232 P. L. Gersper et al. 



TABLE 7-4 Exchangeable Ammonium 

 Nitrogen of a Typical Per- 

 gelic Cryohemist in Moist 

 Meadow, 1973 



Source: Flint and Gersper (1974). 



the upper 10-cm section, with more than 9597o bound in organic matter 

 (FHnt and Gersper 1974). The organic nitrogen can be divided into hy- 

 drolyzable and nonhydrolyzable fractions. The hydrolyzable fraction 

 makes up 80% of the nitrogen in the soil organic matter, while the non- 

 hydrolyzable fraction, which probably represents the most resistant core 

 of the humus, makes up approximately 19.5%. Most of the remaining 

 0.5% is in the form of readily hydrolyzable nitrogen. This latter fraction 

 is seasonally variable, indicating that it may be an integral part of the 

 labile nitrogen in the system. 



The nitrogen content of the living soil microorganisms is uncertain 

 since separation of the organisms from the soil material is difficult. 

 Fungal biomass and nitrogen content of the fungi were both determined 

 (Laursen 1975), but bacterial biomass may exceed fungal biomass by an 

 order of magnitude (Chapter 8), and no measurements exist of the nitro- 

 gen concentrations in the natural bacterial population. 



The inorganic nitrogen in the soil is almost entirely in the form of 

 ammonium ions bound on the cation exchange complex, and in equi- 

 librium with the ammonium and other cations in the soil solution. The 

 vertical distribution of exchangeable nitrogen affects its availability to 

 plants and soil organisms. Although the concentration of ammonium on 

 a weight basis is highest in the surface 5 cm (Table 7-4), the amount in the 

 10-cm rooting zone is only a little more than 25% of the total ex- 

 changeable pool in the active layer (Flint and Gersper 1974). Thus, a 

 large fraction of the nitrogen present in exchangeable form is not physi- 

 cally accessible to most of the plants or microorganisms. 



Soil solutions for nitrogen and phosphorus determinations were ob- 

 tained from sample cores using pressure up to 9.4 bars (Barel and Bars- 

 date 1978). The soil solution contains dissolved and colloidal organic 



