Effects of arbuscular mycorrhizal fungi on productivity and soil nutrients of cultivar and non-cultivar warm-season prairie grasses

Background/Question/Methods

In tallgrass prairie, arbuscular mycorrhizal fungi (AMF) facilitate the dominance of warm-season (C4) grasses.  Because of the extensive loss of tallgrass prairie, restoration of this ecosystem is increasingly common and sources of C4 grass seed can include local, remnant-collected ecotypes and commercially-available, USDA-registered cultivars.  Cultivars of C4 grasses have been selected for traits such as increased germination, forage quality, and drought tolerance.  A greenhouse experiment was conducted to quantify whether the presence of AMF interacts with population sources (cultivar vs. non-cultivar) of dominant C4 grasses to affect net primary productivity (NPP), root architecture, and soil nutrient availability (inorganic N and P).  Cultivar and non-cultivar population sources of Andropogon gerardii and Sorghastrum nutans were grown in the presence or absence of AMF through the application of a fungicide.  We hypothesized that cultivars would have greater NPP than non-cultivars and all population sources would benefit from the presence of AMF, but cultivars would be less dependent on AMF for growth.  
Results/Conclusions

Cultivars of A. gerardii had greater aboveground NPP (P<0.001), belowground NPP (P=0.029), and fine root length (P=0.0605), and less available inorganic N (P=0.0324) in the soil than non-cultivars.  Across both sources, A. gerardii grown with AMF showed higher ANPP (P=0.025), greater fine root length (P=0.0624), and less available soil NH₄-N (P<0.001) and NO₃-N (P<0.001) relative to plants without AMF.  Cultivars of S. nutans had greater NPP (P=0.043) and fine root surface area (P=0.01) than non-cultivars, in the presence of AMF but were not different without AMF.  Furthermore, when grown with AMF, both sources of S. nutans had greater fine root biomass (P=0.003) and fine root length (P=0.002) than without AMF.  Results for both species supported our initial hypothesis that cultivars and AMF will have highest productivity.  In contrast, both species grown with AMF showed greater fine root biomass and length, opposite expected results given that C4 grasses are obligate mycotrophs.  Therefore, population source and AMF influence the productivity of dominant, warm-season grasses commonly used in restoration, but cultivars do not appear to be less dependent on this mutualism.