Arbuscular mycorrhizal fungal diversity and species dominance in a temperate soil with long-term conventional and low-input cropping systems

The aim of this work was to study the effect of long-term contrasting cropping systems on the indigenous arbuscular mycorrhizal fungal (AMF) spore populations in the soil of a field experiment located in western Finland. Conventional and low-input cropping systems were compared, each with two nutrient management regimes. The conventional cropping system with a non-leguminous 6-year crop rotation (barley–barley–rye–oat–potato–oat) was fertilized at either full (rotation A) or half (rotation B) the recommended rate. In the low-input cropping system, plant residues were returned to the plots either as such (rotation C) or composted (rotation D). In the rotation of this system, 1 year with barley was replaced by clover, and oat was cultivated mixed with pea. Thus, the 6-year rotation was barley–red clover–rye–oat + pea–potato–oat + pea. Each rotation was replicated three times, starting the 6-year rotation in three different years, these being designated point 1, point 2, and point 3, respectively. In the low-input system, biotite and rock phosphate were used to compensate for K and P in the harvested yield, while animal manure was applied at the start only. After 13 years, rotation points 1 and 3 were studied. Barley was the standing crop in all plots of rotation point 1, while oat and oat + pea were grown in rotations C and D, respectively. AMF spores were studied by direct extraction and by trapping, sampled on 15 June and 15 August. In addition, a special assay was designed for isolation of fast colonizing, dominating AMF. The cropping system did not significantly affect AMF spore densities, although the low-input cropping system with composted plant residues had the highest density with 44 spores on average and the conventional system with full fertilization 24 spores per 100 cm³ soil in the autumn samples. Species richness was low in the experimental area. Five Glomus spp., one Acaulospora, and one Scutellospora were identified at the species level. In addition to these, three unidentified Glomus spp. were found. Species richness was not affected by cropping system, rotation point, or their interactions. The Shannon–Wiener index of AMF spore distributions was significantly higher in the fully fertilized than in the half-fertilized conventional plots. Glomus claroideum was the most commonly identified single species in the experimental area. It occurred in all the cropping systems and their various rotation points, representing about 30% of the total number of identified spores. In August, G. claroideum accounted for as much as 45–55% of the total numbers of spores identified in the conventional system with halved fertilization. In contrast, Glomus mosseae occurred more commonly in June (26%) than in August (9%). A bioassay using roots as inoculum for isolation and culture of dominating AMF was successfully developed and yielded only G. claroideum. This indicates a high probability of being able to more generally identify, isolate, and culture fast colonizing generalist AMF for use as inoculants in agriculture and horticulture.