This study was carried out in a Mediterranean salt marsh in semiarid Southeast Spain to assess whether a spatial salinity gradient can affect the microbiological and biochemical properties (labile C fractions, biomass C, oxidoreductases, and hydrolases) of the rhizosphere soil of Inula crithmoides and the extent of arbuscular mycorrhizal (AM) colonization in its roots. There were no significant differences in the soluble C fractions (water-soluble C and water-soluble carbohydrates) or the microbial biomass C values of the rhizosphere soil of I. crithmoides among the different zones of the salt marsh. Dehydrogenase activity, hydrolases (urease, protease, phosphatase, and β-glucosidase) and the extent of AM colonization in I. crithmoides roots were greater in the higher salt marsh zones, corresponding to those of lower rhizosphere soil salinity. This study shows that soil salinity is inversely associated with some parameters related to soil microbial activity, such as dehydrogenase activity and some hydrolases, as well as the extent of colonization by AM fungi, which can improve plant performance. However, bioactive organic matter fractions that determine soil productivity are not related to soil salinity.