R

deaminase ammonium

FIGURE 12.5 Breakdown of protein/polypeptides into amino acids and further mineralization to ammonium is a source of N for decomposition, microbial growth, and plant uptake.

the photosynthetic enzyme ribulose bisphosphatase can contain up to 70% of the total tissue N.

Hemicellulose and cellulose are important cell wall polymers composing 10 to 70% of plant residues. The secondary cell wall contains from 5 to 30% lignin, a unique terrestrial plant hydrocarbon that provides a rigid exoskeleton and defense against pathogens. Extractable phenols and tannins are a significant component of some plants, especially in forest systems, and comprise up to 30% of their dry weight. These complex compounds are believed to be deterrents to herbivores and their presence can alter C and N mineralization dynamics dramatically. During later stages of decomposition, when low-N cellulose and lignin are being metabolized, soil nutrients from SOM and mineral, ion-exchange sites are used to metabolize the C compounds in plant litter.

Examination of common, prokaryotic bacteria such as Escherichia coli reveals a variety of C compounds, some similar to plants and others that are vastly different (Table 12.2). Protein makes up 55% of the cell dry weight of this organism. RNA is the next largest N component at 20.5% of its dry weight. The majority (80%) of this is ribosomal RNA (23S, 16S, and 5S RNA). Fungi have similar structures of slightly larger size, e.g., 18S and 25S RNA. Transfer RNA constitutes 15% and messenger RNA 4% of the total RNA. DNA accounts for 3% of the bacterial cell dry weight. Thus the N-containing genetic constituents, such as guanine, adenine, cytosine, and thymine and to some extent uracil, form a significant (24%) source

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