The Soils and Their Nutrients 243 



of 0.06 mg m"^ in summer runoff was calculated by the techniques used 

 for nitrogen. Total phosphorus losses in runoff are 2.28 mg m'^ yr"', less 

 than 0.01 *^o of the total phosphorus in the upper 10 cm of soil. The phos- 

 phorus lost is mainly in the organic form, with particulate matter consti- 

 tuting almost 36% of the runoff loss (Prentki 1976). The high loss of 

 phosphorus in particulate form is in contrast to nitrogen losses, where 

 particulate organics make up less than 10% of the organic nitrogen frac- 

 tion (Barsdate and Alexander 1975). The dissolved organic phosphorus 

 lost in runoff constitutes approximately 10% of the pool of organic 

 phosphorus in the soil solution. The major loss of phosphorus occurs 

 during snowmeh, and the inorganic phosphorus lost during this period is 

 greater than the input of inorganic phosphorus from snow. 



The effects of precipitation and runoff on the nitrogen and phos- 

 phorus pools described above are integrated over a variety of microtopo- 

 graphic units. The effects of precipitation and, in particular, runoff dif- 

 fer among these units but quantitative assessment of this variation is dif- 

 ficult. In the absence of overland flow out of basins of low-centered 

 polygons, these basins accumulate any nutrients that were present in the 

 snowpack above them or on the inner sides of the polygon rims. Polygon 

 troughs, on the other hand, are pathways for water flow, and may there- 

 fore be enriched in inorganic nitrogen and depleted of organic nitrogen 

 and phosphorus by the meitwater. Drier areas retain a greater fraction of 

 the water, and thus lose less of the inorganic nutrients associated with the 

 rainfall or the dissolved or particulate organic matter that is assumed to 

 be lost with the summer runoff. 



Loss of Nitrogen in Gaseous Form 



The soil nitrogen of the coastal tundra at Barrow is depleted by deni- 

 trification as well as by runoff losses. In anaerobic conditions such as are 

 common in the soils, many bacteria can utilize nitrate rather than oxy- 

 gen. This process can lead to denitrification, to the production of 

 nitrogen oxide or nitrogen gas, to assimilation of nitrogen by the bac- 

 teria, or to the production of ammonia (Verstraete 1978). The popula- 

 tion of facultative anaerobes in soils is large. Lindholm and Norrell 

 (pers. comm.) measured the production of nitrite from nitrate in incuba- 

 tions at high nitrate levels. At 5°C the microflora from a polygon trough 

 showed average rates of nitrite production equivalent to the reduction of 

 430 )ug N (g soil)-' day-'. Samples from the tops of high-centered 

 polygons showed a lower average rate, reducing 270 ^g N (g soil)"' day"'. 

 The denitrifying bacteria isolated from the same soils were predomin- 

 antly aerobic Pseudomonas spp. In a test of aerobically isolated bacteria 

 from the upper 2 cm of a wet meadow soil, only 5 to 10% were capable 

 of denitrification, although 68% were facultative anaerobes. 



