Arbuscular mycorrhizal fungi and nematodes are involved in negative feedback on a dual culture of alfalfa and Russian wildrye

The complex biological interactions taking place in soil–plant systems may sometimes alter the functioning of an ecosystem. We examined the relationship between arbuscular mycorrhizal (AM) root colonization, nematode populations, and plant competition in an 8-year-old field experiment comparing alfalfa monoculture to dual culture of alfalfa (Medicago sativa L.) and Russian wildrye (Psathyostachys juncea Fisch. Nevski) (RWR) grown under different soil P fertility levels, in a Brown Chernozemic soil in Saskatchewan. The experiment included three P rates: 0, 20 and 40 kg P₂O₅ ha⁻¹ (0P, 20P and 40P) applied annually and was sampled three times during the cropping season: 30 June, 1 September and 30 September. Higher AM symbiotic development compensated for reduced soil P fertility in alfalfa stands without RWR and forage dry matter yield was not affected by P rates. But in the presence of RWR, reduced soil P fertility at 0P and 20P led to forage yield reduction. Fertilization treatments modified the soil microbial community structure only in the presence of RWR, as revealed by discriminant analysis of the profiles of microbial phospholipids fatty acid in soil lipid extracts and functional nematode groups. Arbuscular mycorrhizal root colonization level was reduced with P fertilizer both in the presence and absence of RWR. In the presence of RWR, lower plant AM root colonization was concurrent with higher abundance of total, fungivorous and omnivorous nematodes. Our results are consistent with a model of negative feedback from the plant-associated soil microflora where the presence of RWR increased the population of fungivorous nematodes and grazing of AM hyphae. Negative impacts were larger in low P fertility soils promoting AM symbiotic development. The unexpected decrease in alfalfa–RWR dual culture yield under 0P and 20P fertilization levels was attributed to a carbon drain created by enhanced nematode feeding on AM fungi in the presence of RWR.