Recommendations for nitrogen fertiliser topdressing rates in rice using canopy reflectance spectra

Three site-specific approaches were developed for dry crop fertilisation: a N fertiliser optimisation algorithm (NFOA), a canopy density (LAI), and a nitrogen nutrition index (NNI) were all modified and calibrated using rice data from different agriculture management practices and then used to determine the nitrogen (N) topdressing rate at the panicle initiation stage based on the real-time canopy reflectance spectra. Rice grain yield, N rate, and the economic and the environmental benefits were compared with the standard N recommendation using N production functions (SN) as a reference. The results showed that these three site-specific approaches could be used to develop rice N topdressing recommendations following modification and calibration. The total N rate in site-specific fertilisation ranged from 166 to 233 kg [N] ha(-1), with almost equal N rates for NNI and LAI approaches, but with lower rates for the NFOA approaches. All were lower than the SN approach (250kg[N]ha(-1)). A 33.6% reduction in the total N rate in the treatment of N topdressing was recommended by the NFOA approach without basal N (OBN1, 0-80-133kg[N]ha(-1)). It slightly increased the yield (about 0.4tha(-1)), and improved the nitrogen use efficiency (NUE) around 20% for agronomic efficiency (AE(N)) and 16% for recovery efficiency (REN) compared with the SN treatment (100-50-100 kg [N] ha(-1)). N apparent loss and soil residual N of all the site-specific N fertilisation treatments were lower than SN, while the net benefit of all site-specific N fertilisation treatments was increased, with the highest increase being 1575Yuan ha(-1) (In 2005 1 Yuan=0.12 US$=0.9(sic)). This suggests that reducing the proportion of N applied at basal and during the early vegetative stage and fine-tuning the N topdressing rate according to the crop growth status with remote sensing could be an effective way to increase NUE and reduce environmental risks in the Taihu Lake Region, China. (c) 2008 Published by Elsevier Ltd. on behalf of IAgrE.