Nondestructively-measured leaf ammonia emission rates can partly reflect maize growth status

A deep understanding of ammonia (NH₃) emissions from cropland can promote efficient crop production. To date, little is known about leaf NH₃ emissions because of the lack of rapid detection methods. We developed a method for detecting leaf NH₃ emissions based on portable NH₃ sensors. The study aimed to (i) determine the performance of the method in detecting leaf NH₃ emissions; (ii) analyze the variation of leaf NH₃ emissions with foliar rank; and (iii) elucidate the relationships between leaf NH₃ emissions and other leaf parameters. Maize (Zea mays L.) was used as the tested plant. The results showed that the NH₃ sensors had good repeatability, accuracy, and selectivity in detecting NH₃. The response time of the method ranged 7-22 s and the NH₃ reading ranged 0.078-0.463 μmol mol^-1. Leaf NH₃ emissions were observed mainly in daytime (negligible at night). Daytime leaf NH₃ emission rates ranged 0.347-1.725 μg N cm^-2 d^-1. The middle leaves (near the ear) were the major contributor to plant NH₃-N loss. There were significant linear relationships between leaf NH₃ emission rates and other nondestructively-measured leaf parameters [e.g., SPAD (soil and plant analyzer development, which reflects the relative concentration of leaf chlorophyll), stomatal conductance, transpiration rate, and net photosynthetic rate] (p < 0.01), as well as with leaf apoplastic ammonium (NH₄⁺) concentration and leaf total N concentration (p < 0.01). Nitrogen application increased leaf apoplastic NH₄⁺ concentration, leaf total N concentration, and leaf NH₃ emission rate. Overall, nondestructively-measured leaf NH₃ emission rates can partly reflect maize growth status and provide information for N management in maize production.