The effect of soil pH and high urea concentrations on urease activity in soil

The rate of hydrolysis of urea in soil over the wide range of concentrations, up to 10 moles N per dm³ soil solution, found in fertilizer practice, was examined in Begbroke sandy loam adjusted to different pH values. On rewetting air-dry soil, urease activity increased rapidly, reached a maximum within the first 24 h and then decreased slowly to level off after about 4 days. Pretreatment of the soil with urea or ammonium had no effect on the urease activity. Urease activity increased with substrate concentration, reached an optimum value and then decreased with rising urea concentration. The results could be explained by substrate inhibition at higher urea concentrations, and the data are well described by a modified Michaelis-Menten equation involving three parameters, V_max, K_m and K_i where K_i is an inhibition constant. K_m decreased linearily with rise in pH whereas K_i increased slightly between pH 4.9 and 7.0 and steeply between 7.0 and 8.4. V_max increased with rise in pH, reached a maximum value at pH 6.0 and then declined at higher pHs. There was a further reaction, reaching a maximum rate at a urea concentration of about 0.2 molar N in the soil solution, that followed Michaelis-Menten kinetics. K_m for this high affinity reaction increased up to pH 7.2 and then decreased at higher pH values; V_max increased up to pH 6.8 and then decreased. The contribution of the high affinity reaction was small except at low concentrations of urea.