Effects of trace elements on urease activity in soils

Disposal of sewage sludges and effluents on agricultural land is becoming a widespread practice. Most sludge samples disposed on soils contain large quantities of various trace elements. Studies of 20 trace elements commonly found in sludge samples showed that they inhibit the activity of urease in soils and that their order of effectiveness as inhibitors of urease depends on the soil. When the trace elements were compared by using 5 μmiol·g^?1 soil, however, some of them showed the same order of effectiveness as urease inhibitors in the six soils studied i.e., for the monovalent and divalent ions, Ag⁺ ≥ Hg²⁺ >Cu²⁺ >Cd²⁺ >Zn²⁺ >Sn²⁺ >Mn²⁺, and generally, Fe²⁺ >Fe²⁺andCu²⁺ >Cu⁺. Other trace element ions that inhibited urease were Ni²⁺, Co²⁺, Pb²⁺, Ba²⁺, As³⁺ B³⁺, Cr³⁺, Al³⁺. V⁴⁺ Se⁴⁺ and Mo⁶⁺. Of the trace element ions studied, only As⁵⁺ and W⁶⁺ did not inhibit urease activity in soils.Studies on the distribution of urease activity showed that it is concentrated in surface soils and decreases with depth. Urease activity was proportional to organic C distribution in each soil profile and was significantly correlated with organic C in the surface soils studied.