Suppression of Ammonia Volatilization from Urea-Based Fertilizers Using Urease Inhibitors: A Reasonably Available Control Technology for Agriculture?

Abstract:

Ammonia loss from fertilizers can impact formation of atmospheric aerosols, as well as contribute to nitrogen (N) deposition in terrestrial and aquatic ecosystems. Urea is the predominant form of N fertilizer used worldwide due to its high N content (46.6% N) and low cost. Once in contact with soil or vegetation, urea is hydrolyzed to ammonium via naturally occurring urease enzymes. Losses of N from surface applied urea as ammonia can exceed 30%. To address this issue, various physical and chemical mechanisms have been incorporated into granular urea. The most common approach is incorporation of urease inhibitors such as N-(n-butyl) thiophosphoric triamide (NBPT). We have been investigating ammonia volatilization from urea granules (+/- urease inhibitors) in various field and laboratory controlled experiments for the past several years. Laboratory experiments are conducted with a customized growth chamber system designed to continuously measure ammonia volatilization. Field measurements are conducted using a passive sampler technology with an acid-coated trap in PVC cylinders, or annular denuder technology using flow-through PVC chambers. Daily exchanges of acid-coated denuder tubes enhance the sensitivity of ammonia volatilization measurements for the urease-inhibitor treated product. Loss of N from commercial urea granules has ranged from 6 - ~ 35%, depending on ambient temperature. This loss typically occurs within the first 5-10 days under field conditions. Some urease-inhibitors can minimize loss of N via volatilization (< 5%) for up to 20+ days in the absence of a rainfall event. Visual observations have confirmed that on bare soil, treated or untreated urea granules quickly “dissolve” and move into the soil. The accompanying urease-inhibitor formulation moves with the urea continuing to provide protection against reaction with naturally occurring urease enzymes. Use of urease-inhibitors does not guarantee increased crop yields or NUE, but the consistency of inhibitors incorporating NBPT suggest that these formulations represent a reasonable available control technology for use in agriculture to reduce ammonia emissions.