| It has become common to use excessive nitrogen（N）fertilizerto speed up the growth of vegetables and maximize the yield,which continuously deteriorates soil environment and seriously threatens the sustainable development of facility vegetable industry.In recent years,with the improvement of living standard,bell pepperas one of the most important facility vegetables become favorite due to its high nutrition.Bell pepper has stringent requirements on water and N nutrition,while farmers still cultivate it with theirown experience of irrigation and N application.Therefore,it is critical in large-scale bell pepper production to optimize the management of water and N,and to improve the yield,water use efficiency（WUE）and the partial factor productivity from applied N（PFPN）.It is of great significance to effectively implement the drip fertigation technology and sufficiently develop the incentive mechanism and synergistic effectbetween water and Nin arid Northwest of China,which will greatly improve the yield,WUE,PFPN,and fruit quality of bell pepper andsimultaneously protectthe water and soil environments.Inthis study,weinvestigated the growth of an American bell pepper variety of "Marcomi"（Capsicum annuum L.var.Grossum Marcomi F1）in a two-year（2014 and 2015）experiment in a sunlitventilated greenhouse at the Ministry of Education Key Laboratory of Agricultural Soil and Water Engineering in Arid Areas,theNorthwest A&F University.Based on daily meteorologicaldata from a mini-meteorological station in the greenhouse,irrigationamountwas determined with the FAO Penman-Montieth equation.The irrigation amount had four differentlevels of 105% ET0（W1）,90% ET0（W2）,75% ET0（W3）,and 60% ET0（W4）,where ET0 was the reference evapotranspiration（ET）.According to the recommended N application rate of 300 kg·hm-2,four N levels were 100%(300 kg·hm-2,N1),75%(225 kg·hm-2,N2),50%(150 kg·hm-2,N3)and 25%(75 kg·hm-2,N4)of recommended N rate.The effects of different irrigation levels and N application rates on multiple variableswere analyzed,including bell pepper growth,physiology,yield and its’ constituent elements,WUE,N fertilizer productivity,plant N accumulation,soil residual N accumulation,and soil moisture.We also studied the growth characteristics,dry matter accumulation,water and N consumption,and soil water and nitrate transport in the greenhouse.Based on theexperimental data,the water absorption and soil moisture migration in the root zoneof bell pepper were simulated and validated with the HYDRUS-3D model.The critical Nconcentration dilution curvewas established and verifiedfor greenhouse bell pepper in Northwest China.Nitrogen nutrition diagnoses were conducted for bell pepper with the critical N concentration dilution curve,N nutrition index model,and N uptake model.Some specific results werelisted as follows.（1）Higher N application level could promote the growth and development of bell pepper;however excessive application of N might cause oversupply of N nutritionand change the microenvironment in root zone,which wasnot good for the absorption and utilization of water and nutrients.Under our experimental conditions,the irrigation levels of W2 and W3 and the application rate of N2 and N3 were able to maintain the “source-sink” relationship and chlorophyll content of plant leavesof bell pepper,which couldprolong leaf function,promote leaf area index（LAI）,increase dry matter accumulation on the ground,and finally increase fruit number per plant,single fruit weight,individual plant yield,and economic yield.The effect of N fertilizer on dry matter accumulation of each organ was greater than those of irrigation.At harvest,the proportion of dry matter partitioning wasfruit>stem>leaf>root.The‘foraging’ performance of bell pepper roots was more obvious when under low irrigation and N fertilization levels.Compared with the irrigation level W1,the economic yield of W2 and W3 were increased by 9.16% and 2.06%,but decreased by 28.04% for W4.Compared with N application rate N1,N2 and N3 increased economic yields by 33.99% and 30.39%;but decreased by 2.21% for N4.The average economic yield of bell pepper was 35.41 t·hm-2for W2（90% ET0）and N2(225 kg·hm-2)in greenhouse in Northwest China.For W3（75% ET0）and N2(75 kg·hm-2),the average economic yield rankedthe second,or 33.11 t·hm-2.For W3（75% ET0）and N3(150 kg·hm-2),the average economic yield ranked the third,or 31.79 t·hm-2.（2）The effects of different irrigation and N levels were significant on NO3-N accumulation of in the soil profile of bell pepper during the whole growing season.The accumulation of NO3-N in soil profile was affected both by the uptake and utilization of pepper plant and the levels of irrigation and N application.The root system of pepper seedling was shallow,so the absorption and utilization of water and nutrient was low.With high N application rate,especially when there was adequate irrigation,it was easy for nitrate N to penetrate into deep soil and pollute soil environment.Under our experimental conditions,the combination of irrigation levels W2 and W3 and application rates N2 and N3,would give rise to higher water consumption,faster growth,and faster biomass accumulation of bell pepper,and finally lead to higher yield.The W3N3 treatment had the highest efficiency of water utilization（17.84 kg·m-3）,while W2N3（17.26 kg·m-3）and W3N2（16.57 kg·m-3）ranked second and third.The plant had more nutrient growth and reproductive growth At the flowering and fruit-bearing stage,or a stage of both vegetative and reproductive growths,the root system of bell pepper developedwell;the capacity of water and nutrient uptake increased;and water andN demandsbecame larger.The difference of nitrate contents in different depths was lower than that at seedling stage.During fruit rotation,the growth of bell pepper weakened and the effect of water on fruit coloring was greater than that of nutrient.Then,fruit arrived at physiological maturity.The results showed thatthe requirements of water and nutrient weakened at the fruit-maturitystage.The effectof N on total accumulation of NO3-N and the effect of irrigation on distribution of NO3-N were significant.Under the same irrigation condition,the N use efficiency,N uptake efficiency,and N fertilizer productivity increased with N level.（3）The proportion of N accumulation in plant organs was fruit>leaf>stem>root.Appropriate water and N supply（such as the combinationW2/W3 and N2/N3）could increase N uptake of plant organsso as to promote crop growth and fruit development,and significantly improve fruit quality but not decreaseyield.The results showed that fruits quality of N3 was the best among all treatments according to a comprehensive evaluation of pepper fruit quality with the principal component analysis.The W3N3 treatment got the highest score.The W4N4 treatmentranked last due to serious water and nutrient stresses.（4）The effect of N on dry matter accumulation of each organ was greater than that of irrigation.At harvest,the proportion of dry matter partitioning in each organ was fruit>stem>leaf>root.The ‘foraging’ performance of pepper rootswas more obvious when under low irrigation and N levels.Irrigation and N application significantly affected the growth and distribution of root system of bell pepper.The number of tips and forks of bell pepper increased greatly with irrigation level and N application rate,while the Tot Surf Area decreased.With the decrease of irrigation amount,the proportion of root length in shallow soil layer（0-40 cm）decreased,whileit increased in deep soil layer（40-80 cm）.The numbers of tips and forks significantly increased.When under water and nutrient stresses,the self-regulating function increased the growth of root system in deep soil and thenumbersof tips and forks.（5）This study simulated and validated the root water uptake and soil moisture migration at the fruit enlargement stage of bell pepper with the HYDRUS-3D model.The results showed that HYDRUS-3D could effectively simulate the root water uptake and soil moisture migration,since the measured and simulated values of soil moisture content evenly distributed along the 1:1 line during the whole fruit enlargement period.The results of T-test showed that there was no significant difference between the simulated and measured values of soil water content.（6）Based on the two-year experimental data,the critical N concentration dilution curve was established and validated for bell pepper.The results showed that with the increase of irrigation level,the total Nuptake,above-ground biomass,yield and water use efficiency（WUE）all decreased after initial increase.The diagnosis results of N nutritionof the greenhouse bell pepperbased on critical N curve were the same between the Nnutrition（NNI）and N uptake models.The optimal N fertilizer rate was between 150 and 225 kg·hm-2,but closer to 225 kg·hm-2.For the two irrigation levels of W2 and W3,economic yields of bell pepper were 34.14 and 35.33 t·hm-2 and WUE was 17.07 and 15.30 kg·m-3,respectively.However,the economic yields and WUE could not achieve optimal results at the same time.The above-ground biomasses of bell pepper ofW2 had no significant difference when N application rate was more than 150 kg·hm-2.The optimal N applicationrates were 194.00 and192.69 kg·hm-2 based on economic yield,while the optimal N application rates were 191.13and191.83 kg·hm-2based on WUE.Considering water shortage in the Northwest China,the irrigation level of75%ET0with190 kg·hm-2 N application rate was suggested for greenhouse bell pepper productionsince it could save water by about 11%while the economic yield only declined by about 3% in comparison with the irrigationlevel of90%ET0.（7）Based on a comprehensive evaluation with the ideal point method,the N2 rate was more favorable to promote the multi-objective comprehensive level,while N4 was least.The W3N2 treatment was the best among all treatments andthe W2N2 treatment was the second best.Based on least square method,the comprehensive benefits were analyzed with the Mathematica 9.0 software.The results showed that the irrigation amounts were195.9-212.9mm（or 78.2-85.0% ET0）and 190.5-167.8mm（or 71.3-80.8% ET0）in 2014 and2015 seasons when the threeindices（yield,WUE,and Vc content）achieved the maximum of85%at the same time,while the N application ranges were 160.7-185.1and164.5-189.5 kg·hm-2,respectively.For different year types,the threeindices of bell pepper were greater than 85% at the same time under our experimental conditions,when irrigation range was 78.2-80.8%ET0and N application range was 164.5-185.1 kg·hm-2,respectively.The determination of irrigation and N application ranges could provide a scientific basis for high quality and efficient water and N management of greenhouse bell pepper. |
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