The Soils and Their Nutrients 249 



7-6). Decomposition may operate first on the material highest in phos- 

 phorus, leading to the release of mineralized phosphorus. If the phos- 

 phorus associated with the more resistant organic matter is insufficient to 

 support microbial growth, decomposition of the accumulated organic 

 matter might lead to lowering of phosphorus availabihty. Thus, the fac- 

 tors that have allowed the gradual accumulation of organic material, 

 such as the occurrence of permafrost and the burial of the sapric organic 

 layer beneath a relatively impermeable mineral horizon, may in fact be 

 acting to increase the availability of phosphorus in the system. Further 

 work would be necessary to evaluate this hypothesis. 



Nitrification 



Although nitrification neither produces nor removes available nitro- 

 gen from the soil, it affects nitrogen transport and utilization. The pro- 

 ducts of nitrification, nitrate and some small amounts of nitrite, are not 

 involved in exchange processes with cation exchange sites. These anions 

 are therefore much more mobile than ammonium in a system with high 

 cation exchange capacity and move vertically and horizontally in the soil 

 by diffusion and are lost by leaching or surface runoff. Nitrate, the ma- 

 jor product of nitrification, is also available to denitrifying bacteria as an 

 oxygen substitute, and may be reduced to dinitrogen gas or nitrous oxide 

 and lost. In agricultural systems, nitrate is the form of nitrogen most 

 readily taken up by plants, but in the tundra, as in natural grasslands 

 (Porter 1975), ammonium may be equally preferred. 



Nitrifying bacteria are scarce in the soils of the coastal tundra at 

 Barrow. Munn (1973) attempted to measure the nitrification potential in 

 soil samples taken from the moist meadows throughout the 1972 summer 

 season. He detected no conversion of ammonium to nitrate in soil sam- 

 ples under aerobic conditions perfused with an ammonium sulfate solu- 

 tion at 23 °C and pH 5.6 to 6.3. No nitrifying bacteria were found among 

 200 aerobic isolates from the 0- to 2-cm horizon of the wet meadow soils, 

 tested at 15 °C (Benoit, unpubl.). However, Viani (unpubl.), using the 

 most probable number technique, was able to detect low numbers of ni- 

 trifying organisms in several soils from polygonal terrain. Norrell and 

 Anderson (unpubl.) measured nitrification in the laboratory and report- 

 ed average rates of 1.5 and 0.75 ^ig N (g soil)"' day"' at 10 °C for dry and 

 wet sites, respectively. These may represent the maximum potential 

 ratesfor these soils, although alternate incubation conditions were not 

 tested. Norrell and Anderson further indicated that temperature was a 

 major limiting factor, nitrification being only occasionally detectable at 

 temperatures below 5 °C. Efforts to isolate psychrophilic nitrifiers from 

 the soils were unsuccessful, with no activity detected after 6 months of 

 incubation at 2°C. 



