Effects Of Low Nitrogen Stress On Soil Nitrogen Converting Enzymes And Microbes Of Different Tartary Buckwheat Cultivars

The typical soil of the Loess Plateau is one of the most important land resources in Shanxi Province.The soil in this area is mostly alkaline and the unique ecological environment causes nitrogen loss.Tartary buckwheat is widely planted in areas with harsh ecological environment due to its short growth cycle and drought tolerance.Its plant protein content is rich,which is a good green manure and animal feed,and it is also a miscellaneous grain with regional characteristics in Shanxi.Therefore,in order to study the nutrient characteristics,nitrogen-converting enzyme activity and microbial change characteristics of the rhizosphere soil of tartary buckwheat under nitrogen stress in the Loess Plateau,this paper comprehensively considered three factors of nitrogen fertilizer concentration（control:CK;low nitrogen:N1;normal nitrogen treatment:N2）,tartary buckwheat cultivar（"Heifeng No.1",HF;"Diqing tartary buckwheat",DQ）and growth period（seedling stage,S;flowering stage,F;maturity stage,M）,studied the rhizosphere the soil physical and chemical characteristics,nitrogen-converting enzymes and functional flora in the soil are expected to reveal the law of soil nitrogen transformation and the internal mechanism of tartary buckwheat in the Loess Plateau,and provide a scientific basis for the optimal management of farmland nitrogen fertilizer in the barren area of the Loess Plateau.The main findings are as follows:（1）Different nitrogen treatments and tartary buckwheat cultivars have significant effects on rhizosphere soil nutrients.1)The results of the variance test of the split zone showed that in the S period,nitrogen treatment,cultivar,period and the interaction of the three all had significant effects on the water content,p H,organic matter,alkali hydrolyzable nitrogen,NH₄⁺,and total nitrogen of the rhizosphere soil;and in the F period,nitrogen treatment,cultivar,period and the interaction of the three have significant effects on the p H,organic matter,alkali hydrolyzable nitrogen,NH₄⁺,total nitrogen,total phosphorus,available potassium,and available phosphorus of the rhizosphere soil;in the M period,nitrogen treatment,cultivar,period and the interaction of the three have significant effects on the water content,p H,organic matter and NO₃^-of the rhizosphere soil.2)After the entire growth period,under N2 treatment,the organic matter,p H,available potassium,ammonium nitrogen,nitrate nitrogen,total nitrogen and total potassium of DQ rhizosphere soil were significantly higher than HF;under N1 treatment,DQ rhizosphere soil p H,alkaline nitrogen,total nitrogen,total phosphorus,and nitrate nitrogen are significantly higher than HF;under CK treatment,DQ rhizosphere soil water content,organic matter,nitrate nitrogen,total nitrogen,total phosphorus are significantly higher than HF.Under three different nitrogen concentration treatments,most of the rhizosphere soil nutrient contents of low-nitrogen tolerant cultivars were higher than that of low-nitrogen-tolerant cultivars.（2）Different nitrogen treatments and tartary buckwheat cultivars have significant effects on the activity of rhizosphere soil nitrogen convertase.1)Except for the urease and protease in the S period,the cultivars have extremely significant differences in the soil nitrogen invertase activity at other different periods;nitrogen treatments have extremely significant differences in the soil nitrogen invertase activity.In addition to urease in the S period,ammonium oxidase and protease in the M period,the interaction between cultivars,nitrogen treatment and period also had a significant impact on soil nitrogen invertase activity.2)After the entire growth period,under the three nitrogen treatments,the urease and ammonium oxidase activities of DQ rhizosphere soil were higher than HF,but under the three treatments,the protease activity of HF rhizosphere soil was higher than that of DQ.（3）Different nitrogen treatments and tartary buckwheat cultivars have significant effects on rhizosphere soil microorganisms.1)The Ace index in theα-diversity of the AOA community is significantly affected by the interaction of cultivars,nitrogen treatments and cultivars.After the end of the entire growth period,the number of OTUs and Chao1 of the two cultivars were significantly higher than that of CK under N2 treatment;under N1 treatment,the Shannon and Simpson indexes of DQ were greater than HF;under N2 treatment,the Shannon index of DQ was greater than HF.The Simpson index of HF is greater than DQ.The Shannon index and Simpson index in theα-diversity of AOB community were significantly affected by different nitrogen treatments.Under N1 treatment,the Simpson index of DQ is greater than that of HF.2)The population structure characteristics of the communities at the AOA genus level and the AOB genus level are also different in different growth periods.At the level of the AOA genus,the communities were Candidatus-Nitrosocaldus with relatively high relative abundance at the three stages,and DQ was greater than HF under the three nitrogen treatments during the growth period;at the level of the AOB genus,the communities were only in during maturity,Nitrosomonas is relatively abundant.Under CK and N2 treatments,the proportion of HF strains was higher than that of DQ,and under N1 treatments,the proportions of DQ strains were higher than HF.（4）Tartary buckwheat rhizosphere soil environmental factors interact with ammonia oxidizing microorganisms and enzyme activities.1)The physical and chemical properties of soil have a significant correlation with the diversity of AOA and AOB.2)The Monte Carlo test and ranking of soil physical and chemical properties and enzyme activities show that total phosphorus,protease,and ammonium oxidase are the key drivers of the AOA-amo A gene community;total potassium,NH₄⁺,organic matter,and total phosphorus are the key drivers of the AOB-amo A gene community.3)Correlation analysis shows that enzyme activities and physical and chemical properties also have a significant correlation.Among them,urease activity,protease activity and ammonium oxidase activity are all positively correlated with NH₄⁺,and NH₄⁺becomes the main influencing factor affecting the activity of nitrogen convertase.In summary,under low-nitrogen stress,nitrogen-tolerant cultivars have higher nutrient content in the soil after harvest than non-nitrogen-tolerant cultivars,which can reduce the fertilizer consumption of the next crop;as the growth period advances,under low-nitrogen stress rhizosphere soil enzyme activities continue to increase,and compared with nitrogen-sensitive tartary buckwheat,soil enzymes in N-tolerant tartary buckwheat are more sensitive to low nitrogen response,which may in turn affect the diversity of microbial communities and substrates in agricultural ecosystems.The decomposition process and nutrient circulation,which in turn affect the availability of nitrogen.Therefore,the nitrogen-tolerant cultivar（Diqing cultivar）has better nitrogen use efficiency and adaptability to stress in the local barren soil than the non-nitrogen-tolerant cultivar（Heifeng cultivar）.Moreover,the nitrogen application rate of 80 mg/kg is the closest to the local nitrogen fertilizer concentration.The above are all provided theoretical references for the nutrient-poor regions of the Loess Plateau.