Aspartase activity in soils: effects of trace elements and relationships to other amidohydrolases

The enzyme aspartase (l-aspartase ammonia-lyase, EC 4.3.1.1) catalyzes the hydrolysis of l-aspartate to produce fumarate and NH3. This enzyme is involved in N mineralization in soils. Recently, the activity of this enzyme was detected in soils, and a method was developed for its assay. The method was used in studies of the effects of the salts of 24 trace elements on the activity of aspartase in three field-moist soils and their air-dried counterparts. At 5 μmol g−1 soil, all the trace elements inhibited aspartase activity in the soils. With most of the elements, greater inhibition was found in air-dried than in field-moist soils. Among the trace elements studied, Ag(I) and Hg(II) were the most effective inhibitors of aspartase activity; >85% when added at 5 μmol g−1 soil. The least inhibition (12%) was with Ni added to the field-moist Harps soil and the greatest (98%) was with Ag(I) in the air-dried Weller soil. Aspartase activity was significantly correlated with the contents of organic C (r=0.85∗∗∗, P<0.001), total N (r=0.73∗∗∗) and clay (r=0.44∗, P<0.05) but not with the content of sand or the pH of 27 surface soils examined, including the three soils used in the studies of the effects of trace elements. The activity of this enzyme in soils was significantly correlated with the activities of asparaginase (r=0.94∗∗∗), glutaminase (r=0.88∗∗∗), urease (r=0.80∗∗∗) and amidase (r=0.44∗).