Heavy metal affects enzyme activity in soil

Recently there has been an increasing concern towards the health hazards associated with heavy metals entering the food cycle. Application of rock phosphate or its products to soil always implies the addition of significant amount of lead and cadmium. There have been several studies on the effect of heavy metals on various enzyme activities. The results indicated that the inhibiting power of a particular element on the activity of a specific enzyme is different for different soils. Since the enzyme activities are considered to be the index of soil health, an attempt had been made to analyse the inhibiting power of Ni, Cd, Hg and Pb on acid phosphatase, alkaline phosphatase, arylsulfatase and urease activities. Soil samples were collected from UPASI tea experimental farm and divided into five parts. The first portion was artificially contaminated with the metal solution of NiCl having Ni at 25 μmole concentration. Similarly, other three parts of the sample were contaminated with the ions of Cd , Hg and Pb . The fifth treatment was an untreated control, where no heavy metal was added externally. The enzyme activities were estimated using standard methodologies. All treatments were replicated thrice. The inhibiting power of Ni and Pb was maximum towards the enzyme arylsulfatase (Fig. 1), responsible for breaking the non available ester compounds of sulphur into available fractions like sulphate in soils. Urease is an important enzyme, which converts applied urea into plant available forms like NH and NO . The results of the current study showed that Hg at 25 μmole concentration inhibited urease to a great extent. Although alkaline phosphatase plays a less significant role on P availability of acid soils, under acute P deficiency its role is considered to be important. Cadmium inhibited the activity of alkaline phosphatase to a maximum extent (70%). The acid phosphatase activity was also inhibited considerably by this metal. The study revealed that Ni , Cd , Hg and Pb inhibited various soil enzymes which are responsible for N, P and S nutrition of tea. The inhibition caused by these metals is expected to be persisting for a long period owing to their non-decaying nature.

Hence, enough care should be taken by ensuring that these metals are preferably
absent in organic manure and mined products that are applied in tea fields from time to time.

(The data mentioned in this article is available on request drsvenkatesan@gmail.com)
Authors : S. Venkatesan and V. K. Senthurpandian