Grazer-Mycorrhizal Interactions in Tallgrass Prairie: Effects on Carbon and Nitrogen Partitioning and Sequestration.

Belowground net primary productivity is the main source of organic carbon for a wide variety of soil ecosystems.  Mycorrhizae play a major role in carbon translocation into the soil. The mutualistic symbiosis between plants and mycorrhizal fungi is an ancient phenomenon on which may be one of the most important but least understood biological associations regulating community and ecosystem functioning.  Interactions among mycorrhizal fungi, grazers (both plant and fungal), and plants are expected because all three depend upon and influence important plant resources.  Changes in belowground C, N, and NPP in turn are predicted to have strong feedback effects on plant, fungal, and consumer species interactions.  The studies focused on the integrative role of mycorrhizal symbiosis and grazing on C allocation above- and belowground, soil C turnover and N availability. The experiments addressed the general hypotheses that 1) AM fungi, by altering allocation of photosynthate, enhance sequestration of C and organic N into the soil, 2) grazers on plant and microbial biomass increase turnover rates and ultimately losses of C and inorganic N in the soil, and 3) these changes produce feedback effects that further influence biotic interactions between plants and the soil biota).  Complementary greenhouse and field experiments assessed mycorrhizal and grazer effects on C and N cycling.  Experimental manipulations include factorial arrangements of arbuscular mycorrhizae, nematodes, and soil nutrient availability.  The approaches in all experiments used a C4 grass into C3-labeled soil and monitor the isotopic composition of plant, soil, and microbial fractions to assess C sequestration and turnover.  Nitrogen acquisition and turnover by plants and microbes was assessed following additions of ¹⁵N.