Construction Of An Inorganic Nitrogen Microelectrode Combined System And Its In Situ Monitoring Of The Transformation Process Of Inorganic Nitrogen In Paddy Soil

China is currently one of the world's largest consumers of nitrogen fertilizer and producers of rice.However,only 30%-40%of nitrogen fertilizer can be effectively utilized in rice production,with large amount of nitrogen loss that has caused hazards to the environment.At present,the main fertilization modes for rice fields in China include surface application and deep application.Surface fertilization is the most commonly used method currently,usually leading to high nitrogen loss to the neighboring environment.Deep fertilization can reduce nitrogen loss through altering the nitrogen cycling in soil,thus raising the nitrogen use efficiency.Three micro-environments(i.e.water-soil interface,rhizosphere and non-rhizosphere)with high heterogeneity would be formed in the in the paddy ecosystem,due to continuous flooding and rice growth.Close interactions with frequent material-energy exchanges exist in these three regions,where the nitrogen transformation processes also significantly differ from each other.Present researches focused on nitrogen transformation process in paddy ecosystems are mostly based on destructive sampling during aerobic or anaerobic incubation which are artificially constructed and thus are ex-situ observations.Information about of the dynamic changes of inorganic nitrogen in different regions of a"same system"provided by in-situ and continuous monitoring still remain lacking.This study successfully constructed an in-situ and real-time system for the monitoring of inorganic nitrogen in paddy soil.The microelectrodes employed in the system were developed with longer life span and better structural robustness.Based on pot experiments,the effects of surface fertilization and deep fertilization on the nitrogen dynamics in paddy soil were investigated.Nitrogen transformation processes were continuously monitored via real-time and in-situ measurements of inorganic nitrogen contents in different regions.The main results are as follows:(1)A fabrication method with high reproducibility of microelectrodes for the measurements of NH₄⁺,NO₂^-,and NO₃^-,respectively,was developed in this study.The detection range of the NH₄⁺microelectrode is 10^-1?10^-5 mol/L.The slope of its standard curve is-56.966 m V/-lg[NH₄⁺],R²=0.9997.The detection range of the NO₂^-microelectrode is 10^-1?10^-6 mol/L.The slope of its standard curve is 37.933m V/-lg[NO₂^-],R²=0.9974.The detection range of the NO₃^-microelectrode is10^-1?10^-5 mol/L.The slope of its standard curve is 56.627m V/-lg[NO₃^-],R²=0.9998.The response times of the three microelectrodes were all less than 5s,and their life spans were at least 7 days.The detection range,response time,ion selectivity,stability and life span of the microelectrode met the requirements of long-term dynamic monitoring in subsequent experiments.In this study,the self-made inorganic nitrogen ion microelectrode in-situ test and the flow analyzer method were used for destructive sampling to simultaneously test NH₄⁺-N and NO₃^--N in soil samples.The results show that the two test methods are different.The measurement based on the developed microelectrodes may be a promising analysis method of the nitrogen transformation processes in paddy ecosystem,which is non-destructive,in-situ,and continuous.(2)A pot experiment with rice cultivation was conducted and the developed microelectrodes were applied in monitor the spatial distribution of inorganic nitrogen and the nitrogen transformation process in different regions,after surface fertilization and deep fertilization using NH₄HCO₃ solution.(1)After surface fertilization,the NH₄⁺-N content was increased only at the water-soil interface,up to 75.1 mg/L.NH₄⁺adsorption in this area was the strongest,with more than 90%of NH₄⁺being adsorbed.The NO₃^--N concentration was increased at the water-soil interface and in the rhizosphere,by 6.6 mg/L and 3.8 mg/L,respectively.Nitrification in these two areas was more active than the non-rhizosphere.In the early stage after fertilization,the rates of NH₄⁺adsorption and nitrogen loss were higher at the water-soil interface and in the non-rhizosphere,whereas those in the rhizosphere were maintained at a low level.(2)After deep fertilization,the highest value of NH₄⁺-N content at the water-soil interface was 22 mg/L.The NH₄⁺-N concentration was also increased in the rhizosphere and non-rhizosphere,by 5.42 mg/L and 2.31 mg/L,respectively,indicating that the N added by deep application could diffuse to the rhizosphere area.The NO₃^--N concentration was increased at the water-soil interface and in the rhizosphere,by 4.17 mg/L and 7.09 mg/L,respectively.NH₄⁺-N adsorption occurred in all the three regions,among which that at the water-soil interface was the strongest.In the early stage after fertilization,the rates of NH₄⁺adsorption and nitrogen loss were higher at the water-soil interface and in the non-rhizosphere than in the rhizosphere,whereas those in the rhizosphere were maintained at a low level.In the rhizosphere,large parts of NH₄⁺were directly taken up by rice or converted into NO₃^-through nitrification,resulting in lower NH₄⁺adsorption and nitrogen loss.The results show that the nitrogen from deep fertilization can reach the rhizosphere zone of the plant more easily.In summary,this study successfully constructed an in-situ and real-time system for the monitoring of inorganic nitrogen in paddy soil,attributed to the development of selective microelectrodes with longer life span and better structural robustness.Using this novel technique,the nitrogen transformation processes were investigated in three different regions formed in paddy ecosystems under different fertilization modes.This study provided a promising approach for the in-situ and real-time analysis of inorganic nitrogen in paddy soils.The findings of this study would help in better understanding the nitrogen transformation processes in paddy ecosystems under different fertilization modes.