Effects of nitrogen application and weather on corn as detected by micrometeorology and remote sensing techniques

Crop yield is a function of slowly-evolving stresses such as nutrient deficiency and periodic environmental stresses such as reduced water availability. Our study attempts to explore this combination of stress agents through the techniques of micrometeorology which provide a nearly continuous monitoring of the temporal variability in water use efficiency (WUE) and CO2 uptake and through snapshots of canopy ecophysiology provided by remote sensing. In a 30 ha corn field near Ottawa, ON (45°N, 75°W), a starter fertilizer (11-30-20) was applied at a rate of 150 kg ha-1. A second fertilizer application (Urea, 46-0-0) created two sub-areas (100% and 60% of 300 kg ha-1) with a third area left as 0% of the second application. Two eddy-covariance systems were installed in the 60% and 100% treatments. Meteorological data, soil respiration and humidity were also monitored. Ground-based hyperspectral reflectance (HR) signatures were collected above the canopy throughout the season using a portable spectroradiometer.

The daily WUE was computed as the ratio of crop net photosynthesis and evapotranspiration. Early in the season and toward the end of the season, the 60%N treatment had a higher WUE than the 100%N treatment. This resulted in a WUE 7% higher than 100%N over the course of the season. The lower N application resulted in lower biomass and leaf development, which reduced the transpiration rate and delayed senescence. A three-week period of reduced rainfall during the early vegetative phase resulted in reduced plant growth which was exacerbated in the more-rapidly developing optimum N treatment. A water band index indicated periods of lower canopy water content which corresponded to low soil moisture and with signals detected by the eddy covariance technique. Structural HR indices were correlated with measures of canopy growth.