Invasive plants may alter soil microbial community composition and thus ecosystem nutrient cycling. Tamarix spp. is an invasive tree/shrub in riparian areas of the western U.S. We examined the spatial variation of soil respiration in a monotypic stand of Tamarix ramosissima on the lower Virgin River floodplain, Nevada. Soil samples were collected from 0–70 cm depths in three different locations; near the river, near the stand edge (60–70 m from the river edge) and at 30–40 m from the river edge in the stand on June, 2012. We also collected soil samples from the surface outside of the stand where a native shrub species was dominated. Soil samples were analyzed for basal respiration and substrate-induced respiration (SIR) as well as soil water moisture, pH, electrical conductivity (EC) and soil organic matter (SOM) content. Basal respiration and SIR at the soil surface were significantly higher in the stand than those outside of the stand, suggesting that soil microbial activity and biomass were higher inside rather than outside of the Tamarix stand. The basal respiration and SIR decreased with depth and those at the surface decreased with increasing the distance from the river. The values were correlated with water holding capacity, pH and SOM but not with water content and EC.