Soil enzyme - soil amendment interactions.

A simple, rapid and reproducible method of assaying 1,3-β glucanase activity in soil has been developed, using laminarin as a substrate. It was founded upon a detailed analysis of the factors involved such as: quantity, type and age of soil; choice of: buffer, substrate concentration, microbial inhibitor, pH, temperature and incubation time, and methods of terminating the reaction. The broader implications of this methodological investigation are discussed in relation to soil enzyme assays in general.

Biochemical characterisation of the soil, 1,.3-β-glucanase revealed a pH optimum of 5.4 (the same as the soil), a temperature optimum of 50° to 55°C, an activation energy of 49 kJ7mole and a Michaelis constant of 0.2 mg/ml. The predominant activity appeared to be exohydrolytic since glucose was the only detectable breakdown product. Activity was irreversibly reduced by upon air-drying but thereafter remained constant for an indefinite period when the soil was stored at room temperature (21 ± 2°C). This remarkable stability was also observed in irradiated soil and long-term incubations of wet soil at a range of temperatures and suggests that 1,3-β-glucanase is a typical accumulated soil enzyme.

In addition, the possibility of using soil enzymes as monitors of agrochemical* effects in soil have been investigated in the laboratory. The choice of test enzymes, 1,3-β-glueanase and urease, was based partly on the considerable input of their substrates into soil; 1,3-β-glucans being cell wall components and cytoplasmic or vacuolar reserve materials in microorganisms and plants, and urea being an agricultural nitrogen fertiliser as well as a mammalian excretory product.

Concentrations equivalent to 5 times the recommended field application rates of the pesticides, 2,4-D, diallate, glyphosate, benzoylprop ethyl and malathion, applied as formulations, had no effect on the activity of either soil enzyme under any of the incubation conditions tested. The latter included unamended air-dried, field-wetness and flooded soil and air-dried soil amended with hPK fertiliser, urea, pig slurry, ground limestone, (all at concentrations equivalent to field application rates) cellulose and glucose. All soil was maintained at 65% WHC (except the flooded soil) and incubated at room temperature for 14 to 90 days.

As far as the pesticides were concerned, these enzyme systems could only be disrupted by unrealistically high dosage rates (100 to 1000 ppm) of some of the active ingredients. Thus, 1,3-p-glucanase activity was enhanced by 2,4-D, inhibited by diallate, benzoylprop ethyl and malathion, but unaffected by glyphosate, whereas urease activity was inhibited by 2,4-D but unaffected by diallate, glyphosate and benzoylprop ethyl.

Of the non-pesticide amendments NPK fertiliser and urea had no effect on either enzyme. 1,3-β Glucanase activity was enhanced by pig slurry, cellulose and glucose but inhibited by ground limestone, whereas urease activity was enhanced by glucose, inhibited by pig slurry but unaffected by ground limestone and cellulose. The activity of neither enzyme was altered by flooding the soil.

The isolation and enumeration of microorganisms capable of producing 1 ,3-β-glucanase and urease is described and the results are discussed in terms of interactions between such microorganisms and the agrochemicals , with particular reference to enzyme origin.

It is suggested that specific soil enzyme activity estimations have certain advantages over some of their more commonly used alternatives (such as counts and composition of microbial populations) in monitoring the effects of agrochemicals on the soil microflora.