250 P. L. Gersper et al. 



While it is impossible to obtain absolute rates of nitrification from 

 fluctuations in the amount of nitrate in the soil, it is possible to estimate 

 rates that are equal to or usually less than the true rate. However, differ- 

 ences in concentration due to field variability or sample treatment may 

 lead to overestimation. Since nitrification is the principal source of ni- 

 trate, an increase in the nitrate pool gives a minimum value for nitrifica- 

 tion, disregarding small spatial transfers. The net rate observed is usually 

 lower than the actual production of nitrate because some nitrate is 

 denitrified or taken up by plants. Therefore, the most rapid rate of in- 

 crease is used for the estimate. With this approach, data for the wet mea- 

 dows from different investigators indicated nitrification rates in the sur- 

 face 10-cm soil layer of 0.024 and 0.0045 mg N m"' cm"' day"' in 1971 

 and 1973, respectively (Barel and Barsdate, unpubl.). Rates in the deeper 

 soil are consistently higher, 0.045 and 0.012 mg N m"^ cm"' day"' in 1971 

 and 1973. Denitrification in the 7- to 15-cm soil layer accounted for a 

 total of 0.05 mg N m"^ day"' at a similar site in 1972. The uptake of ni- 

 trate by the plants can also be added into the rates of change in the ni- 

 trate pool to produce another estimate of nitrification rate. In 1971, an 

 experiment using "N indicated plant uptake rates in the wet meadow of 

 1.7 mg NO3-N m"^ cm"' day"' (Munn, unpubl.). The apparent nitrifica- 

 tion rates in the drier areas are much higher than those in the meadows. 

 In 1973, the estimated rates of nitrification on the rims of low-centered 

 polygons were 1.5 and 2.0 mg N m'^ cm"' day' in the surface 10 cm and 

 the buried organic layer (Barel and Barsdate, unpubl.). The 1973 data in- 

 dicate turnover times for the nitrate pools ranging from 5 to 25 days, 

 with rates on the rims of low-centered polygons lower than those in the 

 meadow soil. 



Transport of Nitrogen and Phosphorus 



Vertical transport of ammonium and nitrate ions in the soil solution 

 should occur by diffusion if a concentration gradient exists with depth. 

 The extremely wet conditions and low bulk densities in most soils of the 

 coastal tundra at Barrow are favorable for diffusion, although it is 

 slowed by low temperatures. The patterns of ammonium and nitrate con- 

 centrations with depth (Figure 7-12) suggest that the diffusive movement 

 of ammonium during the summer of 1973 was into the silt loam mineral 

 layer (8 to 16 cm), from both above and below. The diffusion gradient 

 for nitrate, on the other hand, led to its movement into the surface or- 

 ganic layer (0 to 8 cm) from the mineral and buried peat material. In late 

 September, concentrations of both ammonium and nitrate were highest 

 in the mineral layer, leading to diffusion outward from this layer. 



Preliminary results using "N document that ammonium is trans- 



1 



