244 P. L. Gersper et al. 



Direct measurements of denitrification in the field were made in 

 midsummer. The rates of denitrification per gram of soil were six orders 

 of magnitude lower than the rates of nitrate reduction measured in vitro. 

 Although concentrations of nitrate were considerably lower in the field 

 than in the laboratory incubations, 0.17 mg liter"' vs 69 mg liter"', the ex- 

 treme difference in rates suggests that most of the nitrate reduction that 

 occurred in the laboratory tests did not result in denitrification. Focht 

 (1978) discusses evidence that nitrate losses are significantly greater than 

 denitrification when organic carbon is readily available and ammonium 

 concentrations are low. The mean denitrification rate in the field results 

 in a loss of 52 pig N m"^ day"' from the surface of the wet meadow. The 

 time course of denitrification in situ has not been established. However, 

 the potential for nitrate reduction in soils from the wet meadow re- 

 mained high for 65 days in 1972. If denitrification rates follow the same 

 pattern, a net loss of 3.4 mg N m"^ yr"' would occur, more than five times 

 the amount of nitrate present in the upper 10 cm of the soil. 



The rates of denitrification in other microtopographic units are gen- 

 erally lower than those in the wet meadow. No detectable denitrification 

 occurred in field experiments on the top of a high-centered polygon. This 

 is consistent with the low potential nitrate reduction rate in samples from 

 similar microtopographic units. Nitrate concentrations in soils of tops of 

 high-centered polygons are relatively high (2.5 ppm), indicating that the 

 lack of denitrification activity here is not due to substrate limitation. In 

 the mesic meadow, where nitrate concentrations were intermediate (0.33 

 ppm), denitrification rates in the field were 19 ^g N2 m"^ day"', only a 

 third as high as those from the wetter area. Simultaneous addition of glu- 

 cose and phosphate to these samples produced a more than four-fold in- 

 crease, to 89 ^g N2 m"^ day"', over a 16-day incubation period. Munn 

 (1973) also found a five-fold increase in apparent denitrification when a 

 wet meadow was fertilized with urea. 



Lack of denitrification in the soils from relatively dry polygon tops 

 may be caused by the high aeration. Even though moisture contents re- 

 main high, the high pore volume and permeability of these soils may 

 deter the development of anaerobic microenvironments. The strong re- 

 sponse of denitrifying activity to glucose plus phosphate indicates that 

 either energy or phosphorus is limiting under natural conditions in the 

 mesic meadow. The stimulatory effect of easily decomposable organic 

 matter on denitrification has been shown for temperate soils (Bremner 

 and Shaw 1958). Lack of phosphorus can also inhibit the breakdown of 

 organic matter (Chang 1940, Munevar and Wollum 1977), and this may 

 occur in soils of the coastal tundra at Barrow. No analysis of the effect of 

 pH on denitrification was made, but soil conditions are more acid than is 

 optimal for denitrification in temperate soils (Bremner and Shaw 1958). 



Overall, there is a net gain in the inorganic forms of both nitrogen 



