The Impacts And Mechanisms Of Optimized Fertilization Management On Soil Fertility And Environments

The coordination of food security,soil quality and ecological environment is a major challeng of the global green agricultural development.However,in recent years,excessive chemical fertilizers were applied by farmers for increasing crop yields,which leaded to a series of problems such as low nutrient use efficiency of crop,soil quality decline,and gaseous nitrogen（N）pollution.Therefore,understanding the impact of optimal nutrients management on crop yield,nutrient utilization,soil fertility,atmospheric environment（e.g.,NH₃ volatilization and N₂O emissions）,and soil environment（e.g.,antibiotic resistance genes）can provide fundmental guidance for green crop-soil-environment trinity agricultural production.The field trials were carried out at the Rugao Agricultural Science Research Institute in Jiangsu Province.First,the crop yield,economic benefits and legacy effects under different N application rates were studied to determine the optimal N application rate for the rice and wheat based on a long-term N fertilizer gradient field trial and a greenhouse pot experiment.Second,the changes in the biomass of rice and wheat,nutrient utilization,soil fertility,NH₃ volatilization and N₂O emissions and soil antibiotic resistance genes under different nutrient management models were explored through three-year field trials.Therefore,different nutrient management modes were arranged:1)No fertilization（CK）;2)Phosphorus and pstassium only（PK）;3)Farmers’ fertilization practice（FFP）;4)Optimized N fertilization（OPT）;5)On the basis of optimized nitrogen fertilizer,organic fertilizer was replaced by 20%chemical fertilizer N（OPTMs）,and this fertilization model included three treatments:pig manure（OPTPM）,chicken manure（OPTChM）,and cow manure（OPTCM）;6)Organic fertilization only（M）.Therefore,a total of 8 treatments was included and the effects of different nutrient managements regimies on the yield,biomass,N accumulation and N use efficiencies of rice and wheat were explored.Combined with the soil chemical properties and soil enzyme activities,the soil fertility of rice-wheat season and the changes of soil microbial community in the paddy field were comprehensively evaluated.NH₃ and N₂O emissions from the paddy field were measured using the continuous airflow enclosure method and static closed chamber method were used to measure for two consecutive years.Finally,the succession of ARGs following the repeated application of three types of animal manures（pig,chicken,and cow manure）in consecutive three years to agricultural soil were investigated using high-throughput qPCR.The main findings obtained are as follows:1.As the application of N input increased,the yields and income-increasing benefits of wheat season,rice season,and annuals were increased firstly and then decreased.Non-linear regression analysis showed that the N application rates corresponding to the highest theoretical yields of wheat and rice was 217 kg ha^-1 and 258 kg ha^-1,respectively.When the amount of N applied in wheat and rice season reached 200 kg ha^-1 and 253 kg ha^-1,respectively,the increased economic benefit of N fertilizer was the highest.Five long-term N gradient soils were collected in pots,and wheat seeds were shown without nutrient addition.The results showed that the soil with historical N fertilizer application has obvious legacy effects,which is characterized as significant increasing in the soil alkaline N,total N content and above-ground biomass of wheat.Meanwhile,the soils with historical N inputs harbored a higher aboveground plant biomass and bacterial a-diversity of wheat rhizosphere,and the composition of the rhizosphere bacterial community changed significantly with the historical N application rate.More complexed network of rhizosphere bacteria observed in the soils with higher N inputs(>280 kg N ha^-1),however,increased co-occurrence network size and connectivity were accompanied by increasing aboveground biomass of wheat.2.Optimal fertilization management（including optimized N fertilizer,organic fertilizer replacement of chemical fertilizer N）can achieve the equivalent output level of FFP treatment by optimizing crop growth and N absorption,coordinating yield components.Compared with FFP treatment,the rice and wheat yields in OPT treatment increased by 4.9%-7.1%and 1.2%-12.0%,respectively,in 2015-2018.On the basis of OPT,20%chemical fertilizer N replaced by organic fertilizer（OPTMs,including OPTPM,OPTChM,OPTCM）achieved the similar yield level of FFP treatment,and there was no significant difference in crop yield among the OPTPM,OPTChM and OPTCM treatment.The sustainable yield index（SYI）of rice in OPT and OPTMs was higher than that in FFP.OPT and OPTMs fertilization increased the yield of rice and wheat by optimizing yield components such as increasing the number of panicles and 1000-grain weight.The number of panicles and the number of grains were the main predictors of the yield of rice and wheat.The grain protein content of rice and wheat,and the gluten content of wheat in OPT treatment were higher than FFP treatment,and the protein content of FFP treatment did not changed significantly from that of OPTMs.In the early growth stage（turning green or tillering stage）of rice and wheat,the aboveground biomass and N accumulation in FFP treatment were significantly higher than those in OPT and OPTMs,while the difference in the later growth period gradually decreased.There was no significant difference between the N fertilizer treatments at the mature stage.In the rice season,OPT and OPTMs treatments increased the PFPN by 47.4%and 75.9%,the AEN by 60.9%and 79.7%,and the REN by 75.5%and 85.1%,respectively,compared with FFP treatment.In the wheat season,OPT and OPTMs also significantly increased PFPN,AEN,REN compared to FFP treatment.3.The application of organic fertilizer could improve soil fertility.Compared with application of chemical fertilizers（FFP,OPT）and no fertilizers（CK）,three organic fertilizers instead of chemical fertilizers（OPTMs）and organic fertilizers only（M）treatment significantly increased the content of soil organic matter（SOM）,available phosphorus（AP）and available potassium（AK）,particularly in the last two seasons.The soil enzyme activity including the C-related β-glucosidase and β-xylosidase,N-related leucine aminopeptidase and β-1,4-N-acetyl-glucosaminidase and peroxidase related to oxidative decomposition were increased in OPTMs treatments.The soil integrated quality fertility index（IFQI）in M,OPTPM,OPTChM and OPTCM treatments were higher than that in FFP,OPT and CK treatments in the rice-wheat seasons,and the value in OPTCM treatment was the highest.SOM and C-related enzyme activities were significantly positively correlated with IFQI.In addition,different fertilizer treatments had no significant effect on the diversity and composition of soil microbial community in the rice season.4.Optimal fertilization management could significantly reduce NH₃ volatilization in the paddy soil.Compared with the FFP treatment,the cumulative amount of ammonia volatilization decreased by 20.1%-23.9%in the OPT treatment,and the cumulative amount of ammonia volatilization in the organic fertilizer replacement nitrogen（OPTPM,OPTChM,OPTCM）decreased by 37.2%-43.8%,while there was no significant difference among the OPTPM,OPTChM,OPTCM treatment;ammonia volatilization in paddy fields mainly occurred after the base fertilizer and tiller fertilizer,and the ammonia volatilization flux was significantly correlated with the NH₄⁺-N concentration and pH of the surface water.The application of chemical N could significantly increase soil N₂O emissions,but there was no significant difference in the cumulative N₂O emissions from paddy fields under different N fertilizer treatments,with total N₂O emissions of 0.45-0.79 kg N ha^-1.The soil NO₃-content was positively correlated with N₂O emission rate.5.The application of animal organic manure resulted in the risk of the ecrichment and spread of antibiotic resistance genes（ARGs）in the soil.Compared with CK and OPT treatments,the diversity and richness of soil ARGs in OPTPM,OPTChM,and OPTCM increased significantly.There were no prominent differences in the abundance or diversity of ARGs among the three different manured soils（OPTPM,OPTChM,and OPTCM）.The abundance and diversity of ARGs in manured soils increased over three consecutive years.Additionally,the abundance of mobile genetic elements（MGEs）and bacteria were positively correlated with ARGs,while the changes in the ARGs profiles were dramatically associated with the MGEs and bacterial communities.The profile of soil ARGs was both directly and indirectly regulated by the application of organic fertilizers,as well as by the duration of organic fertilizers and the abundance of MGEs,without being affected by bacterial diversity.In summary,the soil with long-term application of N fertilizer has the potential to promote the growth of crops in later seasons,while increasing the microbial diversity of the rhizosphere and the interaction of microbes.Optimizing N fertilizer management practices and organic fertilizer application can improve the crop N utilization efficiency on the premise of ensuring crop yield.Organic fertilizer application can increase soil chemical and biological fertility and improve soil comprehensive fertility.In addition,the substitution of organic fertilizer for chemical fertilizer N can reduce ammonia volatilization in paddy fields,but increase the enrichment and risk of spread soil ARGs.