Enzymes in soils

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

The inhibition effect exerted by DHA on hydrolytic enzymes in soils was regarded as an additional mechanism by which DHA may indirectly influence hydrolysis reactions. 

One approach to restoration of herbicidal activity in problem soils has been the use of microbial/enzymatic inhibitors (20). Stauffer Chemical Company (now ICI Americas) has provided leadership for the agrochemical industry in this area and provided potential inhibitors to university scientists for validation. Kaufman et al. (21) had earlier suggested the use of inhibitors to improve the performance of herbicides with short residual lives in soils. Kaufman et al. (3) discussed the role of pesticides as inducers, substrates, and/or inhibitors of degradative enzymes in soil microorganisms and gave selected examples of specific pesticides as inducers and others as inhibitors of certain enzymes. They also considered the potential of "multiproblem" soils. 

During decomposition most of the nitrogen is immobilized initially, at least, in microbial cells, which in turn are broken down. In the process, a considerable portion of the nitrogen reacts with phenols, quinones, and other ring compounds derived from lignin, or synthesized by microorganisms, to form humic substances, some of which are heterocyclic. Amino acids and ammonia are subject to such chemical reactions, that may be catalyzed by microbial enzymes. In soils with 2 1 clay minerals any ammonia that is formed is also subject to fixation in the clay lattice (see Chapter 8 and 11). 

The persistence of some enzymic activities in soils, and the possibility that active abiontic enzymes may be stabilized by combination with soil colloids, has led a minority of workers to attempt the characterization of enzymes in soil extracts and in other fractions. Elucidation of the mechanisms by which abiontic enzymes are stabilized in soil may be important in the wider context of understanding the processes which confer biological resistance on soil organic N. 

Individual enzymes or classes of enzymes in soils have been assayed under a variety of conditions, thus limiting the scope for comparing activities reported from different laboratories. Galstyan recently has described assays for five enzymes considered important for assessment of soil biological activity and has called for a standardization of methods. Roberge and Burns have reviewed aspects of methodology regarding pH control, choice of buffer if any, duration of assay, substrate concentration, and soil sterilization. 

Kinetic constants. The activities of many enzymes in soils... 

Vmax vary with soil, with soil physical fraction, and with the changing distribution of enzymes in soil, eg. when accompanying turnover of microbial biomass following organic amendments. Values are also influenced by assay conditions, eg. choice of substrate and buffer, the use of shaken or unshaken soil suspensions. Table 2 shows the ranges of values for kinetic constants calculated for different enzymes in soils and soil fractions. 

These studies on model enzyme complexes imply that the organic compounds which stabilize exocellular enzymes in soil are brown-coloured humic materials, probably aromatic in character. However, knowledge of the nature of the organic ligands and of the manner in which they may complex active enzymes, rests in part on the extraction of enzymes from soil, preferably in high yield and without artefact formation, and their characterization after fractionation and purification. 

The partially-purified extract oxidised a range of phenolic substrates, and also contained proteinases and amino acid decarboxylases. Preincubation of a toluene-treated soil enzyme preparation for 12h at 37°C did not affect diphenol oxidase activities, ie. the oxidases appeared to be resistant to attack by the coextracted soil proteinases. Addition of hyaluronidase before preincubation also was without effect. Preincubation with the microbial proteinase, Pronase for I8h at 37°C decreased diphenol oxidase activities by 307o, and by 100% when both Pronase and hyaluronidase were added. The results suggested that the polysaccharides associated with the extracted soil oxidases protected the enzymes from proteolysis and may play a role in stabilizing exocellular enzymes in soils. 

Enzymic activities of crude soil suspensions have been demonstrated to follow Michaelis-Menten kinetics. Calculated Km values have varied for different soils and for active fractions of soil extracts. The extent to which kinetic constants of soil enzymes are influenced by the state in which the enzymes occur in soils, is unknown. For some activities, two Km values have been distinguished for the one crude soil extract. Fractionation has revealed that enzymes may exist in tightly- and loosely- bound complexes with soil coloured humic compounds. Enzymes freed of coloured materials may nevertheless still be bound in complexes by their association with carbohydrates. These may not only influence enzyme kinetic properties but evidence suggests that they may also confer some degree of stability on the enzymes in soil. Whereas early speculation on the mechanism(s) by which enzymes are protected in soil tended, in the case of abiontic enzymes, to focus on the role of clay or humic colloids, fractionation studies have drawn attention to the potentially important role of soil carbohydrates. The manner in which carbohydrates are bonded to the enzymes in soil has not as yet been established and may be a fruitful line of enquiry. 

LADD J.N. 1978. Origin and range of enzymes in soil. Chapter 2 in Soil Enzymes. Ed Burns R.G. Academic Press, London. 

SKUJINS J. 1967. Enzymes in soil. Chapter 15 in Soil Biochemistry... 

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