Abstract:The growing use of commercial unmanned aerial vehicles (UAV) and the need to adjust N fertilization rates in maize (Zea mays L.) currently constitute a key research issue. In this study, different multispectral vegetation indices (green-band and red-band based indices), SPAD and crop height (derived from a multispectral compact camera mounted on a UAV) were analysed to predict grain yield and determine whether an additional sidedress application of N fertilizer was required just before flowering. Seven different inorganic N rates (0, 100, 150, 200, 250, 300, 400 kg·N·ha −1 ), two different pig slurry manure rates (Ps) (150 or 250 kg·N·ha −1 ) and four different inorganic-organic N combinations (N100Ps150, N100Ps250, N200Ps150, N200Ps250) were applied to maize experimental plots. The spectral index that best explained final grain yield for the N treatments was the Wide Dynamic Range Vegetation Index (WDRVI). It identified a key threshold above/below 250-300 kg·N·ha −1 . WDRVI, NDVI and crop height showed no significant response to extra N application at the economic optimum rate of fertilization (239.8 kg·N·ha −1 ), for which a grain yield of 16.12 Mg·ha −1 was obtained. This demonstrates their potential as yield predictors at V12 stage. Finally, a ranking of different vegetation indices and crop height is proposed to overcome the uncertainty associated with basing decisions on a single index.
Core Ideas
Grain yield, biomass, N uptake, SPAD units, soil N levels and N efficiciencies were affected by the N application rate.
Maximum grain yields required 203 kg N ha−1 of available N in the 0‐ to 30‐cm layer soil.
Sampling to a depth of 0 to 30 cm provided similar correlations than sampling to 0 to 60 and 0 to 90 cm.
Mineral N fertilization increased soil organic C stock.
Nitrogen is a key determinant of growth and grain yield (GY) in maize (Zea mays L.) and is therefore economically and environmentally important. We investigated the performance of maize crops in a 12‐yr experiment (2002–2007, 2010–2015) under sprinkler irrigation in a petrocalcic calcixerept soil in northeastern Spain, with controlled mineral N application rates (0, 100, 150, 200, 250, 300, and 400 kg N ha−1 yr−1). The application rate affected maize GY, biomass, N uptake, SPAD units, soil N levels, N efficiencies, and soil organic carbon (SOC). Average maximum GY's (∼15 Mg ha−1) required 203 kg N ha−1 of available N (defined as initial soil NO3− plus N fertilizer) in the 0‐ to 30‐cm horizon, confirming the importance of the soil N content. Nitrate levels in the 0‐ to 30‐cm horizon for maximum yields achieved a R2 value in the plateau fitting model similar to the 0‐ to 60‐ and 0‐ to 90‐cm horizons. The GY's increased at a rate of 192 kg ha−1 yr−1, suggesting a combination of genetic improvement in the hybrids and also an improvement in agronomic management. The 200 kg N ha−1 fertilizer treatment achieved almost the highest GY's (∼14 Mg ha−1) with simultaneous high nitrogen use efficiency (NUE) (0.83 kg kg−1). Mineral N fertilization also increased the stock of SOC in the 0‐ to 30‐cm horizon.
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