Study On The Mechanism Of Soil Organic Nitrogen Mineralization In Rice-Faba Bean Rotation Field Under The Coupling Of Water And Nitrogen

Rice broad bean rotation is an important planting system in China.At present,the excessive use of nitrogen fertilizer in rice cultivation in China has led to excessive soil nitrogen residue,which can also affect the mineralization and nitrogen supply process of soil organic nitrogen（SON）,an important source of crop nitrogen utilization.However,the mineralization pattern of SON under water and nitrogen regulation,as well as the fate and effectiveness of nitrogen fertilizer in rice broad bean rotation,are still unclear.This article conducted two irrigation modes,namely flood irrigation（FI）and alternate wetting and drying（AWD）,for the rice season.There were three nitrogen application levels,F0（blank control,no nitrogen application）,F1（70%traditional nitrogen application rate,135 kg/ha）,and F2（193 kg/ha,calculated as pure nitrogen）.Traditional irrigation did not apply nitrogen during the broad bean season,with a total of six treatments.The annual rice broad bean rotation experiment was conducted using¹⁵N labeled urea as the source of farmland nitrogen fertilizer.The purpose of this study is to take the main factors affecting the mineralization of SON:the dynamic change law of soil physical and chemical properties and micro ecological environment as a breakthrough,analyze the mineralization law of SON in rice broad bean rotation fields under different water nitrogen coupling modes,identify the key factors affecting SON,and reveal the mineralization mechanism of SON in rotation fields.At the same time,based on the comprehensive assessment and analysis of the impact of annual crop rotation yield,nitrogen availability and nitrogen leaching loss on the groundwater environment under different water nitrogen coupling modes,the optimal water nitrogen coupling mode of rice broad bean rotation system was proposed,which is of great significance for optimizing the water nitrogen regime of rice broad bean rotation and enriching the water nitrogen coupling theory of water dry rotation.The research results are as follows:（1）The AWD model has an average increase of 4.5%in rice yield compared to the FI model.Under the FIF1 treatment,the rice yield was the highest,reaching 10.4 t/ha,while under AWD mode,the rice yield increased with the increase of nitrogen application level.The yield of rotating broad beans after the FI mode in the rice season is 8.3%higher than that after the AWD mode in the rice season,and it is shown that under the nitrogen application level in the F1 mode of the rice season,the yield of rotating broad beans is the highest,reaching 3.9 t/ha.Due to the high economic value of broad beans,the optimal irrigation nitrogen application treatment in terms of yield is AWDF1.（2）Compared with the beginning and end of the annual rice broad bean rotation,the soil nitrate nitrogen（NO₃^--N）and SON content in the 0-40 cm layer decreased by an average of 34.9%and 13.6%,respectively,while soil ammonium nitrogen（NH₄⁺-N）showed a certain accumulation,with an average increase of 228.3%.The content of NH₄⁺-N and NO₃^--N in soil increases with the increase of nitrogen application level,while the content of SON is the highest in the FIF1 treatment,with an average of 2.94g/kg in the 0-10 cm layer.（3）After applying nitrogen fertilizer in the rice field during the rice season,the content of SON increased.The content of SON slowly decreases during the rice season,but significantly decreases during the pod-setting period of broad beans,with a decrease of 0.64 g/kg（24.1%）.The mineralization rate of SON fluctuates positively and negatively during the rice season,with the most severe fluctuation occurring between20-40 cm.The mineralization rate of SON in the broad bean season remains positive,with an average of 0.16 g/（kg 30d）in the 0-10 cm layer,and the mineralization rate gradually decreases with increasing depth.（4）The AWD mode can improve soil temperature and humidity,thereby increasing available nitrogen content and soil enzyme activity,but it is easy to reduce soil p H and aggregate stability.Closely related to the mineralization rate of SON is the abundance of soil urease and fungi（especially Aspergillaceae,Neuroaceae,and unclassified＿o＿＿Helotiales）,while soil invertase,catalase,phosphatase,and bacterial abundance have a relatively small impact.The dynamic changes in soil mineralization-related microbial abundance are strongly influenced by nitrogen application,with the highest increase in Aspergillaceae abundance at the F2 level compared to the F0 level,with an average increase of 90%.（5）In the rotation system,nitrogen fertilizer used in the rice season is mainly utilized during the rice season,with rice absorbing an average of 33.0%-38.1%of the applied nitrogen fertilizer.The effectiveness of residual nitrogen fertilizer in the rice season is very low during the rotation of the broad bean,and the nitrogen fertilizer usedthe in broad bean only accounts for 3.6%-4.0%of the residual nitrogen fertilizer in the rice season.The nitrogen absorbedthe by rice and broad beans is approximately77.0%-96.4%provided by SON mineralization and other nitrogen sources.（6）The concentrations of NO₃^–-N and NH₄⁺-N in the 20 cm,40 cm and 60 cm depths of the paddy field were always high,especially NH₄⁺-N,and the average concentration reached 0.56 mg/L at the three depths,which was much higher than the maximum allowable concentration of NH₄⁺-N in the disturbance flow（0.1 mg/L）.The application of nitrogen fertilizer significantly promoted the leaching loss of NH₄⁺-N in the turbulent flow,and the loss was greater at F2 level than at F1.（7）The comprehensive analysis of SON mineralization,nitrogen fertilizer utilization efficiency,crop yield,and nitrogen leaching loss in the rice broad bean rotation system shows that AWDF1 is a relatively green and efficient water and nitrogen management model.In addition,during the pod setting stage of the broad bean season,in addition,a certain amount of nitrogen fertilizer is required to supplement soil nutrients during the pod stage of broad beans.