Effect Of Nitrogen Application Rate On Nitrogen Uptake And Utilization In A Garlic-Maize Multiple Cropping System And Establishment Of Garlic Nitrogen Diagnostic Model

The Guanzhong plain of Shaanxi province is a traditional garlic production area in China.The multiple cropping of garlic and corn is the main cropping pattern used in this area.In order to achieve high yield and efficiency,the garlic（Allium sativum L.）planting under this cropping system has a common serious problem which is the excessive application of nitrogen（N）fertilizers.This situation can not only lead to the waste of N fertilizers,but also cause a series of environmental problems.The effective measures to solve this problem are to determine the appropriate nitrogen application amount and to establish an effective diagnosis method of garlic nitrogen nutrition.This research was conducted with six nitrogen levels:0（N0）,60（N60）,120（N120）,180（N180）,240（N240）,300（N300）kg·hm^-2（The ratio of fertilizer was 4:4:2）in three-year field experiments with two garlic varieties‘Cangshan’and‘Gailiangsuan,which are two representative varieties of garlic in local area.The effects of different nitrogen application levels on the growth and physiology,dry matter and nitrogen accumulation,water consumption and nitrate migration of garlic were studied.The after-effects of garlic season N application on summer corn were also studied to screen the reasonable N application rate for corn.The critical nitrogen concentration dilution curve model of garlic was constructed by analyzing the relationship between aboveground dry matter and plant nitrogen concentration of garlic.The model and SPAD values were used to establish a nitrogen nutrition diagnosis method for garlic in this research.Meanwhile,the after-effects of N application in garlic season on summer maize and the nitrogen uptake and utilization efficiency of different combinations of N application in 2 seasons were also studied.The following main results were obtained as follows:1.Based on the analysis of growth indexes,physiological response,yield of garlic scapes and bulbs,and nutritional quality of garlic,which were affected by the nitrogen application gradient,the reasonable nitrogen application rate of two garlic varieties were determined.Nitrogen application can significantly increase the leaf area index,plant height,pseudostem diameter,protective enzyme activity,nitrogen metabolic enzyme activity,chlorophyll content and net photosynthetic rate of two garlic varieties in the range of 0-240kg·hm^-2N application rate.With the increase of nitrogen application,the fresh weight and yield of garlic scape and bulb were increased.Compared with the control,the dry weight of scape and bulb of Cangshan could be increased by 109.8%and 80.9%,while the dry weight of scape and bulb of Gailiangsuan increased by 108.5%and 108.8%,respectively.The nitrogen application rate of 180-240 kg·hm^-2can acquire the maximum yield for Cangshan,meanwhile that for Gailiangsuan was 240 kg·hm^-2.However,when the nitrogen application level was over 240 kg·hm^-2,garlic dry matter accumulation and yield did not increase significantly.The soluble sugar,soluble protein,Vc and allicin content of garlic also improved with increasing of nitrogen application.In general,180 kg·hm^-2nitrogen application can ensure the quality indexes reach to better status.2.The effects of different N applications on water consumption,water use efficiency and soil nitrate N accumulation of garlic were clarified.Nitrogen application significantly increased N accumulation,water consumption and water use efficiency of garlic and increased the risk of downward leaching of nitrate N.In the three growing seasons,nitrogen accumulation in Cangshan was increased by up to 156.7%-177.6%and Gailiangsuan was by up to 202.2%-218.5%over the control at harvest date.The water consumption of N300 was the maximum among all N treatments.Compared to those of N0,the water consumption of N300 in Cangshan and Gailiangsuan increased by 77.8-80.9 mm and 76.6-79.2 mm,respectively.The two garlic varieties had reached the highest water use efficiency when nitrogen application was 240 kg·hm^-2.Under the current irrigation system,nitrogen application in excess of 120 kg·hm^-2would result in the leaching of nitrate nitrogen into the soil below 200 cm.While the more nitrogen applied,the greater the accumulation of nitrate nitrogen in the soil between 100 and 200 cm.In the most serious nitrate leaching season,when compared with N0 treatment,nitrate accumulation in 100-200 cm soil layer at bulb harvest period of Cangshan under N60,N120,N180,N240 and N300 treatments increased by 17.3%,64.5%,89.7%,124.1%and 144.6%,respectively.3.The critical nitrogen concentration dilution curve based on dry matter（DM）and leaf area index（LAI）of garlic were also established as follows:Cangshan:N_c=4.86DM^-0.48(DM≥1.02 t·hm^-2),N_c=4.44LAI^-0.51（LAI≥1.04）and Gailiangsuan:N_c=4.52DM^-0.40(DM≥0.98t·hm^-2),N_c=3.91LAI^-0.40（LAI≥1.19）.The nitrogen nutrition index（NNI）calculated based on these models can directly reflect the real-time nitrogen nutrition status of garlic.On this basis,the linear relationship between NNI and relative yield was established for each period,and the results showed that NNI had a good prediction effect on relative yield.The linear relationship between normalized differrential SPAD_ij(NDSPAD_ij)and NNI was established by normalizing SPAD values at different leaf positions.The correlation between NDSPAD₂₄and NNI was the best,and the difference was not significant among different years in this experiment,which means the nitrogen nutrient index can be estimated well by this method.The relationships mentioned above can be represented as Cangshan:NNI=-6.740NDSPAD₂₄+1.21,Gailiangsuan:NNI=-7.453NDSPAD₂₄+1.28,respectively.4.It was demonstrated that N application in the garlic season had a significant after-effect on the subsequent replanted corn.Nitrogen application in garlic growing season can significantly increase the nitrate accumulation in 0-200 cm soil layer at maize harvest stage,and the accumulation of Gailiangsuan was smaller than that in Cangshan.According to the after-effect of nitrogen fertilizer in garlic season,the maximum nitrogen uptake and yield of the two-season crop combination can be obtained by these two combinations:N240（Cangshan）+150 kg·hm^-2（N application in maize）and N240（Gailiangsuan）+200 kg·hm^-2（N application in maize）.At the same time,the cumulative nitrogen use efficiency of the garlic-corn multiple cropping system reached a maximum under this combinations of N application.Furthermore,the N surplus of Cangshan-maize and Gailiangsuan-maize system in the two growing seasons were only 117.0、95.3 kg·hm^-2and 100.0、69.8 kg·hm^-2,respectively.In conclusion,with the consideration of garlic yield,quality,water and nitrogen use efficiency and reduction of nitrate-nitrogen leaching,the reasonable nitrogen application rates in this research were 180-240 kg·hm^-2for Cangshan and 240 kg·hm^-2for Gailiangsuan,respectively.The appropriate N application rates in multiple cropping systems were 240kg·hm^-2in the garlic season+150 and 200 kg·hm^-2in the subsequent summer maize crop,which can achieve the balance between yield and ecological benefit.Moreover,the construction of N nutrition diagnostic model will greatly reduce the difficulty in real-time diagnosis of N nutrition,and provide guidance for N management and accurate N application of garlic in future.