The Soils and Their Nutrients 233 



nitrogen, ammonium, and nitrate, in approximately 10:1.0:0.1 ratios. 

 Most of the organic nitrogen in solution is readily decomposed, but plant 

 uptake is from the inorganic nitrogen in the soil solution, and diffu- 

 sion processes act primarily within this pool. The average concentrations 

 of ammonium and nitrate in the soil solution in 1973 were 145 and 6 ppb, 

 respectively (Bare! and Barsdate, unpubl.). 



The total amount of nitrogen in the soils of the drier microtopo- 

 graphic units is commonly greater than 500 g m'^ (10 cm)'", slightly more 

 than in the moist meadow soils, but the amounts of exchangeable nitro- 

 gen are similar. The average nitrate concentration in the soil solution of 

 the polygon rim was 5.9 ppm NO3-N in 1973, almost three orders of mag- 

 nitude higher than the nitrate concentration in the wet meadow. The am- 

 monium concentration on the rim was 750 ppb, also higher than in the 

 meadow. The ratios of ammonium to nitrate in the soil solution change 

 from 10: 1 in the moist meadow to 0. 1 : 1 on the rims of low-centered poly- 

 gons. Nitrate is also found in greater concentrations than ammonium in 

 the centers of high-centered polygons with mineral soil, but ratios drop 

 below 1 in the other, slightly moister high-centered polygons with peaty 

 soil and in mesic meadows. 



The total soil phosphorus in the upper 10 cm of the moist meadows 

 is approximately 25 g m'^ of which two-thirds is in organic form (Table 

 7-3). Dissolved organic phosphorus is not believed to be available to 

 plant roots but it is apparently susceptible, like dissolved organic nitro- 

 gen, to rapid hydrolysis. The ratio of dissolved to total organic phos- 

 phorus is very low, 0.0008:1 (Barel and Barsdate 1978), even when com- 

 pared to that for organic nitrogen (0.002:1). The organic phosphorus 

 contributed by soil microorganisms has not been determined, but calcu- 

 lations based on decomposer biomass and species composition indicate 

 that the standing crop of decomposers ties up a far larger fraction of soil 

 phosphorus than nitrogen, 3% vs 0.4%. Thus, fluctuations in microor- 

 ganism populations may have a significant effect on the overall distribu- 

 tion of phosphorus. 



The fraction of the inorganic phosphorus that is in equilibrium with 

 the soil solution appears very small when measured by extraction onto an 

 anion-exchange resin (Barel and Barsdate 1978), and the concentration of 

 inorganic phosphorus in the soil solution is correspondingly low, averag- 

 ing 10 ppb in 1973. However, chemical fractionation of the inorganic 

 phosphorus from the moist meadow soils indicated that a large fraction 

 is extractable under reducing conditions (Chang and Jackson 1957). This 

 fraction may contribute considerably more to the exchangeable and dis- 

 solved pools of phosphorus under anaerobic, reducing conditions such as 

 exist in the soils of wet meadows and polygon troughs than is apparent in 

 laboratory analyses performed under aerobic conditions (Khalid et al. 

 1977). Most of the available phosphorus is bound to iron or aluminum 

 ions (Prentki 1976). 



