Effects Of Excessive Nitrogen Application On Soil Organic Carbon Fraction And Microorganisms Of Continuous Cropping Cucumber In Solar Greenhouse

In the cultivation of facility vegetables,the phenomenon of large-scale application of nitrogen fertilizer by farmers in pursuit of economic benefits is serious,which leads to soil acidification,nutrient imbalance and serious harm to crop growth,and has become an important factor restricting the sustainable development of facility vegetable industry in China.However,soil organic carbon is the core material and important supplier of soil fertility,and its fraction are sensitive to nitrogen addition.At present,the effects of excessive nitrogen application on soil organic carbon fraction and microbial communities of cucumber in solar greenhouse are not clear.Therefore,in this study,the cucumber variety’Jinyou 30’in solar greenhouse was used as the test material.Firstly,two different fertilization methods,over-fertilization（OF）and appropriate fertilization（AF）were used to study the soil organic carbon fraction of continuous planting 2,10 and 20 rounds,and the characteristics of soil organic carbon fraction and their key influencing factors under long-term fertilization were clarified.Secondly,two times（N2）and three times（N3）excessive nitrogen application experiments were carried out with appropriate nitrogen application（N1）as the control,and two treatments of N0（no nitrogen application）and N0.5（reduced nitrogen application）were set up.Through in-depth analysis of soil basic nutrients,organic carbon components and chemical composition,microbial community diversity,etc.,the effects of excessive nitrogen application on cucumber growth and yield in solar greenhouse were discussed from the perspective of soil organic carbon fraction.Finally,a simulated acidification verification experiment was carried out,and six different acidity treatments of A0（p H=7.04）,A1（p H=6.68）,A2（p H=6.22）,A3（p H=5.80）,A4（p H=5.55）and A5（p H=4.68）were set up.The effects of soil p H on the changes of soil organic carbon components and cucumber growth in solar greenhouse were discussed,which provided theoretical basis and technical guidance for scientific fertilization of greenhouse vegetables,maintaining soil health and promoting the sustainable development of solar greenhouse.The achievements are as follows:1.It was cleared that excessive chemical fertilizer application led to soil acidification and nutrient enrichment of cucumber in solar greenhouse.The large accumulation of inorganic nitrogen was the main reason for the decrease of soil labile organic carbon fraction.Compared with the appropriate fertilization,the soil p H was significantly decreased by2.04 units and the inorganic nitrogen content（NH₄⁺and NO₃^-）was significantly increased by284.97%after cultivation round of 20.The proportion of microbial biomass carbon（MBC/SOC）and dissolved organic carbon（DOC/SOC）in labile organic carbon was significantly decreased by 265.38%and 69.33%,respectively,compared with appropriate fertilization,while the proportion of soil stable organic carbon（ROC/SOC）and ROC content were significantly increased by 128.11%and 35.95%.Two-factor variance analysis and RDA clutter analysis showed that microbial biomass carbon（MBC）was the most sensitive to fertilization,and soil NH₄⁺and NO₃^-were the main regulatory factor of soil organic carbon changes caused by long-term fertilization.2.It was cleared that excessive nitrogen application aggravated soil acidification of cucumber in solar greenhouse,which led to the decrease of soil microbial community diversity and microbial biomass carbon,and then led to the decrease of cucumber yield.Compared with appropriate N application,excessive N application（N2 and N3）significantly inhibited the growth of cucumber,and the plant height,aboveground biomass,underground biomass and yield of cucumber decreased by 28.45%-36.02%,19.67%-20.35%,25.81-27.27%and 31.06%-33.61%,respectively.Over-application of nitrogen significantly reduced soil p H by 1.08-1.49 units and promoted the accumulation of nitrate ions and the increase of soil electrical conductivity.With the increase of nitrogen application rate,the contents of MBC,LHOC,POC,CO₂-C,HFOC and Ca-SOC in organic carbon fraction decreased significantly,while the contents of DOC and Fe（Al）-SOC increased significantly in high nitrogen treatment.The loss of Ca-SOC in heavy fraction organic carbon is the main reason for the decrease of SOC.Excessive nitrogen application significantly reduced the relative content of carbohydrate functional groups（8.51%-8.86%）in organic carbon chemical composition,and increased the relative contents of aliphatic and aromatic functional groups（7.40%-11.63%and 3.88%-5.99%）,so that soil organic carbon changed to humus.In addition,the deterioration of soil environment（acidification and nutrient imbalance）significantly inhibited soil microbial diversity,changed the community composition of fungi and bacteria,and reduced the activities of soil BG,NAG,PPO and CAT,thus affecting the acquisition and regulation of carbon sources by microorganisms,which was ultimately not conducive to the formation of cucumber yield.3.Simulated acidification confirmed that severe soil acidification（p H<5.5）had a negative impact on soil organic carbon sequestration,soil microbial characteristics and cucumber yield formation in solar greenhouse.Similar to excessive nitrogen fertilizer application,cucumber growth,soil organic carbon fraction,enzyme activities and microbial diversity were inhibited to varying degrees in strong acid treatment（A5）with p H<5.5.Microbial community structure was significantly different from that of the other four treatments（A0-A3）with p H>5.5.Soil p H and exchangeable Al³⁺had significant effects on bacterial and fungal community composition and soil organic carbon composition.At the genus level,Humibacter in bacteria and Saitozyma in fungi could well explain the change of organic carbon fraction under the addition of acid.Through microbial function prediction,we found that strong acids addition would significantly inhibit lipid transport and metabolism,biosynthesis of secondary metabolites,transport and catabolism,amino acid transport and metabolism,energy production and conversion,coenzyme transport and metabolism and other functions.In addition,multiple stepwise linear regression analysis showed that MBC explained 68.00%of the yield variation through screening,further confirming that MBC,as a carbon fraction closely related to microbial activity,plays a key role in regulating cucumber yield.