The species composition of vesicular-arbuscular mycorrhizal (VAM) fungal communities changed during secondary succession of abandoned fields based on a field to forest chronosequence. Twenty-five VAM fungal species were identified. Seven species were clearly early successional and five species were clearly late successional. The total number of VAM fungal species did not increase with successional time, but diversity as measured by the Shannon-Wiener index tended to increase, primarily because the community became more even as a single species, Glomus aggregatum, became less dominant in the older sites. Diversity of the VAM fungal community was positively correlated with soil C and N. The density of VAM fungi, as measured by infectivity and total spore count, first increased with time since abandonment and then decreased in the late successional forest sites. Within 12 abandoned fields, VAM fungal density increased with increasing soil pH, HO soluble soil C, and root biomass, but was inversely related to extractable soil P and percent cover of non-host plant species. The lower abundance of VAM fungi in the forest sites compared with the field sites agrees with the findings of other workers and corresponds with a shift in the dominant vegetation from herbaceous VAM hosts to woody ectomycorrhizal hosts.

The effect of plant diversity (1, 2, 8, or 16 species) on arbuscular mycorrhizal fungi (AMF) was assessed at the Cedar Creek Long-Term Ecological Research site at East Bethel, Minnesota, from 1997 to 1999. At each of the five samplings, AMF in 16-species plots produced from 30 to 150% more spores and from 40 to 70% greater spore volumes than AMF in one-species plots. Regressions of spore numbers and volumes with percent plant cover, plant diversity, and soil NO3 as independent variables suggest that midsummer plot soil NO3 was the best single predictor of AMF spore production in these plots. Plant diversity influenced spore volume in four samplings and spore numbers in the first three samplings. Plant cover was predictive of spore volume throughout the experiment but of spore number only in the first year. Sporulation by larger-spored AMF species (Gigaspora spp. and Scutellospora spp.) increased significantly with increasing plant diversity, while sporulation of the smaller-spored species varied in response to host diversity. Spore numbers of several AMF species were consistently negatively correlated and none positively correlated with midseason soil NO3 concentrations, demonstrating the adaptation of these AMF species to nitrogen-limited conditions.Key words: mycorrhiza, community, grassland, sporulation, nitrogen, specificity.

Earlier studies showed that mycorrhizal fungi selectively proliferate in soils cropped in monoculture to corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. This study evaluated whether the dominant mycorrhizal fungi, based on spore numbers present in soil, affected growth and nutrient uptake of the following crop. Plots at two locations in Minnesota with a continuous corn or continuous soybean history were planted to both corn and soybean. The relationship between spore numbers of proliferating species of mycorrhizal fungi and crop yield and nutrient concentrations were assessed using simple correlation analysis. Spore populations of mycorrhizal fungi which proliferated in corn were generally negatively correlated with the yield and tissue mineral concentrations of corn, but were positively correlated with the yield and tissue mineral concentrations of soybean. Spore populations of soybean proliferators exhibited the reciprocal relationship, although less clearly. We suggest that, compared to other fungi, proliferating VAM fungal species may be less beneficial (or perhaps detrimental) to the crop in which they proliferate. We propose a mechanism to explain how vesicular‐arbuscular mycorrhizal (VAM) fungi could cause yield depressions associated with monoculture, and outline research needed to test this hypothesis.

SUMMARYCommunities of vesicular-arbuscular (VA) mycorrhizal fungi were studied in a long-term crop rotation experiment at two locations (Waseca and Lamberton, Minnesota, USA). Spores of mycorrhizal fungi were counted and identified in experimental plots with a cropping history of either corn (Zea mays L.) or soybean \Glycine max (L.) Merrill], Mycorrhizal fungal communities were affected by both location and cropping history. At Waseca, Glomus aggregatum Schenck & Smith, G. leptotichum Schenck & Smith and G. occulttmi Walker spores were more abundant in soil with a corn history than a soybean history, while spores of G. microcarpum Tul. & Tul. exhibited the reciprocal pattern. Approximately 90 % of the spores recovered at Lamberton were G. aggregatum and did not vary with crop history. However, the spores of three other species: G. albidum Walker & Rhodes, G. mosseae Gerdemann & Trappe, and G. occultum, were more abundant in plots with a corn history than a soybean history. Densities of G, aggretatiim spores were negatively correlated with soil pH at Waseca, but were unrelated to pH at Lamberton were the mean soil pH was lower. Our results indicate that mycorrhizal fungal species are individualistic in their responses to cropping history and edaphic factors.

Crop and edaphic factors influence arbuscular mycorrhizal (AM) fungal species composition and populations. This study was conducted to determine the effect of management history, crop, and input system on species composition of AM fungal spore populations. Corn (Zea mays L.)‐soybean [Glycine max (L.) Merr.] sequences receiving no inputs (NI), organic inputs (OI), minimum inputs (MI), and conventional inputs (CI) were established in two adjacent areas with differing management histories: one area, the Koch Farm, had received no fertilizer or herbicide inputs for the past 25 yr; the other area, the experiment station, received recommended herbicide and fertilizer inputs. Fifteen AM fungal species were found in a survey of mycorrhizal fungal spore populations. Glomus aggregatum populations were positively correlated with Setaria spp. populations (r = 0.56), pH (r = 0.47), and K (r = 0.25) and negatively correlated with soil P (r = −0.57). Populations of Gl. geosporum, Gl. leptotichum, Gl. macrocarpum, and Gl. occultum were also positively correlated with soil pH and negatively correlated with soil P. Gigaspora margarita spore populations were positively correlated with soil P level (r = 0.272). Although species richness was greater (13 species vs. 10), species diversity (Hw) was lower at the Koch Farm than the experiment station (0.57 vs. 0.64) because of large Gl. aggregatum spore populations. The correlation of AM fungal spore populations with Setaria spp. and P, K, and pH indicates that management practices influence AM fungal species composition through both biotic and abiotic factors.