[R] adonis(), design

[Kay Cichini]

hello list,

i'd appreciate help regarding formula expression in adonis(), package vegan.

i'm interested in the effect of an impact on species composition and if this
effect is the same for different sites.
i have an impact survey with plot pairs, one plot with impact the other with
similar ecolgical conditions but without impact, which is the control 
(factor1: impact). 
two different sites were sampled, at each 6 plot pairs (factor2: community)
with a final N of 24. 
so this would be a 2-factorial crossed design, right? thus, i called: 

adonis(speciesdata~Community*Impact)

....but do i have to a account for the identity of pairs by blocking them -
if so, how to achieve this in adonis?
i'm not sure about the reasons why a blocking would be needed and if the
rather small size of my data would allow a blocking, without loosing all
statistical power.

greetings,
kay
 
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