Effects Of Alfalfa/Maize Rotation And Nitrogen Application On Phospholipid Fatty Acid Of Soil Microbial In The Rhizosphere And Yield

Nitrogen（N）is the most important limiting factor for plant growth.However,long-term N addition has resulted in issues such as lower crop yields and soil fertility loss,which reduces ecosystem services.Agricultural development is needed to shift from intensive agriculture to a more efficient use of nitrogen.Maize（Zea mays L.）,an annual gramineae crop,is one of the major food crops in farming and pastoral areas of northeast China.Alfalfa（Medicago sativa L.）,a perennial leguminous forage with high nitrogen-fixing capacity,is frequently used to restore degraded soils.Maize and alfalfa rotation not only ensures food supply,but it also has far-reaching ecological environmental implications.In the soil,crop output and soil nutrient availability are strongly influenced by soil organisms.Researchers frequently use phospholipid fatty acid（PLFA）analysis to quantitatively microorganisms.Crop rotation patterns of annual legumes with gramineae grain crops have been primarily the focused of current research,but there are still unclear how perennial legume rotation with grain crops affect community crop yield and phospholipid fatty acid of soil microorganisms,what is the relationship between them under different N levels.This experiment studied the rotation of maize and alfalfa from 2016 to 2021,for a total of 6 years.Two N application levels were set:no nitrogen（N₀）and N application（N₁）,and two crop rotation patterns were used:alfalfa-maize rotation（MA-ME）and maize-alfalfa rotation（ME-MA）,resulting in a total of four treatments,with each treatment being repeated four times.From 2016 to 2018,maize and alfalfa were consecutively planted for 3 years under nitrogen application and no nitrogen application conditions,respectively.In 2019,crop rotation treatment was conducted,with alfalfa planted on the original maize plots and maize planted on the original alfalfa plots,with no change in nitrogen fertilization treatments or crop planting areas.The study aimed to investigate the effect of maize and alfalfa rotation on yield,and the relationship between changes in soil physicochemical properties and PLFA of rhizosphere microorganisms with crop yield.The main results of this study are as follows:（1）In the analysis of the total crop yield over 6 years,the MA-ME rotation pattern under N₁condition achieved the highest yield in this study.In terms of the 3-year average maize yield,compared with the ME-MA pattern,the MA-ME pattern significantly increased the average yield of the subsequent maize crop due to the planting of alfalfa.The average annual maize yield of the MA-ME rotation pattern was 4.72 t ha^-1under N₁condition and 6.67 t ha^-1under N₀condition,which represented an increase of 74.81%and 23.9%,respectively.（2）In the MA-ME pattern,the planting of alfalfa as a cover crop significantly increased accumulation of nutrients in the rhizosphere soil of subsequent maize crops.Compared to the ME-MA pattern,the MA-ME pattern significantly increased oil organic carbon（SOC）and total nitrogen（TN）due to the planting of alfalfa.In N₀condition,the rhizosphere soil TN and SOC of maize in the MA-ME pattern were 0.86 g kg^-1,23.57 g kg^-1,which were 145.71%and177.62%higher than that in the ME-MA pattern.In N₁condition,the rhizosphere soil TN and SOC of maize in the MA-ME pattern were 0.92 g kg^-1,23.04 g kg^-1,which were 109.10%and20.75%higher than that in the ME-MA pattern.（3）In the MA-ME rotation pattern,the planting of alfalfa altered content of bacteria,fungus,actinomyces,Arbuscular mycorrhizal fungi PLFA,TPLFAs,and gram-positive bacteria/gram-negative bacteria and bacteria/fungus in the rhizosphere of maize.Among them,compared with the ME-MA pattern,alfalfa as a cover crop in the MA-ME pattern has the most significant impact on the PLFA of bacteria.Compared to the ME-MA pattern,planting alfalfa in the MA-ME pattern under N₀and N₁conditions increased the maize rhizosphere soil by 201.71%、202.26%for bacteria PLFA,and by 32.32%and 44.69%for bacteria/fungus.（4）There were significant positive correlations between yield of maize and NH₄⁺-N,NO₃^--N.In the MA-ME pattern,there were significant positive correlations between NH₄⁺-N,NO₃^--N and content of fungus PLFA without N application.At the same time,there were significant positive correlations between NH₄⁺-N,NO₃^--N and content of bacteria PLFA under N application.Therefore,in the MA-ME the increase of fungal PLFA increased soil nitrogen accumulation and thus increased maize yield,under nitrogen application conditions.While under no nitrogen application conditions,the turnover of nitrogen increased yield of maize.In summary,the planting of alfalfa had increased the quantity of soil microbial PLFA,which had a positive effect on soil nitrogen and increased the yield of the subsequent crop,maize.The results of this study provide a theoretical basis for exploring the effects of nitrogen application and rotation with perennial legume forages on soil microbial PLFA,soil physicochemical properties,and crop yields in the Northeast China farming-pastoral zone.By establishing a rotation pattern of alfalfa and maize,We make reasonable use of perennial leguminous forage and achieve precise fertilizer control for subsequent crops.This study provides scientific theoretical support for food security and forage supply in the Northeast China farming-pastoral zone.