In soils the major part of the organic N derives from decaying vascular plant material. Additional sources of N-containing compounds are the residues from the microbial biomass. Detailed descriptions and accounts of the composition and structure of plant and microbial components are given by de Leeuw and Largeau (1993), Stevenson (1994), Paul and Clark (1996), and Kogel-Knabner (2002). Of the plant dry mass, 2-15% is assigned to N-containing compounds (Haider 1992). Nitrogen is a compo nent of three very important biological macromolecular structures, i.e. proteins/polypeptides, polymers of amino sugars and DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Minor biological sources of organic N are smaller molecules, such as porphyrins (mainly chlorophyll and hemoglobin) and various N-containing secondary plant metabolites, such as alkaloids and sphingolipids (Stevenson 1994). Of the bacterial biomass, 50-60% can be assigned to N-containing compounds (Haider 1992). The ratio of protein to RNA is around 5 and the ratio of protein to DNA about 2. The relatively high content of N-containing biomolecules is responsible for the low C/N ratio of 5-8 of bacterial biomass (Paul and Clark 1996). Fungi contain approximately 14-52% N-containing compounds. Most of the N-containing compounds in plants and microorganisms are known to be highly sensitive to microbial degradation and are therefore expected to be quickly mineralised and/or reutilised for biological production.
Rhizodeposition, i.e. all organic matter released by living roots, accounts for a substantial input of organic N in soils (Whipps 1990). The amount of data for soils is limited, mainly because most of the data obtained from sterile soil experiments or nutrient solutions cannot be applied to natural soils. Root exudates containing N are mainly amino acids and amides, but nucleotides and flavonones are also reported (Uren 2001; Hutsch et al. 2002; Kuzyakov et al. 2003). Most of the exudates are rapidly consumed by soil microorganisms and subsequently immobilised in microbial residues (Mayer et al. 2003).
The majority of organic P inputs into soils from both plant and microbial origin are in the form of orthophosphate diesters (Turner et al. 2002). Microbial inputs are dominated by nucleic acids with 60% of total P and phospholipids, accounting for 5-30% of the microbial and fungal input, and even more in plant input to soils.
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