Study On The Environmental Safety Thresholds Of Nitrogen Inputs In The Key Greenhouse Vegetable Production Regions Of Huang-Huai-Hai

The issue of agricultural non-point source pollution resulting from excessive nitrogen（N）fertilizer inputs in greenhouse vegetable fields（GVF）has attracted increasing attention.Striking a balance between yield and the environment,and identifying scientifically safe thresholds for N fertilizer inputs is of paramount importance for advancing green development in the vegetable industry.This study employed a combination of literature review,field investigation,and Denitrification-Decomposition（DNDC）model simulation to examine the GVF in the Huang-Huai-Hai region（HHH）.A regional nitrogen management database was built,the spatial variations in N input and output were determined,the response equations for the N input-derived effects for different watersheds were quantified,the regional environmental safety N application thresholds were identified,and a multi-objective N application optimization strategy for yield and environmental cooperation was developed in the HHH.The main results were as follows:（1）There were significant spatial variations in both greenhouse vegetable yields,N application and reactive N losses in the HHH.The mean variation in yield,organic N,synthetic N and total N applied in different provinces（municipalities）ranged from 46.6 to 88.2 t ha^-1 season^-1,107 to 274 kg N ha^-1 season^-1,116 to 324 kg N ha^-1 season^-1 and 292 to 579 kg N ha^-1 season^-1,respectively.Fruit vegetables exhibited considerably higher yield and N application rates compared to roots/stems/leafy vegetables.On a county level,the mean yield and N application rate for GVF were 76 t ha^-1 season^-1 and 429 kg N ha^-1 season^-1,respectively,with a yield ranging from 11-364 t ha^-1 season^-1 and the total N application ranging from 0-2003 kg N ha^-1 season^-1,respectively.The result of the integrated literature revealed that the relationship between N₂O emission,NH₃ loss and NO₃^--N leaching and organic and synthetic N application could be expressed by a highly significant binary linear equation respectively.The proportion of counties with N₂O emissions,NH₃ volatilization and NO₃^--N losses below 9,20 and 200 kg N ha^-1 season^-1 respectively,accounting for more than 90%.（2）The simulation results of DNDC demonstrated that the response relationships between yield,GNr emissions,NO₃^--N leaching,N use efficiency,apparent N balance,and the limit of NO₃^--N leaching and N application in GVF can be expressed by highly significant linear+plateau,piecewise regression models,piecewise regression models,exponential functions,linear equation and piecewise regression models,respectively,at the third-level watershed cells of the HHH.These equations could be directly applied to estimate the yield and environmental effects of greenhouse vegetable fields in each watershed across the HHH.（3）Based on the analytic hierarchy process weighting and multi-objective optimization algorithm（MAHP+GWO）,the optimized N application rates of various watersheds in the GVF of the HHH were generally consistent with the inflection point where the N application rates reached the maximum yield.The optimized N application rates of each watershed ranging from 311.45-523.22 kg N ha^-1 season^-1 and211.73-359.61 kg N ha^-1 season^-1 for spring-summer and autumn-winter cultivation,respectively.The mean optimized N application rates in the spring-summer cultivation were significantly higher than those in the autumn-winter cultivation.（4）Based on the quality requirement of Class III groundwater(NO₃^--N≤20 mg L^-1)in the"Groundwater Quality Standards"as the upper limit of NO₃^--N concentrations for leaching in the root zone,the upper limit of NO₃^--N leaching loss could be defined as the multiplying of the water leaching volume by the maximum allowed NO₃^--N concentrations from the field.By comparing critical limit curve of NO₃^--N leaching and the NO₃^--N leaching curves,the maximum permissible NO₃^--N leaching of GVF in each watershed ranged from 121.64-128.86 kg N ha^-1 season^-1 and 45.39-79.29 kg N ha^-1 season^-1 for spring-summer and autumn-winter cultivation,respectively.The corresponding environmental safety N application thresholds was 527.77-852.05 kg N ha^-1 season^-1 and 289.15-545.38 kg N ha^-1 season^-1,respectively.The mean environmental safety N application thresholds in the spring-summer cultivation was significantly higher than those in the autumn-winter cultivation.In conclusion,this study found that there were obvious spatial variations in yield,N input and reactive N loss in GVF in the HHH.Incorporating the production characteristics of each watershed within the regional environmental safety threshold of N input,by using the N response equation and the optimized N application strategy,we can accurately estimate the vegetable yield and environmental effects and other indicators,and achieve the multi-objective cooperation of yield and environmental effects.These guidelines serve as a framework for the clean production of greenhouse vegetables in the HHH region.