[R] Orthogonal contrasts in mixed-effect models

[xavi sole]

Hello all,

I am wiriting here to look for help!!!

I am running a mixed-model effect lmer() relating the total species richnees
observed in one place with some landscape indicators depending on three
positions at field scale (Boundary, edge, centre)  nested to the location
where the survey was conducted. The model follow the next instruction

lmer(a~position*%naturalvegetation + position*P/A+ (1|location)

However, running the model in this way it is not possible to compare
position between them and I have to run orthogonal contrast for this
porpouse.

my data matrix has the following structure


   Richness      Location        %NV    P/A   Location
         3                       1                      34.3      0,43     
B
         5                      2                      45.2      0.33      B
        12                      3                      21.2      0.21      E 
        6                        4                      11.2      0.21     E
                               ....
                               ....
        8                     135                 76.3     0.29      C
 
How can I run the orthogonal contrast. Somebody now how tu run it and the
necessary code.

Excuse me because I am not an stadistic people. I am botanic....

Thanks!

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