Combined Leaf and Canopy Reflectance Measurements Indicate Crop Utilization of N Fertilizer in Real-Time and Predict Response to N Topdress in Spring Wheat.

Although there has been much work demonstrating the utility of various reflectance technologies for predicting the productivity of wheat and other staple crops, few studies have examined simultaneous reflectance measurements of a cropping system taken from multiple scales of the canopy and at various stages of development. We hypothesized that simultaneous leaf and canopy reflectance measurements in wheat, combined with soil nitrate-N measurements from the top 30 cm of the soil profile, would result in additive information about the N fertilizer utilization and protein yield of wheat crops grown under varying N fertilization management schemes. We measured leaf and canopy reflectance and soil nitrate-N across 6 site-years at the tillering, boot, and flowering growth stages. These measurements were regressed against both rates of N fertilization and protein yield outcomes and characterized statistically using a combination of classification and regression and mixed-effects linear and non-linear models. During the vegetative growth period, canopy NDVI, leaf chlorophyll, and soil nitrate-N measurements each added unique information regarding the likelihood of a post-measurement N fertilization response. As the crop transitioned into the reproductive growth stage, canopy NDVI and leaf chlorophyll measurements became relatively more important than soil nitrate-N for indicating productivity outcomes. Despite recording correlated information, the combination of measurements taken at multiple canopy scales produced additive information about the likelihood of crop response to in-season N fertilization and the overall protein yield response to N fertilization in spring wheat. Given the relatively low cost of these sensors, they may be best used in tandem both for agronomic research and for creating robust decision support tools for N management in spring wheat.