Mycorrhizal symbiosis in the tallgrass prairie: Above-and belowground linkages

Arbuscular mycorrhizal fungi, through their external hyphal network, provide a direct physical link between their host plants and soil resources. The hyphal network contributes to host plant uptake of nutrients, may provide an interconnection for nutrient exchange between host plants, and may act as a stabilizing agent in the formation and maintenance of soil structure. A series of complementary experiments was conducted to examine the role of mycorrhizae in tallgrass prairie ecosystems. Experimental microcosms containing an assemblage of tallgrass prairie grasses and forbs were maintained under mycorrhizal and nonmycorrhizal conditions, with and without native soil nematodes, and with and without foliar herbivory, to assess the effects of these factors on plant community structure. This research indicated that mycorrhizal symbiosis is a key mechanism for maintaining dominance of warm-season grasses, while foliar and root herbivory are processes that increase plant diversity. A second study assessed interplant mycelial transfer of 32P from the root system of a 'donor' plant to a 'receiver' plant of the same or different species. Mycorrhizal-mediated P transfer was greater in Sorghastrum nutans than in Artemisia ludoviciana . However, direct root uptake of nutrients was greater in A. ludoviciana than S. nutans. These plant species appear to have evolved to utilize different primary nutrient acquisition strategies, with the facultative mycotroph, (A. ludoviciana) relying principally on root absorption, and the obligate mycotroph (S. nutans ) absorbing and transferring nutrient via mycorrhizal hyphae. A third experiment assessed the long-term effects of N enrichment coupled with annual burning and mowing on mycorrhizal symbiosis, soil aggregation, and carbon and nitrogen storage in soils. Nitrogen enrichment increased the activity of mycorrhizal fungi and these increases were strongly correlated to increases in soil macroaggregate formation, as well as the amount of C and N sequestered into the soil. Suppression of the symbiosis (via fungicide applications), decreased hyphal networks, with a subsequent reduction of C transport into the soil, and a loss of macroaggregates. This study provides strong evidence that soil structure and stability are largely mitigated by mycorrhizal hyphae. These studies underscore the multiple roles of plant-fungal symbiosis and indicate that alterations in mycorrhizae can have large effects on plant community composition and soil structure in grasslands.