Low spatial mobility of associated microbes along the hyphae limits organic nitrogen utilization in the arbuscular mycorrhizal hyphosphere
Abstract
Bacground: Arbuscular mycorrhizal (AM) fungi enhance plant nutrient acquisition from soil; however, their ability to exploit organic nutrient forms in the absence of associated microbes capable of mineralization remains unclear.
Methods: To test if the AM fungi carry their beneficial bacterial partners into nutrient-rich zones, we conducted three controlled experiments manipulating the microbial inputs, diversity and composition in plant-AM fungus-soil systems, ranging from open pots to semi-sterile mesocosms. We manipulated soil microbial diversity by imposing a microbial diversity gradient (complex communities fractionated by size, resulting in fractions passing through 1 µm to 1000 µm sieves) and cultivated Andropogon gerardii in previously sterilized substrate together with a bacterial-free Rhizophagus irregularis. In each experiment, 15N-labeled chitin or mineral nitrogen (N) compartments were installed in the root-free zone of each mesocosm.
Results: With decreasing microbial inputs into the root-free zone, the N uptake from chitin to plants, facilitated by the AM fungal hyphae, decreased. Upon complete absence of microbes in the root-free zone, AM hyphal foraging preferences assessed by quantitative PCR indicated that exploration of the mineral N compartments was more effective than that of the chitin compartments. The AM fungal hyphae were ineffective in priming mineralization of organic N even if provided with complex soil microbiomes at a distance from the…
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