The effect of trees on arbuscular mycorrhizal fungi and nutrient acquisition of grassland plant communities in three temperate silvopastoral systems

Abstract Aims Despite their importance for plant nutrient acquisition, the role of arbuscular mycorrhizal fungi (AMF) in tree-crop interactions in silvopastoral systems is unclear. The aim of this study was to investigate the effect of trees on AMF biomass and grassland root colonisation in relation with nutrient acquisition of the grassland plant community (GPC) in temperate permanent grazed silvopastures. Methods Samples were collected at two soil depths (0–20 cm and 20–60 cm) in three 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. Results Apple trees had an overall negative effect on AMF biomass, colonisation, and GPC root biomass. Trees increased soil mineral N and N concentrations in GPC roots, which were likely N-limited at all sites as indicated by N:P ratios comprised between 5 and 9. Conclusions Rather than mediating interactions between trees and the GPC, AMF colonisation and biomass likely followed direct effects of trees on nutrient cycling and consequential changes in root biomass. We speculate that trees lifted nutrient limitations in the topsoil due to ‘hydraulic lift’ and ‘nutrient pumping’ mechanisms which would have decreased root biomass by shifting the functional equilibrium of the GPC towards above-ground production.