Ammonia Volatilization, Urea Hydrolysis, and Urease Inhibition with the Application of Granular Urea in Agroecosystems

Synthetic nitrogen (N) fertilizers play a key role in human nutrition and crop production. The most widely used N source globally is urea; however, N loss via ammonia volatilization can be great in agricultural systems where urea is surface-applied. The objectives of the experiments reported in this dissertation were: 1) evaluate the performance of a new laboratory ammonia volatilization measurement system for measuring ammonia volatilization from coated granular urea; 2) determine if urease can be extracted from corn and soybean residues; 3) determine if differences in urease activity are present in corn and soybean residues; and 4) evaluate N content and yield of corn treated with surface-applied coated urea fertilizers. The laboratory ammonia volatilization system had a system recovery efficiency (SRE) of 97% of the applied N and the lowest variation in mg N captured in the acid traps when the air flow rate was 1.00 L min⁻¹, at 26°C, and an acid trap volume of 100 ml 0.02M phosphoric acid. Ammonia volatilization was greatest from 12-24 h after N application with a total of 17% of the applied N being lost during that period. The urease inhibitor N-(n-butyl) thiophosphoic triamide (NBPT) was the most effective ammonia volatilization control treatment and reduced ammonia losses 30-40% compared to urea in the laboratory trials. Urease was extracted from soybean residue and retained activity during extraction; however, urease from corn residue could not be identified in extracts. The agronomic field trials indicated that NBPT increased N concentration in corn ear leaves; however the effect on corn grain yield was masked by environmental conditions. The data from this study suggests that ammonia volatilization from granular urea can be effectively controlled using NBPT, and corn tissue N content in the field indicates that NBPT allows for more N to be utilized by the plant. The urease extraction showed that there may be differences in urease activity in different crop residues. Further research is needed to determine if varying levels of volatilization control are needed for urea applied to different crop residues in no-till systems.