Nitrogen deficiency and recovery in sustainable corn production as revealed by leaf chlorophyll measurements

Assessing economic and environmental impacts of nitrogen (N) fertilization in production agriculture is important for preventing unnecessary application of N fertilizer and avoiding losses of this N into water body. Chlorophyll meters are often used to evaluate N management practices and diagnose deficiencies of N in corn (Zea mays L.). Chlorophyll meter readings (CMRs) are usually interpreted relative to reference readings taken from plants having ample N to maximize rates of growth. Although measurements taken with chlorophyll meters provide a scale for estimating the sufficiency of N for corn growth, effects of above-optimal supplies of N on the measurements have not been studied when diagnosing N deficiencies during vegetative growth stages. Here, temporal trends in chlorophyll measurements were monitored in trials where various rates of N were applied soon after planting and (or) after symptoms of N deficiency had developed due to the changes of N status in soil and demand for N during corn growth. Divergence of chlorophyll measurements from the reference readings occurred in situations where plants having too little N were compared with plants having adequate N. In contrast, convergence of chlorophyll measurements with the reference readings after application of fertilizer N during the growing season indicated partial or complete recovery of the plants from the deficiency of N. The recovery can be explained by considering that luxury production of chlorophyll occurred at higher rates of fertilization or by interactions of N with soil water and other nutrients supplied during corn growth. Observations that plants can partially or completely recover from periods with inadequate N for chlorophyll production suggest that the leaf chlorophyll measurements taken early in the season should not be always expected to highly correlate with final yields of grain. Therefore, it is important to recognize the possible recovery of chlorophyll production and to avoid N losses to the environment.