% Water-filled pore space

FIGURE 13.2 The relationship between water-filled pore space (a measure of soil moisture availability) and relative amount of microbial activities (redrawn from Linn and Doran, 1984).

populations of bacteria and fungi, and they can create or modify habitats for a wide array of organisms, e.g., earthworms create burrows and termites macerate wood. Mineralization and immobilization are widely distributed because they are so fundamental—all heterotrophic soil organisms consume organic materials for energy and C and immobilize and mineralize N as a by-product.

The widely distributed nature of mineralization and immobilization means that the environmental regulation of these processes is relatively straightforward. Rates of activity increase with temperature and are optimal at intermediate water contents, similar to respiration in Fig. 13.2, although it is important to recognize that significant activity is likely to occur at extremes of both temperature and moisture. In most soils the quantity and quality of detrital inputs are the main factors that control the rates and patterns of mineralization and immobilization. When moisture and temperature are favorable, large inputs of organic matter lead to high rates of microbial activity and the potential for high rates of mineralization and immobilization.

Water-filled pore space (WFPS) is a useful measure of moisture's influence on soil biological activity because it includes information about the impact of soil water on aeration in addition to information on water availability per se. The calculation of %WFPS is

™ ,„r„n soil water content X bulk density X 100 %WFPS =---.

TABLE 13.2 C:N Ratios in Various Organic Materials (from Tisdale et al., 1993 and Hyvonen et al., 1996)

Organic material

C:N ratio

Soil microorganisms


Soil organic matter


Sewage sludge


Alfalfa residues


Farmyard manure


Corn stover


Grain straw


Oak litter


Pine litter


Crude oil


Conifer wood


Soil water content is determined gravimetrically (g H2O/g dry soil), bulk density (g cm-3) is the oven dry weight of a given soil volume, and the value 2.65 is the density (g cm-3) of rock—sand grains and other soil mineral particles.

What controls the balance between N mineralization and N immobilization? The answer is primarily organic matter quality—the availability of C in the material relative to its available N. Consider the effects of adding various materials with different C:N ratios to soil (Table 13.2). When one adds manure, with a relatively low C:N ratio (ca. 20:1), to soil, for example, the microbes have no trouble obtaining N and as a result mineralization dominates over immobilization, and plant-available N increases in soil. This is why manure is frequently used as a fertilizer. On the other hand, were one to add sawdust to soil, a material with a high C:N ratio (625:1), the microbes are keen to obtain the energy and C in the sawdust but cannot degrade this material without additional N because the sawdust does not have sufficient N to allow the microbes to build proteins. So the microbes must immobilize N from their environment, resulting in a decrease in plant-available N in the soil.

The balance between mineralization and immobilization is also affected by organism growth efficiency. For example, fungi have wider C:N ratios in their tissues than bacteria and, therefore, have a lower need for N and will thus mineralize N more readily. As a general rule of thumb, materials with a C:N ratio >25:1 stimulate immobilization, while those with a C:N ratio <25:1 stimulate mineralization (Table 13.2). The exception to this rule is highly decomposed substances with a low C:N ratio, e.g., soil organic matter (humus or compost) in which labile C and N have been depleted and the remaining C is in complex forms inherently resistant to decomposition (see Chap. 12) and therefore resistant to mineralization.

There are a wide variety of methods for measuring mineralization and immobilization (see Hart et al., 1994; Robertson et al., 1999). Measurement of net mineralization and immobilization rates is much easier and more common than is measurement of gross rates. Measurement of net rates usually involves quantifying changes in inorganic N levels in some type of whole soil incubation. In most cases these incubations are in containers, with no plant uptake or leaching losses, and changes in inorganic N levels are measured by periodic extractions of the soil. Incubation methods vary widely, from short (10-day) incubations of intact soil cores buried in the field to long (>52-week) incubations of sieved soils in the laboratory. Gross rates are measured using isotope dilution methods whereby small amounts of 15N-labeled ammonium are added to the soil and the subsequent dilution of the label is used as a basis for calculating the gross production and consumption of ammonium.


Nitrification is the microbial oxidation of reduced forms of nitrogen to less reduced forms, principally NO- and NO33. Autotrophic bacteria, first isolated in the late 1800s, gain as much as 440 kJ of energy per mole of NH3 oxidized when NO3 is the end product. We know now that heterotrophic microbes can also nitrify, although autotrophic nitrification appears to be the dominant process in most soils.

The importance of nitrifiers to ecosystem function is substantial: although some nitrate enters ecosystems in acid rain or as fertilizer, in most ecosystems nitrate is formed in situ via nitrification. Because nitrate is an anion, it is more mobile than ammonium, the ionized source of NH3 in soil water:

As a positively charged ion, ammonium can be held on cation-exchange sites associated with soil organic matter, clay surfaces, and variable-charge minerals. Nitrate, on the other hand, can be easily transported out of the rooting zone by water when precipitation exceeds evapotranspiration.

Nitrate is also subject to denitrification, in which denitrifying bacteria convert nitrate to N gas. Additionally, nitrification in many soils is a major source of soil acidity, which can have multiple effects on ecosystem health, including the hydrologic loss of base cations as hydrogen ions displace other cations from exchange sites. And in soils dominated by variable charge minerals, which include most highly weathered tropical soils, soil acidity largely controls cation-exchange capacity (CEC), and nitrifier-generated acidity can drive CEC to nil. Further, some plants and microbes appear better able to take up ammonium than nitrate, and vice versa, implying a potential effect of nitrifiers on plant and microbial community composition. Finally, nitrifiers can also be direct sources of the atmospheric gases NOx and N2O.

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