The effect of trees on arbuscular mycorrhizal fungi and grassland root biomass: Case study of two temperate silvopastoral systems

Despite their importance for plant nutrient acquisition and inter-species interactions, the role of arbuscular mycorrhizal fungi (AMF) in tree-crop interactions in temperate silvopastoral systems has not been studied. The aim of this study was to investigate the effect of trees on AMF biomass and grassland root colonisation in relation with root biomass and root nutrient stocks of the grassland plant community (GPC) in temperate permanent grazed silvopastures. Samples were collected at two soil depths (0–20 cm and 20–60 cm) in two paired sites on commercial farms, each combining an apple ( Malus domestica )-based silvopasture adjacent to a grassland managed identically excepting the presence of trees. Soil chemical and physical properties were determined. AMF biomass was measured by extracting and quantifying Neutral Lipid Fatty Acids (NLFA). GPC roots were isolated from the soil samples and their colonisation by AMF, biomass, and N and P concentrations were measured. Our results showed that apple trees had a consistent negative effect on AMF biomass and AMF colonisation of the GPC at both sites. This was despite site-specific effects of trees on soil organic carbon (SOC) stocks and soil water content. Generally, we could not identify strong correlations between AMF and measured soil properties in the topsoil, while AMF biomass was correlated with SOC stocks, pH and Olsen P in the subsoil. We hypothesize that the promotion of competitive microbial communities by trees in the topsoil at the expense of AMF is a possible mechanism explaining these results. We also found a consistent negative effect of trees on GPC root biomass, likely resulting from competition for resources. The lack of correlations between AMF biomass/arbuscular colonisation and soil properties, GPC root biomass and the N:P ratio in the topsoil, may also suggest that AMF were not a mediator between trees and the GPC, but rather that both AMF and the GPC were affected by plants through different mechanisms. Our approach capitalising on collaboration with farmers to characterise tree-crop interactions in two commercial farms provides realistic observations of a negative effect of apple trees on AMF biomass and colonisation. Mechanisms can, however, only be speculative. Our results call for more observations at different sites and additional mechanistic studies to confirm these results and understand the role that AMF play in tree-crop interactions in silvopastoral systems.