1. Arbuscular mycorrhizal fungi (AMF) have been proposed as a mechanism to reduce nutrient inputs in agriculture, thereby reducing costs and increasing environmental sustainability. However, before this can be achieved, we need to gain a better understanding of the importance of the prolonged selective pressures acting on indigenous AMF communities.
2. Much research concentrates on short-term ecological soil × plant × AMF interactions. However, we have little understanding of how long-term manipulations of abiotic conditions can be strong selection agents for AMF communities. Here, we ask how the long-term management of soil fertility and fertilizer use can influence the AM symbiosis. More specifically, we investigated whether 70 years of consistently imposed nutrient limitations affected the structure and symbiotic functioning of indigenous AMF communities.
3. Using the long-term Static Nutrient Deficiency Experiment carried out since 1937 in Thyrow, Germany, with and without nitrogen (N) and phosphorus (P) additions, we addressed the following questions: (i) Do different soil fertilizer treatments affect the overall abundance and diversity of indigenous AMF in an agricultural field; and (ii) Does the depletion of a nutrient select for an enhanced AMF ability to supply the deficient nutrient?
4. We assessed AMF spore diversity in the field and established a common garden experiment where soil nutrient treatments were calibrated against those in the long-term field experiment. For each soil nutrient treatment, we compared the growth responses of barley plants to the indigenous AMF communities isolated from the different soil fertilization treatments in the field.
5. We found that the long-term use of specific soil fertilization treatments altered the effects of the AMF symbiosis on plant and fungal growth. Consistent with the optimal foraging theory, AMF from N- or P-deficient soils grew larger but reduced plant growth more in those conditions relative to AMF isolated from non-deficient soils. This could result from both community-level changes and/or adaptations within species.
6. Thus, we propose that the ongoing agronomic management of abiotic selective pressures such as soil fertility needs to be considered as a strong determinant of AMF symbiotic functioning.