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.