Effect Of Elevated Atmospheric O₃ On The Bioavailability Of Soil Trace Elements In Rice-Wheat Rotation Ecosystem

In recent years,the increase of atmospheric O3 concentration has been increasing year by year,which has affected the growth of crops in the rice-wheat ecosystem and has indirectly or directly affected the morphological transformation and bioavailability of micronutrients in soil.Research on the bio-availability of soil trace elements has become a focus of attention in the fields of agro-ecology and environmental science.Rice-rice rotation is the main mode of water and drought planting in most parts of southern China.The degree of influence of ozone stress on the bioavailability of trace elements in rice and wheat soil is clearly defined and revealed,which can provide a theoretical basis for future agricultural production and farmland water and fertilizer management.Therefore,in this paper,OTCs with normal atmospheric control and elevated O3 concentrations treatment(about 40 nmol·mol-1 higher than control).The rice-wheat pot experiment was conducted to explore the changes of trace elements in wheat-soil soil with different morphological changes,available morphological contents,rhizosphere environmental changes and plant growth under elevated O3 concentrations,and analyze the influencing factors,research the response mechanism of bioavailability of trace elements in rice and wheat soil to ozone pollution,the main results are as follows:(1)Ozone stress significantly reduced the biomass of wheat roots,straw,and grain during ripening,with a decrease of 16.35%(P<0.05),13.57%(P<0.01),and 14.63%(P<0.01),respectively.Ozone stress reduced the biomass of different organs of rice at maturity stage,but reached a significant level only in the grain part,a decrease of 17.22%(P<0.05).Ozone treatment significantly reduced the 1000-grain weight of wheat and rice,resulting in a decline in rice and wheat yield.(2)Elevated O3 concentration significantly reduced the total root length and total root surface area of wheat at the heading stage and the mature stage,but increased the average root diameter.Ozone stress had no significant effect on rice root index,and the change of root average diameter was consistent with that of wheat.(3)Elevated ozone concentration promoted the absorption of soil trace elements by wheat plant roots,significantly increased the content of Fe,Mn and Cu in wheat grains,and significantly reduced the content of Zn in the above-ground parts.Ozone stress significantly decreased Fe content in rice roots and straw,significantly increased Mn content in straw,significantly increased Cu content in various organs,and significantly reduced Zn content in various organs.(4)Elevated ozone treatment increased the bioavailability of Fe,Mn,and Cu in the plough layer of wheat season,and reduced the bioavailability of Zn.The soils in all depths have the same trend of variation and show a certain vertical distribution pattern,and they gradually decrease from top to bottom.The BCR and DTPA method have similar laws in characterizing the bioavailability of trace elements.(5)Ozone fumigation significantly increased DTPA-Cu content in rice ripening period and WAS-Mn content in jointing stage,and decreased WAS-Fe content in ripening stage and WAS-Zn content in jointing stage.Both the BCR and DTPA methods show that elevated atmospheric ozone concentration has the effect of increasing the bioavailability of trace elements Mn and Cu in the soil-rice system,while decreasing the trend of Fe and Zn bioavailability.The difference between different treatments in the middle and later stages is even more pronounced.(6)Ozone stress reduced the pH,electrical conductivity,total carbon content and total nitrogen content in the rhizosphere soil at different growth stages of wheat,and reached a significant level at maturity(P<0.05),but increased its redox potential.Ozone stress significantly increased the pH and total N of the rhizosphere soil during rice maturity,significantly increased the conductivity at the tillering and heading stages,significantly reduced the Eh at the heading stage,and did not significantly increase the total C.(7)From the results of BCR,ozone stress significantly increased the proportion of WAS-Fe,Mn,and Cu in the total wheat rhizosphere soil and promoted the transformation of Fe,Mn,and Cu into more bioavailable forms.The decrease of the ratio of WAS-Zn and RED-Zn reduces the effectiveness of Zn.For rice rhizosphere soil,O3 reduced the proportion of WAS-Fe,Mn,and Zn,and increased the proportion of WAS-Cu and RED-Cu.O3 stress significantly increased the content of DTPA-Fe,Mn,and Cu in rhizosphere soil at the ripening stage of wheat,but significantly reduced DTPA-Zn.O3 stress significantly increased the content of DTPA-Fe,Mn,Cu and Zn in the rhizosphere soil at the heading stage of rice.O3 stress significantly increased the content of DGT-Fe,Mn,and Cu in the rhizosphere soil at the ripening stage of wheat,but significantly reduced DGT-Zn.O3 stress significantly increased the content of DGT-Mn and Cu in the rhizosphere soil at the ripening stage of rice,but significantly reduced DGT-Zn,but had no significant effect on DGT-Fe.(8)In the rhizosphere soil of rice and wheat,elevated ozone concentration increased the activity of catalase,polyphenol oxidase,dehydrogenase and invertase in mature soil to varying degrees.The order of organic acid content in rhizosphere soil was as follows:oxalic acid>malic acid>citric acid.Elevated ozone concentration promoted the secretion of organic acids in wheat rhizosphere,and ozone stress had no significant effect on the content of organic acids in the rhizosphere soil of rice.