Effects Of Straw Returning Depth On Photosynthetic Carbon Fixation And CO₂ Emission In Maize-soil System

With the increasingly severe global warming problem,“fixing carbon and reducing discharge”has become a research hotspot,and improving soil organic carbon fixation and reducing soil greenhouse gas emissions are effective ways to mitigate climate change.The photo-contracted fixed carbon（photosynthetic carbon）is closely related to the atmospheric environment and the cyclic turnover of the soil organic carbon pool,as the hub of the carbon cycle of the atmosphere-plant-soil-microbial system.Straw returning has the great potential of carbon sequestration to improve soil organic carbon and it is considered as an effective measure to improve soil fertility.Therefore,studying the effects of straw returning on the fixation of photosynthetic carbon and CO₂ emissions in plant-soil system is of great significance for alleviating global climate change,scientific assessment of farmland soil carbon cycle and sustainable development of soil and ecology.In this research,¹³C pulse-labeling technique and static box-gas chromatography were used in field micro-zone test and ¹³C pulse-labeling was carried out during the jointing stage of maize,to study the effect of different straw returning depths on photosynthetic carbon in maize-soil system.At the same time,the soil CO₂ emission of maize in the whole growing season was measured to explore the effects of different straw returning depths on soil CO₂ emission.The main conclusions are as follows:（1）After ¹³C pulse-labeling being carried out during the jointing stage of maize,the total assimilation rate of photosynthetic carbon in the“maize-soil”system gradually decreased with the growth of maize.The total assimilation rate of photosynthetic carbon in the above ground（stems and grains）and underground（roots and soils）treated with straw returning in deep soil（S）,straw returning in shallow soil（Q）,straw returning on surface of soil（F）and without straw returning（CK）decreased from 69.23%,73.18%,61.78%,and 57.18%on the second day of the labeling to 45.22%,44.34%,39.78%,and 38.22%in the period of harvest,respectively.The loss rate of assimilated photosynthetic carbon in the above four treatments were 34.70%,39.42%,35.60%,and 33.18%,respectively.The amount of assimilation and loss of photosynthetic carbon in straw returning treatments were all higher than that in treatment without straw returning.（2）On the second day after the labeling in jointing stage,the average distribution proportion of photosynthetic carbon to underground and photosynthetic carbon to the stems and leaves were 19.79%and 80.21%,respectively.And the amount of ¹³C in microbial biomass carbon accounted for 33.77%of the amount of ¹³C in soil organic carbon.At the maturity stage,the average distribution proportion of photosynthetic carbon to the underground increased to 26.41%,the average distribution proportion of photosynthetic carbon to stems and leaves decreased to 69.40%,and the average proportion of microbial biomass carbon(¹³C)to soil organic carbon(¹³C)increased to 39.13%.As the maize growing,the assimilated photosynthetic carbon gradually transfers from the above ground to the underground.（3）At the maturity stage,the proportions of photosynthetic carbon in maize roots treated with S,Q,F and CK were 15.62%,15.53%,14.42%and 13.96%,respectively.The contents of fixed active organic carbon in each treatments were 77.75μg kg^-1、76.04μg kg^-1、72.55μg kg^-1and 55.14μg kg^-1,respectively.Straw returning treatments increased the proportion of photosynthetic carbon in the root system and the content of fixed active organic carbon.In particular,in treatment S and treatment F,the distribution proportion of photosynthetic carbon to maize roots were significantly increased（P<0.05）.（4）The emission flux of soil CO₂ showed seasonal change,which was characterized by the highest in summer,followed in spring,and the lowest in autumn.The cumulative emissions of CO₂ in the treatment S,Q and F were increased by 6.7%,26.4%and 15.9%,respectively,compared with treatment CK.Among them,the cumulative emissions of CO₂ in treatment Q was significantly increased（P<0.05）and in treatment S was decreased significantly（P<0.05）,compared with treatment Q.The emission flux of CO₂ in soil had extremely significant positive correlation with atmosphere temperature,soil temperature,soil conductivity,soil DOC and ¹³C-DOC,and had extremely significant negative correlation with soil moisture content.（5）The total surface area,total volume,total length and average diameter of roots in straw returning treatments were all higher than those in treatment CK.The growth of roots whose length in 0¹ mm was significantly promoted in treatment S（P<0.05）,and the total volume of roots whose length greater than 4 mm was significantly greater than that in other treatments（P<0.05）,indicating that straw returning could promote the growth of maize root.Under the conditions of this research,the straw returning treatments can provide favorable conditions for assimilation utilization of photosynthetic carbon in maize-soil system.The depth of straw returning affected the distribution of photosynthetic carbon in the underground and the fixation of photosynthetic carbon by microorganisms.Straw returning in deep soil and straw returning on surface of soil are more conducive to the growth of maize roots and promote the distribution of photosynthetic carbon to the underground.Soil DOC content and CO₂ emission increased after straw returning.In particular,straw returning in deep soil（depth in 20⁴0 cm）can significantly reduce emissions of soil CO₂,and at the same time obtains higher maize yield and biomass,it is the better measure of straw returning compared with straw returning on surface of soil and straw returning in shallow soil（depth in0²0 cm）.