| At the regional,continental and global levels,global problems related to biodiversity loss,water resource shortage,climate change and acceleration of the nitrogen cycle are increasingly affecting and restricting the sustainable development of human society.Looking back over the 100 years of world fertilizer use,fertilizer has made a significant contribution to increasing grain production and solving food shortages.However,with the massive application of fertilizers,serious ecological balance destruction and environmental pollution problems have also occurred.Application of nitrogen fertilizer is likely to cause greenhouse effect by nitrogen oxide volatilization and pollution of groundwater by nitrate leaching.Vadose zone is a necessary way for nitrate and other pollutants to enter groundwater,where atmospheric water and surface water contact with groundwater and exchange water.It is also an important place for nitrate storage.Therefore,it is of great significance to study the migration and storage of nitrate in vadose zone,especially to predict the time of peak nitrate leaching arriving water table,which can provide a strong basis for policy adjustment.This paper quantified the lag time of peak nitrate leaching arriving water table and nitrate storage in the global vadose zone based on improving the nitrate time bomb?NTB?model and published data sets,including global soil nitrate leaching in 20th century,vadose zone thickness,groundwater recharge,and porosity.In order to determine whether the peak soil nitrate leaching entered the vadose zone from the basis of the soil in the 20th century,the input and output of global soil nitrogen cycle from 1900 to 2000 were obtained based on the simulation results of g Integrated Model to Assess the Global Environment?IMAGE?.N cycle inputs included biological N fixation,atmospheric N deposition,N compound fertilizer application,and animal manure application.The inputs increased obviously from1900 to 2000,which was the main factor causing the increase of N surplus.The loss of N in the process of denitrification,leaching,remove with harvesting and grazing and NH3 volatilization constituted N outputs.Denitrification was the main process of nitrate reduction and accounted for a large proportion in N output.The leaching amount was relative to the change of climate and soil conditions.Leaching rates were4.8%and 6.1%of total N inputs in the Middle East and North America,lower than other areas.The leaching rate was higher in humid regions,such as South Asia and Europe,which accounted for 17.9%and 18.5%of the total N inputs respectively.N recovery was 58.8%in North America,48.2%in South America,and higher than 60%in the Middle East and Africa.The N recovery in South Asia was 36.5%,significantly lower than that in other regions.High N fertilizer input and low N recovery was one of the reasons for the increase of nitrate leaching into groundwater.Soil nitrate leaching in different countries increased significantly in the 20th century,especially in the United States,Western Europe,India and China.Until 2000,the leaching amount in the United States remained stable,while that in Western Europe declined significantly compared with 1980.But China and India maintained upward trend.Peak nitrate leaching has not been entered in vadose zone in 56 of the248 countries.Developed and developing countries accounted for 17.9%and 82.1%respectively.The leaching amount of basic volcanics and acid plutonic was still on the rise.The peak nitrate leaching in unconsolidated sediments,siliciclastic sediments,mixed sedimentary,carbonate sediments and evaporite rocks has been entered in vadose zone.Of the 37 major aquifer systems in the world,the peak nitrate leaching in 30 aquifers has entered in vadose zone.To simulate the time that peak nitrate that has entered the vadose zone arriving water table,retardation factor was introduced in the nitrate time bomb?NTB?model.The retardation factor represented influencing factors such as permeability,pore size,diffusion,dispersion and adsorption.Taking the unaffected area by the peak nitrate leaching of chalk aquifer in the UK as reference,the global retardation factor value was determined by Monte Carlo simulation.Based on the improved NTB model,the velocity and storage of nitrate in the global vadose zone and the time of peak nitrate leaching arrived the water table were simulated for the area that peak nitrate leaching have entered the vadose zone.The velocity after introducting of retardation factor was closer to the measured value than that calculated by the original NTB model,which improved the accuracy of the nitrate leaching velocity and lag time simulation in the vadose zone.The regions with higher velocity were mainly located in eastern Canada,northern South America,central Africa,Norway and northern Britain.The time of the peak nitrate arrived the water table was calculated by taking the country,lithology,major large aquifers and types of aquifers as units.The time varies greatly,which can be up to 815 years from 2019.The largest nitrate storage area located at North America,China and Europe,which have relatively thick vadose zones and extensive long-term agricultural activities.In these areas,the long lag time of nitrate in vadose zone may delay the effect of changes in agricultural practices on groundwater quality.Taking typical latitude zone and countries as examples,the correlation and influencing factors of soil nitrate leaching and its lag time in vadose zone were analyzed.Among the many factors affecting the nitrate leaching,the effects of human factors were greater than that of natural factors,especially the N fertilizer application.Increasing grain yield per unit area by N fertilizer application was the main driving factor of N fertilizer input.Population,urbanization rate and proportion of cultivated area have less influence than N fertilizer application.Countries with high urbanization rate have less labor force to devote to agricultural activities,and can obtain economic income through other means.The economic pressure on agricultural activities would be relatively small.China and India have significantly higher numbers of people earning economic income from agricultural activities than other countries,leading to increased pressure for economic income from agricultural activities.Increasing grain yield per unit area by applying N fertilizer was the main driving factor of N fertilizer input,thus increasing leaching amount.The United States,China and India have significantly more arable land than other countries.However,the proportion of cultivated area in China was lower than that in the United States,France and India,which was related to the unsuitability of cultivation in western China.The high values of nitrate leaching in China were concentrated in the eastern areas with large cultivated area and dense population.With the increase of nitrate leaching,the change of cultivated land area was not obvious in America,India and China.However,nitrate leaching in Brazil and other developed countries did not change significantly with the increase of cultivated land area.The results showed that the higher leaching amount in China and India was caused by the higher N fertilizer input and lower N utilization rate.After nitrate leaching entered the vadose zone,its lag time was mainly affected by groundwater recharge,vadose zone thickness,effective porosity and retardation factor.The groundwater recharge was the most important factor affecting the lag time,the impacts of vadose zone thickness and porosity on lag time were relatively small.This study showed that the vadose zone was an important migration channel and storage area of nitrate in the whole world,and the lag time of peak nitrate in vadose zone determined when it would start to affect groundwater quality.Nitrate storage was significant in areas with extensive history of agricultural development.In some of these areas,peak nitrate leaching did not enter the vadose zone in the 20th century..Even in areas where the peak has already entered,it would take at least several decades arriving water table and begin to affect groundwater quality.Therefore,when planning pollution mitigation measures,policy makers should consider the lag time of peak nitrate and nitrate storage in vadose zone. |
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