The molecular processes of urea hydrolysis in relation to ammonia emissions from agriculture

Abstract

Ammonia emissions from the agricultural sector give rise to numerous environmental and societal concerns and represent an economic challenge in crop farming, causing a loss of fertilizer nitrogen. Ammonia emissions from agriculture originate from manure slurry (livestock housing, storage, and fertilization of fields) as well as urea-based mineral fertilizers. Consequently, political attention has been given to ammonia volatilization, and regulations of ammonia emissions have been implemented in several countries. The molecular cause of the emission is the enzyme urease, which catalyzes the hydrolysis of urea to ammonia and carbonic acid. Urease is present in many different organisms, encompassing bacteria, fungi, and plants. In agriculture, microorganisms found in animal fecal matter and soil are responsible for urea hydrolysis. One strategy to reduce ammonia emissions is the application of urease inhibitors as additives to urea-based synthetic fertilizers and manure slurry to block the formation of ammonia. However, treatment of the manure slurry with urease inhibitors is associated with increased livestock production costs and has not yet been commercialized. Thus, development of novel, environmentally friendly and cost-effective technologies for ammonia emission mitigation is important. This mini-review describes the challenges associated with the volatilization of ammonia in agriculture and provides an overview of the molecular processes of urea hydrolysis and ammonia emissions. Different technologies and strategies to reduce ammonia emissions are described with a special focus on the use of urease inhibitors. The mechanisms of action and efficiency of the most important urease inhibitors in relation to agriculture will be briefly discussed.

Phosphorus Leaching from Soil Profiles in Agricultural and Forest Lands Measured by a Cascade Extraction Method

“…Application of urea for 25 yr was reported to decrease the pH of calcareous soil by 0.9 units (Zhang et al, 2017). Urea hydrolyzes to NH 4 + under the catalysis of urease (Sigurdarson et al, 2018). The roots secret H + when the crops absorb NH 4 + , resulting in an imbalance of Ca 2+ , Mg 2+ , and H + on the surface of soil colloidal particles and inducing soil acidification (Zhang et al, 2017).…”

“…The difference in soil Olsen P was determined with respect to depth and land use ( Table 2). The critical agronomic P values as indicators for crop P deficiency and P excess are 20 and 40 mg kg −1 Olsen P in calcareous soil, respectively (Shen, 1998). Based on these critical values of topsoil Olsen P, 79.2% of agricultural land is at risk of P excess and 12.5% at risk of P deficiency.…”

Development of a M9‐based urea medium (M9U) for sensitive and real‐time monitoring of ureolytic activity of bacteria and cell‐free urease

“…(Bekheet & Syrett, ; Booth & Vishniac, ; Cook, ; Frankenberger & Tabatabai, ; Hanlon, ; Sumner, ) Urease in livestock animal feces is a concern in agriculture because it hydrolyzes the urea found in livestock animal urine, causing emissions of NH 3 that can damage the environment, reduce air quality, and represent a loss of fertilizer nitrogen. (Sigurdarson et al, ) Urease is also an important virulence factor of several pathogens. The pathogenic ureolytic bacterium Proteus mirabilis can infect the urinary tract and cause complications such as the formation of kidney or bladder stones, which is a direct result of ureolytic activity.…”

Tracking AdBlue properties during tests of selective catalytic reduction (SCR) systems ‐ the suitability of various analytical methods for urea content determination

“…Independent hydroanalysis of AdBlue samples concluded that increasing value of conductivity is a cause of increasing concentration of carbonate ions and ammonia. It is known from the literature that urea can hydrolyse forming cyanatanes and consequently carbonate ions and ammonia . The subsequent considerable decrease in the electrical conductivity with runtime in the case of Test 1 (spraying), where the AdBlue solution is in intensive contact with air, could be attributed to the decomposition of ammonium carbonate.…”

“…It is known from the literature that urea can hydrolyse forming cyanatanes and consequently carbonate ions and ammonia. 13,23 The subsequent considerable decrease in the electrical conductivity with runtime in the case of Test 1 (spraying), where the AdBlue solution is in intensive contact with air, could be attributed to the decomposition of ammonium carbonate. Changes in alkalinity of AdBlue (expressed as a mass concentration of ammonia) during the tests of SCR systems clearly illustrate increasing concentration of ammonia due to undergoing chemical conversion of urea.…”