Soils microbiological activity

In microbiologically active soils degradation is rapid. Because of reasons mentioned above, the length of time of the herbicidal efficiency of 2,4-D at the usual application rate varies considerably. Generally, it is claimed to be 1-4 weeks (Audus, 1964 Sheets and Harris, 1965). In the maize fields of Mexico, Agundis... 

The partial pressure of C02 in the soil air controls the concentration of both dissolved C02 and undissociated carbonic acid. At 0.003 atm of C02 (g) as a reference level for soils, [H2C03°] is about 1.04 x 10 4 M (Lindsay, 1979). At a normal atmospheric level of 0.0003 atm C02 (g), [H2C03°] is approximately 1.04 x 10 5 M. In most soils, C02 (g) is higher than in the atmosphere. C02 is released from soil and plant root respiration. In flooded soils, C02 (g) partial pressure increases to 0.01-0.3 atm, about 1000-fold higher than normal upland soils due to strong microbiological activity (Lindsay, 1979). 

There is a need to measure the activities of enzymes and to correlate these measured activities with microbial diversity in soil. It is conceptually wrong to assume a simple relationship between a single enzyme activity and microbiological activity in soil (Nannipieri et al. 2003). Most of the assays used to determine microbiological activities in soil present the same problem measuring potential rather than real activities (Nannipieri et al. 1990). Indeed, assays are generally made at optimal pH and temperature and at saturating concentration of substrate. Furthermore, synthetic rather than natural substrates are often used, and soil is incubated as a slurry (Nannipieri et al. 1990). 

Soil pH Soil pH should be in the range of 6 to 8, to maintain cell turgidity and promote enzymatic reactions. Soil buffers, such as carbonate minerals, can be valuable in neutralizing acidic groundwaters as a result of high C02 concentrations because of microbiological activity. 

The composition and reactivity of the liquid phase (known as the soil solution) is defined by the quality of the incoming water and affected by fluxes of matter and energy originating from the vicinity of the solid phase, microbiological activity, and the gas phase. To understand the properties of the subsurface hquid phase, it is first necessary to consider the structure of the water molecule. 

Soil and water are essential to an effective disposal system. The soil is a source of microorganisms, a food source and a great aid to containment because of its high adsorption capacity. The water is an aid in monitoring and for microbiological activity. 

A second source might be the products of the soil microflora, which then directly indicate a variation in the composition of the microflora in the root zone for the two cultivars. A third possible source for the observed variation is degradation of the components in the whole soil system. In this case different levels or kinds of microbiological activity are recorded. Probably we have a combination of the three suggested sources. 

Roots modify their environment quite extensively in many ways. The most important of these are pH change, exudation and microbiological activity in the rhizosphere. Root exudates contain compounds such as hydroxycarboxylic acids and amino acids and these are capable of complexing trace metals. Bowling (1976), Farago (1986) and Streit and Stumm (1993) have discussed the theories of mineral uptake by plant roots the first suggests that there are four links in the uptake chain movement of ions or complexes in the soil to the roots uptake into the root transport across the root to the vascular system and movement to the shoot. 

The difficulty in interpreting this particular type of data is further compounded by its application in the upper soil zone where the most active plant and microbiological activity takes place. Many organic and inorganic compounds (humic acids CO2, N2O, NO2, etc.) are produced in this zone, all of which are rapidly adsorbed by activated charcoal. These compounds are present in macro concentrations (parts per thousand to percent) and produce fragment patterns which overlap the much lower concentrations of hydrocarbons, which are generally in the ppm range. 

There are, of course, several processes that can frustrate the detection of such expressions. It is important to consider the rate at which gas enters and leaves the soil air. Baver (1972) quotes several authors and estimates that there would have to be a complete renewal of soil air every hour to a depth of 20 cm in a normal cropped soil in order to maintain its usual average composition and microbiological activity. If mineral deposits are to have adequate expression in the soil air, sulphide oxidation must clearly influence its composition at a rate commensurate with such rapid aeration. 

Microbiological activity may continue in the samples and change the constituents, e.g. changes in nitrate content of soil and extracts unless they are stored at below 4 °C. Samples may be altered if they are dried too severely. Silage material, for example, may show a reduction in N content if it is dried at 100 °C. 

The anthropogenic H2 is emitted into the air in automotive exhaust gases, which contain H2 in the range of 1-5 % by volume. The nature of the oceanic source is not entirely clear but it is probably due to microbiological activity. However, the supersaturation of ocean waters unambiguously indicates hydrogen gas formation there. The emission from soils is caused by the fermentation of bacteria. 

Field studies with single applications of phenoxyalkanoic acid herbicides have indicated that breakdown is rapid under temperature and moisture conditions that favour microbiological activity (5). Enhanced degradation of these herbicides, under field conditions, was first noted in the late 1940s. The use of plant bioassay procedures, led to the discovery that the persistence of 2,4-D, but not 2,4,5-T, was decreased,by pretreatment of the soil with 2,4-D (26, 27). This enhanced breakdown was later confirmed using (14C)2,4-D and radiochemical analytical techniques (29). The breakdown of the (14C)2,4-D being more rapid in soil from the treated plots, tested 8 months after the last field application, than in soil from plots treated for the first time. 

Arsenic uptake from soil (i.e., arsenic bioavailability) depends on soluble arsenic species present in soil, on soil properties (i.e., the type and amount of the sorbent components of the soil), redox (Ej,) and pH conditions and microbiological activity (see Section 6.4.2.1). Uptake into plants further depends on phosphate (often originating from fertilizers) and vanadate levels, as the behavior of arsenate is similar to that of phosphates and vanadates (see also Section 6.6.1). 

Soil temperature affects microbiological activity in soils, and therefore can modify biotransformations of arsenic in soils (46). We found that the Penicillium sp. mediated transformation of MMAA into TMA was optimum at 20°C. Similarly, more arsines were produced in a silty clay loam at 25°C than at 5°C (19). Methylation of arsenic is pH dependent with the highest rates occurring at pH... 

---