Abstract Although soil organic matter decomposition is an important process determining nutrient transformations and availability in wetland ecosystems, few studies have attempted to assess which environmental factors are most important in controlling spatial differences in decomposition rates found along environmental gradients. Relative soil decomposition was determined in a Phragmites australis Cav. Trin ex Steudel dominated wetland in northern Jutland, Denmark along a natural salinity gradient, where nutrients, soil moisture, temperature and salinity among other factors also varied. Our objective was to identify which edaphic factors most limited rates of relative soil decomposition, as evaluated by measuring cellulose decomposition with the cotton strip technique. Replicate cotton strips were placed at seven marsh sites along the salinity gradient, and soil and interstitial water samples were collected and analyzed for major macro- and micronutrients (NH 4–N, NO 3–N, P, PO 4, K, Mg, Ca, Na, S, Fe, Mn, Zn, Cu, Mo, B, Si), pH, Eh, conductivity, temperature, and soluble sulfides. Cellulose decomposition, expressed as cotton tensile strength loss (CTSL) per day, decreased with increasing salinity, except at the highest salinity site where a significant increase occurred. Mean CTSL values, averaged for each marsh site, varied 3-fold from 1.8 to 5.5% loss per day. Principal component and multiple regression analyses were used to prioritize the importance of the various factors that might control this spatial difference in CTSL rates. Although soil conductivity (salinity) accounted for the large percentage (45%) of the variation in the environmental data, soil fertility- and soil reduction-associated variables explained the greatest percentage (56%) of the spatial variation in cellulose decomposition. Univariate correlation analyses supported the conclusion that soil fertility, primarily inorganic nitrogen and phosphorus, is the major environmental factor determining soil cellulose decomposition rates along this salinity gradient.