Table 2. The effects of the composition of contaminating material on estimates of root N concentration, and on the relationship between N estimates and sample ash.

                                        #           *      
              Contam-           Property        Sig.      @
              inating     ____________________ regres-  ^   
Species       material      No      Ao    C/N  sions    N  
_____________________________________________________________
P. smithii    Indirectly  .00367   .927  10.0    1    .0125
              estimated

     "        Whole soil  .00170   .965  10.2    4    .0165

     "        Sand-free   .00387   .923   9.9    1    .0119
              fraction
            
     "        Ash         0.       1.      -     4    .0192

B. gracilis   Indirectly  .00235   .962   8.0    4    .0060
              estimated

     "        Whole soil  .00091   .984   8.8    4    .0109

     "        Sand-free   .00409   .931   8.4    3     < 0
              fraction

     "        Ash         0.       1.      -     7    .0135
_____________________________________________________________

# The properties of the contaminating material are:

• No = N content (fraction).
• Ao = ash content (fraction).
• C/N = C/N ratio of organic matter.

* Number of regressions between N estimate and sample ash significant at P < .05. There were 14 data sets for each species, as described in the text.

@ The estimate of root N in the single sample with the greatest sample ash, chosen as a sensitive indicator of the effect of contaminating material.