Colonization of plant roots by arbuscular mycorrhizal (AM) fungi is a primary factor determining mycorrhizal associations. This study aimed to investigate the variation in AM colonization among maize genotypes and in response to plant breeding programs. Three types of maize (Zea mays) germplasms composed of 141 inbred lines, 38 hybrids, and 76 landraces were grown in replicated field experiments in Sapporo, Japan, for two cropping years to evaluate the percentage of root length colonized by indigenous AM fungi. The percent colonization varied greatly and continuously among maize genotypes. Inbred lines that originated (released) in particular locations (e.g., Tokachi, Japan) and years (e.g., 1960s) showed significantly larger values than other lines. Inter-location differences were also observed for landraces. The direction of the year-ofrelease effect on colonization depended on the origin. No significant differences were observed between leaf-blight-disease-resistant near-isogenic inbred lines and their parents. Modern hybrids showed significantly greater values than inbred lines and older landraces. Evaluating numerous, diverse genotypes demonstrated that AM colonization of maize plants varies with germplasm type, origin (country and location), and year of release, and that modern plant breeding programs do not necessarily lead to the suppression of colonization.

The effects of fallow and cultivation of sunflower, maize, soybean, potato, wheat, sugar beet, and rape on arbuscular mycorrhizal (AM) colonization and growth of succeeding maize were examined under field conditions from 1990 to 1992. All the crops except for sugar beet and rape were mycorrhizal. Shoot weight and grain yield of maize were much higher in the plots following sunflower, maize, soybean, and potato than in those succeeding rape, fallow, and sugar beet. It was thus concluded that maize growth after cultivation of mycorrhizal crops was more enhanced than after the cultivation of nonmycorrhizal crops. Phosphorus uptake and AM colonization in succeeding maize were closely correlated with shoot weight and grain yield. The results showed that the cultivation of mycorrhizal crops in the previous season promoted AM formation on roots of succeeding maize, which in turn enhanced the P uptake and growth, and finally increased the grain yield.

Soil type is one of the key factors affecting soil microbial communities. With regard to ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), however, it has not been determined how soil type affects their community size and soil nitrification activity. Here we quantitatively analyzed the ammonia monooxygenase genes (amoA) of these ammonia oxidizers in fields with three different soil types (Low-humic Andosol [LHA], Gray Lowland Soil [GLS], and Yellow Soil [YS]) under common cropping conditions, and assessed the relationships between soil nitrification activity and the abundance of each amoA. Nitrification activity of LHA was highest, followed by that of GLS and YS; this order was consistent with that for the abundance of AOB amoA. Abundance of AOB amoA showed temporal variation, which was similar to that observed in nitrification activity, and a strong relationship (adjusted R 2 =0.742) was observed between the abundance of AOB amoA and nitrification activity. Abundance of AOA amoA also exhibited a significant relationship (adjusted R 2 =0.228) with nitrification activity, although this relationship was much weaker. Our results indicate that soil type affects the community size of AOA and AOB and the resulting nitrification activity, and that AOB are major contributors to nitrification in soils, while AOA are partially responsible.

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