Establishing A Fertilizer Recommendation Method And Proposing Nitrogen Application Limits For Radish

Balanced fertilization is an important practice to ensure good quality,high yield,and nutrient efficient of vegetables.The establishment of scientific fertilizer recommendation method is the key of balanced fertilization.However,there are few studies on fertilizer recommendation method of radish.Based on field experiments from multiple sites/years in radish-growing areas of China,the Quantitative Evaluation of the Fertility of Tropical Soil（QUEFTS）model was adopted to study the characteristics of nutrient requirement parameters of radish and the characteristics of soil indigenous nutrient supply,yield response,and agronomic efficiency were analyzed.Based on which,Nutrient Expert（NE）for radish,a fertilizer recommendation method based on yield response and agronomic efficiency was established combined with 4R nutrient management strategy.Field experiments for NE system were carried out to validate its agronomic,economic,and environmental benefits.Meanwhile,based on the recommended N application rate of NE system for radish,a five-season N fertilizer dosage fixed site field experiment of spring and autumn radish was carried out.The DNDC model was used to simulate radish yield and nitrate leaching,field management practices for synergistic agronomy and environmental effects that were optimized by sensitivity analysis,and the limit of N application was determined.The main findings were as follows:（1）Estimating nutrient uptake requirements for radish based on the QUEFTS model.Totally247 field experiments in radish planting areas of China from 2000 to 2017 were collected to investigate the relationship between fleshy root yield and nutrient accumulation in radish plant using the QUEFTS model.The QUEFTS model predicted a linear increase in fleshy root yield if nutrients were taken up in balanced amounts until yield reached about 60%–70%of the potential yield.The balanced N,P,and K requirements in radish plant were 2.15,0.45,and 2.58 kg to produce 1000 kg of fleshy root,and the corresponding internal efficiencies for N,P,and K were 465.1,2222.2,and 387.1 kg/kg,respectively.The simulated balanced N,P,and K removal by fleshy root to produce 1000 kg fleshy root were 1.34,0.30,and 1.93 kg,respectively.Approximately 62%,67%,and 75%of N,P,and K in radish plant were accumulated in the fleshy root and removed from the soil.Field validation experiments confirmed the consistency between the observed and simulated nutrient uptake values,and thus,the QUEFTS model could be used to estimate nutrient requirements for radish under a certain target yield.（2）Establishment of fertilizer recommendation methodology and Nutrient Expert decision support system based on yield response and agronomy efficiency for radish.On average,soil indigenous N,P,and K supply were 118.7,28.2,and 208.8 kg/ha,yield response to applied N,P,and K fertilizer were 17.7,10.4,and 10.3 t/ha,relative yield to N,P,and K were 0.73,0.86,and 0.85,agronomic efficiency of N,P,and K were 104.7,105.0,and 69.5 kg/kg,respectively.Yield response coefficients of indigenous nutrient supply of low,medium,and high were 0.36,0.21,and 0.11 for N,0.18,0.12,and 0.06for P,and 0.21,0.13,and 0.06 for K,respectively.A significant negative exponential relationship was observed between yield response and soil indigenous nutrient supply,a significant negative linear relationship was detected between yield response and relative yield,and a significant quadratic relationship was obtained between yield response and agronomic efficiency.A fertilizer recommendation method based on yield response and agronomic efficiency for radish was constructed,where N fertilizer rate was yield response divided by agronomic efficiency,P and K fertilizer rates considered the amount of P or K required by yield response,the maintenance of soil nutrient balance,and the residual effects of previously applied fertilizers as well as the return of crop residues.Nutrient requirements for maintaining soil balance were calculated based on the optimal nutrient requirements simulated by QUEFTS model.Using the information technology,the complex fertilizer recommendation principles was simplified into a user-friendly computer software named“Nutrient Expert（NE）”.（3）Field validation of Nutrient Expert for radish.Compared with farmers’practice（FP）,NE treatment significantly reduced N,P,and K application rates by 98 kg N/ha,110 kg P₂O₅/ha,and 47 kg K₂O/ha,respectively,and significantly reduced N and P surplus by 105.1 kg N/ha和115.1 kg P₂O₅/ha,respectively,and apparent N loss by 110.8 kg N/ha.NE treatment significantly decreased N and P inputs by 48 kg N/ha and 44 kg P₂O₅/ha,respectively,compared with soil testing（ST）.Compared with FP and ST treatments,NE treatment significantly increased radish yield by 4.2%and 4.0%and economic benefit by 5948 and 3072 yuan/ha,respectively,and significantly improved AEN,REN,and PFPN by 42.4 and31.0 kg/kg,11.4 and 7.0 percent points,and 162.9 and 96.8 kg/kg,AEP,REP,and PFPP by 67.4 and 50.9kg/kg,14.1 and 7.5 percent points,and 488.0 and 327.3 kg/kg,and AEK,REK,and PFPK by 20.3 and12.3 kg/kg,11.3 and 6.3 percent points,and 86.9 and 22.4 kg/kg,respectively.Organic fertilizer can substitute a maximum of 30%chemical fertilizer N recommended by NE,which ensured both radish yield and nutrient use efficiency.（4）Nitrogen application limit simulated by DNDC model for radish.DNDC model performed well in simulating radish yield,N uptake,soil temperature and moisture,leached water amount,and nitrate leaching.The results of sensitivity analysis showed that radish yield and nitrate leaching were most sensitive to the amount of N application and irrigation,and planting date had a greater impact on radish yield in autumn.Optimized field management practices generated from sensitivity analysis included agronomic and environmental critical N rates of 150 and 180 kg N/ha,respectively,irrigation amount of200 mm and 150 mm in spring and autumn,respectively,N split ratio of 3/10:4/10:3/10,and planting date of early to mid-April in spring and late July to early August in autumn,which could reduce N rate of current farmers’practice by 120-150 kg N/ha,irrigation amount by 33.3%and 50.0%,and nitrate leaching by 86.0%-95.0%for spring and autumn radish.The acceptable nitrate leaching losses in spring and autumn radish production to minimize the risk of water environment pollution were 14.0 and 20.6 kg N/ha,respectively.The environmental critical N rates was consistent with the recommended N rate by the NE system which further verified the rationality of the N fertilizer recommendation in terms of environmental benefits.In summary,the fertilizer recommendation method based on yield response and agronomic efficiency was proved to be a feasible tool in fertilizer recommendation for radish production in China.The approach can improve not only radish yield and profitability,but also reduce fertilizer rate and the environmental risk caused by N over application.