The diversity of microorganisms may depend on the type of plant growing in the soil and changes in soil nutrients can alter the biological activity in it (Torstesson et al, 1998). On the other side, factors that affect soil development also influence the type of plant community (Pritchett and Fisher, 1987).

Soil properties (Musil 1993), microbial community structure and function and nitrogen dynamics in soils can be altered by the presence of exotic species (Ehrenfeld et al 2001, Kourtev 1999, 2002).

N-fixing trees, such as Acacia species, can promote enrichment of soil, particularly in systems where soils are naturally poor (Milton 1981, Musil and Midgley 1990, Musil 1993) as sand dunes. Biological activity and chemical and physical properties of soils will be evaluated.

3.1.1 SIR (substrate induced respiration), BIOLOG and some soil enzymes (cellulase, chitinase) were analyzed to evaluate the microbiological functional potencional

3.1.2 nitrogen cycling - nitrogen is one of the main limiting nutrients in dune systems. As Acacia is a N-fixing species, the N cycle will be studied. Biological mineralization and nitrification will be measured.

Salinity, pH, nitrogen (total N, nitrate and ammonium) and organic C, were determined in the soil and litter. Several cations (K+, Na+, Ca2+, Mg+) were also analysed.

Soil microorganisms are critical in creating and maintaining good soil structure (Torstensson et al., 1998). Texture and moisture will be studied.