Activity of soil enzymes

1 Effect of soil types and management, and environmental conditions 

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. 

Values for Vmax were dependent upon soil moisture levels and were achieved at substrate concentrations of about 200 pi H2 1 For a sand, the Vmax was approximately doubled when the water content was decreased from 16.3 to 11.3%. The first-order rate constants for H2 utilization in soils were directly proportional to the amounts of soil used. Decreased activities at increasing soil water contents were considered due more importantly to decreased concentrations of suitable electron acceptors rather than to effects of anaerobiosis or to effects on H2 diffusion. 

Shaking soil suspensions during assay decreased Km values and increased Vmax values for soil ureases, phosphatases and arylsulphatases . Frankenberger and Tabatabai showed that Km and Vmax values for soil amidases, active against formamide, acetamide and propionamide, ranged with soil type as well as assay substrate. Km values also varied with pH of assay, being lowest at the pH optimum (pH 8.5). 

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GALSTAYAN A.Sh. 1978. Standardization of methods for determining the activity of soil enzymes. Pochvovedenie, 2, 107-114. 

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. 

Assays of soil enzyme activities are usually carried out in soil slurries, since efficiencies of enzyme extraction from soil and purification are still low (49). Such assays, under these conditions, will only give a measure of potential rather than actual activities moreover, they constitute integrated measures of activity as enzymes come from a variety of sources and are in several states in the soil (50). Enzyme activities may vary substantially with the season according to the synthesis, release into soil, and persistence of plant, animal, and microbial enzymes (57). 

Baldrian, P., der Wiesche, C., Gabriel, J., Nerud, E, and Zadrazil, F., Influence of cadmium and mercury on activities of ligninolytic enzymes and degradation of polycyclic aromatic hydrocarbons by Pleurotus ostreatus in soil, Appl Environ Microbiol, 66 (6), 2471-2478, 2000. 

The enzymatic activity in soil is mainly of microbial origin, being derived from intracellular, cell-associated or free enzymes. Only enzymatic activity of ecto-enzymes and free enzymes is used for determination of the diversity of enzyme patterns in soil extracts. Enzymes are the direct mediators for biological catabolism of soil organic and mineral components. Thus, these catalysts provide a meaningful assessment of reaction rates for important soil processes. Enzyme activities can be measured as in situ substrate transformation rates or as potential rates if the focus is more qualitative. Enzyme activities are usually determined by a dye reaction followed by a spectrophotometric measurement. 

Further studies are necessary to define the field parameters which afford maximum activity of the enzyme (e.g., soil type, amount of moisture, whether or not multiple applications of enzyme are needed). 

In soils, electrons are produced by the metabolic activity of soil biota. These electrons are usually accepted by O2 dissolved in the soil solution which is then replaced by O2 from the soil air. Oxygen may, however, become deficient if all pores are filled with water as in waterlogged or compacted soils. Fe in Fe oxides may then function as an alternative electron acceptor and Fe ions will be formed according to eq. (16.3). The electrons are transferred from the decomposing biomass to the Fe oxide by microbially produced enzymes. Other potential electron acceptors in soils are nitrate, Mn and sulphate. 

In the past the mineral matrix was considered as inert, only providing stabilization support for enzymes and humic substances however, due to the overwhelming amount of evidence at the molecular level, there is no doubt that minerals participate in abiotic catalysis of humification reactions in soils. Naidja et al. (2000) referred to mineral particles as the Hidden Half of enzyme-clay complexes, which not only prolong the activity of immobilized enzymes but also are readily able to participate in electron transfer reactions. Many environmental factors can negatively affect the... 

Mucilage has protective functions for the root meristem and improves root-soil contact by inclusion and aggregation of soil particles. It may also contribute to P desorption and to the exclusion of toxic elements (Al, Cd, Pb) by complexation with galacturonates, mainly in exchange with Ca2+ (Neumann and Romheld, 2002). Secreted enzymes contribute to the extracellular enzyme pool it has been shown that the activity of extracellular enzymes, such as phophatases, proteases, and aryl-sulfatases, exhibit more activity in the rhizosphere relative to the bulk soil and may have a dramatic effect on the cycling of nutrients such as P, N, and S (Badalucco and Nannipieri, 2007). 

EPTC and Butvlate Fluorescein Diacetate Assay. Spectrophotometric determinations of the hydrolysis of fluorescein diacetate have been shown to be simple, rapid, and sensitive methods for determining microbial activity in soil (18). Essentially, the hydrolytic cleavage of diacetate from fluorescein is responsible for the reaction products including fluorescein, which may be detected spectrophotometrically at 490 nm. This method is somewhat nonspecific in that it is indicative of overall activity of several enzymes (protease, lipase, esterase) rather than of a specific class of enzymes. Enzyme activity may be influenced by subtle pH changes in the sample since abiotic hydrolysis of fluorescein diacetate may occur. Also, an associated lag phase in soil hydrolytic activity must be accounted for in each assay. 

The activity of soil dehydrogenase at the beginning of the soil dilution experiment in 0, 10, 50, and 90% diluted waste-pile soil was 18, 0, 65, and 138%, respectively, of the activity in CHECK soil. After 21 days, soil dehydrogenase was still inhibited in the 0 and 10% diluted waste-pile soil. The inhibition of enzyme activity suggests that high concentrations of alachlor may be toxic, but microbial bioactivity can be restored if contaminated soil is diluted enough. 

F. Holz, Automatic, Continuous-Flow, Photometric Determination of Soil Enzyme Activities by the Use of (Enzyme) Oxidative Coupling Reactions. Part 1. Determination of Catalase Activity [in German]. Landwirtsch. Forsch., 39 (1986) 139. 

Phenol oxidase activity is highly sensitive to the availability of oxygen. The activity of this enzyme may be of great significance in soils with high organic matter content. Because of its oxygen... 

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