Soil cropping selects for nutrient efficient but more costly indigenous mycorrhizal fungal communities

Abstract

Conventional agriculture has been suggested to promote less mutualistic arbuscular mycorrhizal fungi (AMF). The main aim of this study was to test this assumption by a detailed functional analysis of the plant mycorrhizal benefits and costs. A cross-inoculation experiment was established with Plantago lanceolata as a host plant and inocula of AMF sourced from four pairs of conventionally managed arable fields and neighbouring grasslands. Mycorrhizal effects were determined for a range of plant parameters including fluxes of isotopically labelled phosphorus (P), nitrogen (N) and carbon (C), and related to root colonization and composition of the different AMF communities. The association of P. lanceolata with arable-field inocula was less beneficial in terms of plant growth promotion and it also led to more pronounced P accumulation in plant biomass, as compared to grassland inocula. Furthermore, arable-field AMF increased <sup>15</sup>N depletion in soil and <sup>15</sup>N transfer to shoots, and induced higher <sup>13</sup>C drain to soil. These differential functional parameters were related to consistent compositional differences between arable-soil and grassland AMF communities in the roots. Differential effects of the AMF inocula on N and C partitioning in the soil–plant system suggest faster foraging for nutrients by arable-soil AMF and higher demand for C, which are characteristics associated with ruderal AMF. This implies that arable-soil AMF may be less beneficial in conditions of plant growth limitation by C than the grassland AMF.