144 



R. T. Prentki et al. 



Pond B sediment. This indicates that all of these soils had a much smaller 

 phosphate buffering capacity than the Barrow sediment. 



The Temkin isotherm slope dX(d\og Cj"' can be approximated by a 

 one point estimator, the phosphate sorption index (PSI), equal to the ratio 

 of X to log C obtained by adding phosphate to a sediment at a ratio of 

 1500 Mg P to each gram of soil. Bache and Williams (1971) found that 

 this estimator was highly correlated with isotherm slope (r=:0.97) in 42 

 soils. A PSI of 545, calculated for the Pond B sediment in Figure 4-22, 

 underestimated the real isotherm slope by 21%. This underestimate is not 

 severe and the relative error involved in comparing two PSI values would 

 normally be less, especially if similar phosphate concentrations were in 

 equilibrium with the two sediments before addition of phosphate. The PSI 

 determinations on to 3 cm sediments from ponds in the IBP watershed 

 (Figure 4-29) ranged from 318 to 728 (average of 532). The PSI of the 

 intensively studied ponds A-B, C, D, E, J, and X ranged only from 359 to 

 540. In comparison, the highest PSI obtained by Bache and Williams 

 (1971) from 42 soil samples was only 382 (our units). Meyer (1979) in a 

 study of Bear Brook in the Hubbard Brook Experimental Forest found 

 that sediment PSI averaging 10.3 for silt and 2.1 for sand were sufficient to 

 buffer dissolved phosphorus concentrations at 2 Mg P liter"'. The 

 phosphate buffering intensity of pond sediments is 50 to 250 times higher. 



2.0 



-N- 



FIGURE 4-23. Concentrations of DRP (\xg P liter') in the 

 water of ponds. Concentrations in Pond B and in Pond F (*) 

 are averages from August 1970 and 1971. The concentration 

 in Pond 21 (**) is from 28 August 1970. 



