Symbioses between plants and arbuscular mycorrhizal (AM) fungi are among the most abundant but least understood biological associations in grasslands. Plant roots and mycorrhizas are conduits between atmospheric carbon dioxide and soil organic carbon. Consequently, a better understanding ofAM ecology and discovery of mechanistic linkages between the structure of mycorrhizas and their symbiotic and ecosystem functioning will advance our understanding of soil carbon dynamics. We combined multiple data sets from field and greenhouse studies of mycorrhizas in North American grasslands and sub-Saharan African savannas to test three hypotheses. Because plants face a trade-off between allocation to fine roots or to mycorrhizal fungi, we hypothesize that allocation to mycorrhizas will be greater when soil resources are limited, but not when they are in luxury supply. Second, we hypothesize that relative availability of soil nitrogen and phosphorus influences AM structure and functioning. Mycorrhizas are predicted to be most beneficial to plants when phosphorus is limited and nitrogen is not limited because nitrogen availability controls photosynthetic capacity and fixed carbon is the main currency for mycorrhizal trading partnerships. Finally, we hypothesize that grassland management that encourages the production of AM fungal hyphae will increase storage of organic carbon in the soil.
Our analyses support all three hypotheses. We found a strong inverse relationship between root fibrousness (measured as specific root length) and the density of AM fungal hyphae in the soil. This was observed across multiple field studies including experiments involving single plant species and studies of entire plant communities. More hyphae were produced in phosphorus limited soil than in phosphorus rich soil. Also, phosphorus availability mediated whether nitrogen fertilization increased or decreased allocation to AM fungi. A series of greenhouse experiments show that plants benefit most from AM mutualisms when phosphorus limitation exceeds nitrogen limitation and AM parasitism is likely when both phosphorus and nitrogen are in luxury supply. In phosphorus limited soils, allocation to AM fungal hyphae is positively correlated with plant biomass. In contrast, in phosphorus rich soils, allocation to AM fungal hyphae is not correlated, or it is negatively correlated, with plant biomass. Several studies show that burning and fertilizer management influence densities of AM fungal hyphae. This is important to consider when designing management strategies to maximize belowground carbon sequestration because AM fungi comprise a large and potentially very important belowground carbon sink.