Effects Of Carbon And Nitrogen Additions On Soil Carbon And Nitrogen Dynamics In Lucerne-Winter Wheat Rotation Systems

The stability of the soil organic carbon pool plays a critical role in soil carbon and nitrogen dynamics,as well as soil fertility.Lucerne（Medicago sativa L.）is a perennial legume crop with nitrogen fixation ability.Due to its high input and retention of organic matter,it has great potential in improving soil carbon fixation Introducing grain and forage crop rotations with lucerne can enhance the agronomic traits,nutritional quality,and yield of the subsequent rotation crop while improving soil fertility.Investigating the mineralization and stability of soil organic carbon and nitrogen transformation processes in agricultural soils holds significant research value.However,studies on soil carbon and nitrogen transformations in grain and forage crop rotations in the Loess Plateau and arid regions under exogenous carbon and nitrogen addition conditions are still limited.In this study,we selected soil from a 3-year lucerne rotation followed by 3-year continuous winter wheat（Triticum aestivum L.）and a 2-year lucerne-1-year winter wheat rotation in the Loess Plateau and arid regions as research subjects.We explored the impact of exogenous carbon and nitrogen addition on soil carbon and nitrogen dynamics through indoor cultivation experiments.Three carbon substrates were set:glucose,citric acid,L-glutamine,and a control（water）.Additionally,two nitrogen substrate gradients were set:potassium nitrate added and a control（water）.We investigated the effects of exogenous carbon and nitrogen addition on soil carbon and nitrogen dynamics in lucerne-wheat rotation systems separately,as well as the effects of simultaneous addition of carbon and nitrogen on soil carbon and nitrogen dynamics in lucerne-wheat rotation systems.The main findings of this study are as follows:（1）The addition of carbon significantly increased nitrate nitrogen and soil organic carbon content（p<0.05）.Compared to the control,the nitrate nitrogen content increased by 9.0%and 27.2%with glucose and citric acid addition,respectively,while the organic carbon content increased by 7.7%,5.8%,and 6.9%with glucose,L-glutamine,and citric acid addition,respectively.The 2-year lucerne-1-year winter wheat rotation system increased Fe（II）and Fe（III）content,and under L-glutamate treatment,the Fe（II）content in the 3-year continuous lucerne rotation was significantly higher than in the other two rotation systems.Exogenous carbon addition significantly increased complex iron oxide content and significantly decreased free iron oxide content,while amorphous iron oxide content did not show significant differences.As lucerne planting years increased,soil soluble organic carbon content gradually decreased.Redox potential changes followed a pattern of"increase-decrease-slow increase-tendency to stabilize,"with peak values appearing on the second and fourth days of the experiment,reaching 486 mv and 474 mv,respectively.（2）Under nitrogen addition,the ammonium nitrogen content in the 3-year lucerne system significantly increased by 20.8%（p<0.05）compared to the 3-year winter system.As lucerne planting years increased,available phosphorus content gradually increased,while labile organic carbon and soil organic carbon content gradually decreased.Complex iron oxide,amorphous iron oxide,and free iron oxide contents gradually increased.In the lucerne-wheat rotation system,Fe（II）and Fe（III）content increased significantly by 24.3%and 37.8%（p<0.05）,respectively.The basic trend of redox potential change was"increase-decrease-no change,"and the redox potential in the nitrogen addition treatment was higher than the control in all rotation systems.（3）When carbon and nitrogen were added simultaneously,both citric acid and L-glutamine significantly increased carbon dioxide（CO₂）emissions（p<0.05）and promoted soil organic carbon mineralization and total nitrogen content in the three different rotation soils.The lucerne-wheat rotation system significantly increased nitrate nitrogen and soluble organic carbon content and decreased labile organic carbon content（p<0.05）.L-glutamine significantly increased nitrous oxide（N₂O）emissions（p<0.05）,and the soil N₂O emissions in the 3-year continuous lucerne rotation was higher than in the 3-year continuous winter wheat rotation and the 2-year lucerne-1-year winter wheat rotation.When citric acid and potassium nitrate were added simultaneously,the soil organic carbon content in the rotation soil was significantly reduced（p<0.05）.In summary,this study demonstrated that exogenous nitrogen addition significantly increases the free iron oxide content in the 2-year lucerne-1-year winter wheat rotation soil,contributing to the stability of soil organic carbon and nitrogen.When exogenous carbon and nitrogen were added simultaneously,citric acid and L-glutamine promoted soil organic carbon mineralization.In practice,adopting the lucerne-wheat rotation system can enhance the stability of soil organic carbon and nitrogen,providing valuable insights for agricultural soil carbon and nitrogen dynamics and nitrogen fertilizer application.These findings offer crucial theoretical support for optimizing soil carbon and nitrogen management in grain and forage crop rotation systems in loess plateau and arid areas.This can aid in improving soil fertility and reducing greenhouse gas emissions,providing significant scientific guidance for achieving national dual carbon goals and fostering green development.