PNNL

Abstract

Interactions between microorganisms and plants can alter plant growth, nutrient cycling, and soil function. Arbuscular mycorrhizal (AM) fungi and free-living nitrogen (N) fixers (FLNF) are common in the rhizosphere of many plants where they help increase bioavailability of nutrients, especially nitrogen, phosphorus, and carbon (N, P, and C). Here, we conducted a systematic literature review to synthesize the current understanding of tripartite interactions between AM fungi, FLNF, and plants, and identify important knowledge gaps. Existing research suggests that this kind of tripartite mutualism can support increased plant biomass and nutrient uptake, in some cases on a scale similar to increases resulting from N and P fertilization. AM fungi increase the abundance and/or function of certain FLNF groups, and in some cases FLNF increase AM fungal colonization of roots. A review of studies to date on these tripartite relationships elucidated three prominent knowledge gaps: 1) identifying drivers of inconsistent plant, AM fungi, and FLNF responses to co-inoculation, 2) clarifying whether these tripartite relationships are mutualistic for AM fungi and FLNF as well as the plant, 3) identifying major controls on mutualistic nutrient exchange between all three groups of organisms. Nutrient exchange based relationships between rhizosphere microbiota and plants holds much potential for improving the outcomes of soil management, sustainable agricultural practices, and ecological restoration efforts. Closing these knowledge gaps will provide information essential to the development of effective inoculation application methods across these fields.