49.16%,40.42%and 18.12%,respectively.ECO₂ reduced the content of N in winter wheat by 12.97%,but had no significant effects on the content of phosphorus（P）and potassium（K）.In contrast,the accumulation of N,P and K increased by 22.04%,38.11%and 36.93%,respectively.No matter the ACO₂ or ECO₂ treatment,increase or decrease of 20%conventional fertilization had no significant effects on grain N content,N accumulation of grain and whole plant,grain K content,accumulation of P and K.The dilution effect could not be completely explained changes in nutrient content.（5）ECO₂ reduced soil p H by 4.37%in the anthesis stage,but no significant effects were observed at the heading and mature stages,but soil organic matter increased slightly.There were no significant differences between soil p H and soil organic matter in the mature stage.The increased uptake of N,P and K in winter wheat under elevated CO₂ corresponds to the decrease of available N（including ammonium and nitrate N）,P and K in soil,indicating that plants with increased CO₂ tend to absorb more N,P and K,thereby reducing the availability of N,P and K in soil.ECO₂ reduced the soil alkali-hydrolyzed N by 35.46%,7.52%and 17.04%,ammonium N by 42.88%,25.05%and 17.28%,nitrate N by 67.81%,83.77%and 84.30%,available P by 0.51%,14.24%and 34.78%,and available K by15.36%,4.12%and 7.38%,in the heading,anthesis and mature stages,respectively.The increase or decrease of 20%of conventional fertilization rate did not affect soil alkali-hydrolyzed N and available K.（6）ECO₂ increased the soil sucrase activity by 22.31%,13.04%and 4.56%,soilβ-glucosidase activity by 17.54%,11.55%and 8.63%,soil protease activity by 13.18%,16.70%and 19.96%in the heading,anthesis and mature stages,respectively.ECO₂ increased the soil urease activity by 12.5%and 13.6%in the heading and mature stages,respectively.Compared with no-fertilization control,fertilization increased the activity of soil sucrase,β-glucosidase and protease significantly,but not the urease activity.In conclusion,results in the present study showed that under the projected future ECO₂,a reduced fertilization did not affect the yield of winter wheat and the accumulation of N,P and K in grains.ECO₂ increased the net photosynthetic rate of winter wheat in the heading and anthesis stages,which promoted the biomass production of different tissues and the translocation of dry matter after anthesis,increased the number of panicles and grain number per spike,and thus increased the grain yield.Meanwhile,the soil enzyme activity increased with ECO₂,which might be helpful to the absorption of nutrients in winter wheat.There were no obvious interactive effects between CO₂ concentration and fertilization on yield and its components of winter wheat.A 20%reduction of the current fertilization rate would maintain,while a 20%increase of such a rate could not further improve,the present yield production of winter wheat.