Effects of arbuscular mycorrhizal fungi on grassland productivity are altered by future climate and below-ground resource availability

Due to their key position at the soil–root interface, arbuscular mycorrhizal fungi (AMF) may play an important role in determining the impact of global change on plant communities. We studied the effects of mycorrhizal colonization and “future climate” on productivity of model grassland ecosystems. In 2007 and 2008, artificially assembled grassland communities, grown on pasteurized soil inoculated with or free from AMF, were exposed to ambient conditions or to combined elevated atmospheric CO2 concentration and temperature. In the full soil accessibility experiment (“full soil”, 2007) plant roots could explore the total soil volume. In the partial root-exclusion experiment (“partial soil”, 2008), a nylon membrane of 25 μm mesh size allowed AMF to explore an additional soil volume that was not accessible to the roots. In the latter experiment, planting density was higher which increased the competition for nutrient uptake by roots compared to the “full soil” experiment.The main findings were the following:
• “Future climate” had a positive effect on AMF root colonization.
• Under “future climate”, the stimulated AMF suppressed root biomass in both experiments.
• AMF increased plant nutrient stocks and above-ground biomass, regardless of the climate, but only in the “partial soil” experiment, when plant competition for nutrients likely was higher and AMF could increase the available nutrient pool by exploring an extra root-free soil compartment.

► “Future climate” increased AMF root colonization.
► Under “future climate”, increased AMF suppressed root biomass.
► AMF increased plant nutrient stocks at likely higher nutrient competition.
► At likely lower nutrient competition AMF had no positive effect on grassland productivity.
► “Future climate” did not strengthen the positive AMF effect on above-ground biomass.