Establishment And Validation Of Winter Oilseed Rape Nitrogen Fertilizer Topdressing Model Based On Hyperspectral Remote Sensing

Rapeseed production in China accounts for 20%of the world’s production,with winter oilseed rape being the primary oil crop in the Yangtze River Basin.However,the crop has a long growth period and high nutrient demand,particularly for nitrogen,which is one of the essential elements for photosynthesis and yield improvement.Thus,real-time and accurate monitoring of nitrogen status in winter oilseed rape is important to achieve high yield and efficient utilization of nutrients.This study conducted a field experiment to analyze the canopy spectral characteristics in winter oilseed rape under different nitrogen application gradients,and developed remote sensing estimation models for plant nitrogen content(PNC)and aboveground biomass(AGB)during the key growth period of direct-seedling winter oilseed rape.Based on estimations of PNC and AGB and the critical nitrogen concentration dilution curve,the recommended topdressing amount of winter oilseed rape during the key growth period was calculated.An additional field experiment was conducted to carry out the real-time estimation of N topdressing rate based on hyperspectral remote sensing.The study evaluated the impact of variable topdressing on the oilseed yield,providing better decision support for accurate fertilization and green production of direct-seedling winter oilseed rape.The main results are as follows:(1)To identify the best vegetation indexes(VIs)for quantitatively estimating PNC and AGB in winter oilseed rape during the overwintering and bolting period,we analyzed different combinations of ratio vegetation index(RVI)and the existing VIs that were sensitive to PNC and AGB.Results showed that RVI(R669,R769)based on the reflectance at 669 and 769 nm wavelengths was the best VI for estimating the PNC in winter oilseed rape during the overwintering period(y=-464.12x+489.96,R~2=0.88).Cater index 1based on the reflectance at 695 and 420 nm wavelengths provided the most accurate estimations of PNC during the bolting stage(y=-3.98x+31.04,R~2=0.96).RVI(R1597,R1636)based on dual bands of 1597 and 1636 and RVI(R665,R1599)based on bands of665 and 1599 nm were the best VIs for estimating AGB during overwintering and bolting period,respectively.The model equation was y=-69012.08x+63349.96(R~2=0.97)for RVI(R1597,R1636)and y=-24038.96x+11342.21(R~2=0.94)for RVI(R665,R1599).(2)Based on the established estimation models of PNC and AGB at different key growth stages of winter oilseed rape,we calculated the recommended topdressing amount during the corresponding period.The study recommended 62.3,33.3,and 24.6 kg N/hm~2fertilizers during the overwintering period for N60+spectral diagnosis topdressing,N120+spectral diagnosis topdressing,and N180+spectral diagnosis topdressing treatments,respectively.Similarly,the study recommended bolting fertilizers of 56.3,27.6,and 19.7kg N/hm~2 respectively.(3)This study examined the actual effect of spectral diagnosis topdressing on the yield,AGB,agronomy efficiency,seasonal utilization rate of nitrogen fertilizer,and fertilizer contribution rate.The results showed that the highest yield of 3068 kg/hm~2 was observed in the N240(conventional topdressing)treatment.Among the spectral diagnosis topdressing treatments,N180+spectral diagnosis topdressing had the best effect with a yield of 2955 kg/hm~2,which was comparable to the N240 treatment but saved 16 kg N/hm~2fertilizer,the decrease of 6.7%.This finding suggested that spectral diagnosis topdressing can be an effective alternative to conventional topdressing for improving the yield and reducing fertilizer use in winter oilseed rape cultivation.The analysis of AGB showed that there was no significant difference in AGB between the N240(conventional topdressing)treatment and the N180+spectral diagnosis topdressing treatment.Analysis of the agronomic efficiency,seasonal utilization rate of nitrogen fertilizer,and fertilizer contribution rate demonstrated that the N180+spectral diagnosis topdressing treatment was comparable to the N240(conventional topdressing)treatment,with higher agronomic efficiency than the N240 treatment.The study concludes that the N180+spectral diagnosis topdressing treatment is a suitable alternative to local farmers’conventional topdressing methods and is more environmentally friendly.The findings from this study are expected to guide farmers and agronomists in improving the efficiency of topdressing in direct-seedling winter oilseed rape cultivation.