SUGAR-BEET CANOPY TYPE AND ACCUMULATION OF PLANT NITROGEN AS DELINEATED BY AERIAL-PHOTOGRAPHY AND GLOBAL POSITIONING SYSTEMS

Considerable within-field variation in the N content and appearance of sugarbeet (Beta vulgaris L.) leaf canopies is found often at root har vest. Since this heterogeneity can affect subsequent soil N mineraliza tion, a study was initiated to determine if the within-field variation could be identified and located by aerial photography and a global po sitioning system (GPS). An aerial color photograph was taken of a comm ercial sugarbeet field prior to harvest. Three reflectance patterns as sociated with different degrees of canopy greenness (''green'' [G], '' yellow-green'' [YG] and ''yellow'' [Y]) were identified readily. Four areas, each approximately 0.8 ha in size, with these three canopy type s were selected. The canopy subsites were located by use of a GPS unit . The G, YG, and Y canopy types contained 277, 138, and 85 kg N ha(-1) . respectively, in sugarbeet tops. The corresponding leaf N concentrat ions were 30.6, 20.1, and 15.8 g kg(-1), respectively Postharvest soil NO3-N levels in the upper 120 cm of soil were 57, 14, and 10 kg ha(-1 ) for the G, YG, and Y canopy sites, respectively. Nitrate-N in sugarb eet tops returned to the soils was at least partly responsible for the higher soil NO3-N at the G subsites. In contrast to dry matter yield, recoverable sugar yield was significantly less for roots associated w ith the G-canopy type than for roots associated with the YG-and Y-cano py sites. Aerial photography and GPS technology can increase precision of soil testing for NO3-N after a sugarbeet crop, help to identify ar eas of increased soil N mineralization for a subsequent crop, and decr ease subsequent N fertilizer use if a variable rate N-fertilizer appli cator is used.