Abstract
In agricultural calcareous soils, hydroxyapatite (APA) may well represent an important support for urease immobilisation and could be present in both mineral and organo-mineral complexes. In this paper we studied the formation of APA-urease-humic acid (HA) complexes after the addition of urease either before or after HAs. We then proceeded to evaluate the role of HAs on the activity and stability of the complexes as compared to the APA-urease complexes and free urease. When increasing amounts of HAs were added after urease, they did not affect the activity of the enzymes that had already adsorbed onto the complexes. On the contrary, adding the same amount of HA before the enzyme caused a significant reduction in the amount of enzyme adsorbed. However, when urease adsorption onto the APA-HA complexes was carried out in the presence of NaCl, the enzyme activity of the complexes increased sharply to 86% of the initial activity. The immobilisation of the enzyme on the support increased urease stability against pronase treatment as well as directly in soil over time. The inhibition of urease activity by Cu2+ was reduced by urease immobilisation. However, the presence of HA did not alter the stabilisation capability of APA when alone.
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