Toshihiko Karasawa

Effect of previous crops on arbuscular mycorrhizal formation and growth of succeeding maize

Abstract The effects of fallow and cultivation of sunflower, maize, soybean, potato, wheat, sugar beet, 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, 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 non-mycorrhizal 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, finally increased the grain yield.

Variable response of growth and arbuscular mycorrhizal colonization of maize plants to preceding crops in various types of soils

Abstract. The effects of the preceding crops, sunflower (mycorrhizal host) and mustard (nonhost), on arbuscular mycorrhizal (AM) colonization and growth of succeeding maize were examined in 17 soils in an attempt to clarify the influence of soil characteristics on the effects of preceding crops. Shoot weight and P uptake of maize planted after sunflower were much higher than those after mustard in 14 soils, although the preceding crop had little effect on soil-P availability. AM colonization of maize after sunflower was much higher than that after mustard. The effect of the preceding crop was eliminated by soil sterilization. These results suggested that the differences in maize growth were caused by differences in the AM colonization. Correlation analysis of the effect of the preceding crop and soil properties showed that the difference in the effects on maize growth could not be explained by soil chemical properties, but only by the AM colonization of the preceding sunflower crop. In one of the 17 soils, however, the effect was not evident despite the higher AM colonization of sunflower. This soil was sterilized, and the effect of inoculation by AM fungi (AMF) on maize was examined. However, it was found that the inoculation increased AM colonization but did not improve maize growth at any P level, suggesting that the effect of AMF was unusually inhibited in this soil by unknown soil physicochemical properties. In most soils, however, the preceding mycorrhizal host crop, sunflower, improved the growth and AM colonization of maize depending on the AM colonization of sunflower.

How does arbuscular mycorrhizal colonization vary with host plant genotype? An example based on maize (Zea mays) germplasms

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-of-release 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.

Temporal or spatial arrangements of cover crops to promote arbuscular mycorrhizal colonization and P uptake of upland crops grown after nonmycorrhizal crops

AimsA field experiment was conducted where maintenance of indigenous arbuscular mycorrhizal (AM) fungal populations was attempted using AM host cover crops arranged temporally or spatially during growth of nonmycorrhizal crops.MethodsTo arrange AM hosts temporally, sunflower or oat was grown as a cover crop after non-host cropping (cabbage) or fallowing. In order to arrange AM hosts spatially, red clover, white clover or vetch was intercropped during growth of non-host cabbage.ResultsThe AM colonization and growth of maize with previously introduced sunflower or oat were much greater than those without introduction of cover crops or those with introduction of non-host cover crops. The AM colonization and yield of winter wheat grown after cabbage with AM host intercropping were greater than those after cabbage only cropping, suggesting that arrangement of AM hosts between cabbage rows is effective for maintaining the AM fungal population in soil during non-host cropping.ConclusionsMycorrhizal hosts cropped after or during non-host cropping is an effective means to increase indigenous AM fungal populations. The results show that AM colonization, P uptake and productivity of crops after cultivation of nonmycorrhizal crops can be improved by arranging AM hosts temporally or spatially as cover crops.

Effects of previous crops on arbuscular mycorrhizal formation and growth of maize under various soil moisture conditions

Abstract The effects of previous crops on arbuscular mycorrhizal (AM) formation and growth of succeeding maize (Zea mays L.) were examined at different soil moisture levels. Maize was grown in pots filled with soils (Andosol) from the plots, where mycorrhizal (sunflower, maize, soybean, kidney bean, adzuki bean, potato, wheat) and non-mycorrhizal (mustard, radish, sugar beet, and buckwheat) crops were cultivated in the previous season. Soil water potential was adjusted to around -10 (wet: W), -50 (moist: M), < -63 kPa (dry: D) from 11 d after sowing. The soils after cultivation of mycorrhizal plants in the previous season contained more AM spores than those after cultivation of non-mycorrhizal plants. The influence of the previous crops on AM formation was pronounced on drier soils, in which AM colonization of maize following mycorrhizal crops occurred more frequently than that after non-mycorrhizal crops. Arbuscular mycorrhizal colonization of maize, however, improved with increasing soil moisture status even after non-mycorrhizal crops, despite the differences in the AM spore population. The influence of previous crops on maize growth was also distinct, but it declined markedly with the increase in the soil moisture status. Phosphorus uptake and maize growth were positively correlated with AM colonization. The increase in AM colonization with the increase in the soil moisture status despite the low AM spore population suggests that a higher soil moisture status improved the efficiency of AM colonization, thus promoting AM formation. Such effects may have, in turn, stimulated P uptake and enhanced plant growth, thereby reducing the influence of the cropping history.

Maintenance of soybean yield with reduced phosphorus application by previous cropping with mycorrhizal plants

Abstract Previous cultivation of arbuscular mycorrhizal (AM)-host plants promotes AM colonization, phosphorus (P) uptake and the growth of succeeding AM-host plants. Three field experiments were conducted to investigate whether reduced application rates of P fertilizer could maintain soybean (Glycine max (L.) Merr. cv. Tsurumusume) yields when fields were previously cropped with AM-host plants. In the experiments, soybean was cropped after growing both AM-host (wheat, sunflower, vetch, maize and azuki bean) and non-AM-host plants (radish, white mustard, sugar beet and buckwheat) or fallow with different rates of P fertilizer (0–200 kg P2O5 ha−1). The results showed that previous cropping with AM-host plants increased AM colonization of soybean roots, soybean growth, shoot P content and yield compared with either previous cropping with a non-AM host or fallow treatment. Soybean yields following AM-host plants did not decrease with reductions in the P application rate from 150 to 50 kg P2O5 ha−1, although soybean yields decreased in some cases after non-AM-host plants or fallow treatment. A general linear model analysis revealed that soybean yield following AM-host plants was less affected by a reduction in P application rate than plants following non-AM-host plants. As a result, P application rates can be reduced from 150 kg P2O5 ha−1 (the rate recommended by the Hokkaido Government) to 50 kg P2O5 ha−1 for soybean cultivation in soils after AM-host plants.

Introduction of various cover crop species to improve soil biological P parameters and P uptake of the following crops

Field experiments were conducted to clarify whether the introduction of several cover crop species increases P uptake of the following wheat and soybean. Four summer cover crops (sorghum, buckwheat, groundnut and crotalaria) and four winter cover crops (oat, rye, vetch and lupin) were tested. Growth and P uptake of succeeding wheat were significantly increased by P fertilizer application and tended to be increased by sorghum, groundnut or crotalaria incorporation, whereas buckwheat did not show positive effects. Growth and P uptake of succeeding soybean were significantly increased by oat or vetch incorporation and tended to be increased by P fertilizer or other cover crop incorporation. These positive effects of cover crops were attributed to the large amount of P incorporation, increase in the P-solubilizing fungal population and/or biomass P in soil. Sorghum, oat, rye and vetch were thought to be suitable cover crops to accelerate P uptake of the following crops since a large amount of P would be incorporated. Sorghum, groundnut and lupin were thought to be suitable cover crops because they increased the indigenous P-solubilizing fungal population in soil. Soil biomass P correlated with P uptake of wheat. Incorporation of suitable cover crops as a P source and activation of indigenous soil microorganisms by the carbon supply were thought to have accelerated P uptake of the following wheat and soybean. It is therefore thought that introduction of suitable cover crops could be an effective means to reduce P fertilizer application for the following crops.

A DGGE analysis shows that crop rotation systems influence the bacterial and fungal communities in soils

A better understanding of the relationships among different cropping systems, their effects on soil microbial ecology, and their effects on crop health and productivity is necessary for the development of more efficient, sustainable crop production systems. We used denaturing gradient gel electrophoresis (DGGE) to determine the impacts of crop rotations and crop types on bacterial and fungal communities in the soil. The communities of bacterial 16S rRNA genes and fungal 18S rRNA genes were analyzed in experimental field plots that were kept under 4 different crop rotation systems from 1999 to 2008 (continuous cabbage (Brassica oleracea var. capitata L.), cabbage–lettuce (Lactuca sativa L.) rotation, cabbage–radish (Raphanus sativus L. var. longipinnatus L.H. Bailey) rotation, and a 3-year crop rotation). A principal component analysis (PCA) and a canonical correspondence analysis (CCA) revealed that both the bacterial and fungal communities in bulk soils were influenced by the crop rotation systems. However, the primary factors influencing each community differed: bacterial communities were most affected by soil properties (especially carbon content), while fungal communities were influenced most strongly by rotation times. To elucidate factors that may cause differences in crop rhizosphere microbial communities, the microbial communities in the harvested cabbage rhizospheres were also analyzed. The results suggest that the fungal communities in bulk soil are related to the rhizosphere fungal communities. Our present study indicates that the microbial communities in bulk and rhizosphere soils could be managed by crop rotation systems.

Arbuscular mycorrhizal (AM) effects on maize growth and AM colonization of roots under various soil moisture conditions

Abstract The effects of arbuscular mycorrhizal (AM) inoculation on growth, P uptake, and AM formation of maize (Zea mays L.) were examined at different soil moisture levels. Arbuscular mycorrhizal inoculum was added to pots filled with sterilized Andosol at concentrations of 0, 10, 50 g kg-1 dry soil. From 11 to 75 d after sowing, the soil water potential was adjusted to around -10 (wet: W), -50 (moist: M), or < -63 kPa (dry: D). The effect of inoculation on maize growth and P uptake was distinct in dry soil, in which AM colonization of roots occurred significantly more frequently in inoculated than in non-inoculated soil. The effect, however, was less pronounced with the increase in the soil moisture status, despite wide differences in the AM spore population and AM colonization. The increased AM colonization with the increase in the soil moisture status indicated that a higher soil moisture status improved the efficiency of AM colonization of roots. The increase in the shoot weight of non-inoculated maize with the improvement in the soil moisture status was presumably due to a higher P availability under a higher soil moisture status. Such effects may in turn stimulate P uptake and enhance plant growth, thereby masking the influence of the AM population.

Relationships between ammonia oxidizers and N2O and CH4 fluxes in agricultural fields with different soil types

Abstract Nitrous oxide (N2O) is a greenhouse gas that contributes to the destruction of stratospheric ozone, and agricultural soil is an important source of N2O. Aerobic soils are sinks for atmospheric methane (CH4), a greenhouse gas. Ammonia monooxygenase (AMO) can oxidize CH4, but CH4 is mostly oxidized by methane monooxygenase (MMO), and CH4 oxidation by AMO is generally negligible in the soil. We monitored the N2O and CH4 fluxes after urea application in fields containing different soils using an automated sampling system to determine the effects of environmental and microbial factors on the N2O and CH4 fluxes. The soil types were Low-humic Andosol (Gleyic Haplic Andosol), yellow soil (Gleyic Haplic Alisol) and gray lowland soil (Entric Fluvisol). Cumulative N2O emissions from the yellow soil were higher than those from other soil types, although the difference was not significant. The CH4 uptake level by Andosol was one order of magnitude higher than that by other soils. There were significant relationships between the ammonia oxidation potential, AOB and AOA amoA copy numbers, and the CH4 uptake. In contrast, the gene copy numbers of methane-oxidizing bacteria (MOB) pmoA were below the detection limit. Our results suggested that the AMOs of AOB and AOA may have more important roles than those previously considered during CH4 oxidation in agricultural soils treated with N fertilizers.