Environmental Engineering Reference
In-Depth Information
processes go on under aerobic and anaerobic conditions, and at extremes of temperature,
pH, salinity, water availability, and other environmental conditions.
Recent research showing that plants can take up simple, soluble amino acids (a type of
dissolved organic N) has motivated discussion about expanding the concept of mineraliza-
tion to include not just the production of NH 4 1 , but any simple, soluble, N-containing
compound available to plants. Such a redefinition would not alter the basic concept that
mineralization involves simplification of organic compounds into forms available to plants.
The balance between mineralization and immobilization regulates the size of the soluble
N pool and is thus a critical regulator of N availability to primary producers and for
hydrologic losses. If immobilization dominates over mineralization, the soluble pool
decreases and vice versa. A good predictor of this balance is the C:N molar ratio of the
substrate being decomposed. If microbes are degrading substrates (a molecule upon which
an enzyme acts) rich in N, their needs for N are easily met and mineralization dominates.
As a general rule, substrates with molar C:N of 25:1 or lower lead to net mineralization,
while substrates with C:N greater than 25:1 lead to net immobilization ( Table 7.2 ). Thus, it
is easy to understand why adding materials such as manure stimulates plant growth
whereas adding materials such as sawdust inhibits plant growth (See Figure 4.8 in
Chapter 4). Some materials, such as soil organic matter, have low C:N ratios but do not
lead to increases in the soluble N pool because they are extensively degraded and chemi-
cally altered (humified) and are therefore not susceptible to microbial use.
In addition to switching from mineralization to immobilization to meet their N needs,
microbes have the ability to produce N-acquiring enzymes in response to N limitation of
their growth and activity. Thus, addition of N-poor substrates will stimulate production of
such enzymes as chitinase, which releases N from chitin (N-acetylglucosamine) or
TABLE 7.2 Molar C:N ratios in various organic materials.
Organic Material
C:N Ratio
Soil microorganisms
8:1
Sewage sludge
9:1
Soil organic matter
10:1
Alfalfa residues
16:1
Farmyard manure
20:1
Corn residue
60:1
Grain straw
80:1
Oak
200:1
Pine
300:1
Crude oil
400:1
Conifer sawdust
625:1
From Tisdale et al. 1993; Hyvo¨nen et al. 1996 .
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