Kinetic Determination of Urease Activity in Fresh Pig Feces and Slurry and the Effect on Ammonia Production at Different Conditions

Ammonia (NH3) emissions have become a serious environmental pollution problem, and livestock production is an important source of NH3 emissions, especially pig farming. The origin of NH3 release is the hydrolysis of urea in urine that is catalyzed by urease present in feces. This research determined the urease activity in fresh feces by Michaelis–Menten kinetics and then compared the process of urea hydrolysis and ammonia production in fresh slurry. For feces, the kinetic parameters Vmax and K’m were calculated by determining the concentration of ammonium in initial 5 minutes in closed vessels, and the resulting Vmax and K’m were 26.9 ± 1.2 mmol·[urea]·kg−1·min−1 and 99.7 ± 3.5 mmol·[urea]·l−1, respectively. In fresh slurry, the rate of urea hydrolysis determined directly was higher than the ammonium formation rate in the early stage (0–8 h) and was accompanied by a rapid rise in pH. In addition, we further explored the effects of temperature, pH, and mixing rate on urease activity within different periods (0–5 min, 5 min–2 h and 2 h–8 h). Our observations show that the optimal urease activity occurred at 35 °C, pH 6.71, and 821.83 rpm of stirring, indicating that microbial species and communities associated with urease production are affected by environmental conditions.