Research on arbuscular mycorrhizal fungi (AMF) suggests that their effect on plant growth is a continuum that changes from positive to negative along environmental gradients as conditions for plant growth improve. Prairie wetlands were used to test this mycorrhizal continuum model. First, three saline and three freshwater wetlands were surveyed for AMF, to investigate the hypothesis that AMF colonization would be greater in wetlands with high soil salinity, lower nutrient levels, and during periods of drought. Second, a greenhouse experiment on Carex stricta and Calamagrostis canadensis tested whether AMF improved plant growth and nutrient uptake under five hydrologic regimes. Third, another greenhouse experiment tested whether AMF improved the competitive ability, productivity, and macronutrient uptake of the facultative mycotroph, C. stricta, along nutrient and soil moisture gradients while growing with Phalaris arundinacea and Typha latifolia;AMF colonization in Iowa and North Dakota ranged from 0.2 to 72% among all plant species and was strongly dependent on the plant host regardless of environmental factors or the plant's life stage. Colonization was greatest at the peak growing and fruiting period (June) on many early season grasses and sedges. AMF colonization was also greater in plants growing in the poor soil conditions found in North Dakota--low phosphorus and high soil pH. Soil moisture did not have a significant effect on AMF colonization. However, the survey was done in a wet year and may have underestimated the importance of soil moisture on AMF colonization. AMF colonization levels measured in the field supported the mycorrhizal continuum model;High soil phosphorus levels in the hydrology experiment probably prevented AMF colonization of C. stricta and C. canadensis. The lack of AMF colonization suggests that mycorrhizae are not important to these plants growing in natural wetlands with high soil phosphorus levels, even in dry conditions. Non-mycorrhizal fungi reduced biomass of C. canadensis, but not C. stricta, indicating that C. stricta may have antifungal properties;Nutrient levels and interspecific competition explained 35% of the total variation on C. stricta productivity and macronutrient concentration, whereas soil moisture explained only 7% and AMF 0% of the variation. AMF did not increase the competitive ability, productivity, or nutrient uptake of C. stricta.