This study was done to determine microbial interactions between nitrogen-fixing bacteria (NFB) and different indigenous bacteria, both of which were isolated from the same samples of sago palm. Co-culture of NFB and indigenous bacteria, isolated using nutrient agar medium, into a nitrogen-free Rennie medium showed significantly higher nitrogen-fixing activity (NFA) than single inoculations of NFB did in almost all combinations. A reduced oxygen status also enhanced the NFA of NFB. All NFB preferred simple sugars as their substrates for NFA and showed extremely low levels of NFA in starch, hemicellulose, and pectin-containing media. NFA was markedly stimulated in the consortium of starch-degrading Bacillus sp. strain B1 and NFB, ranging from 0-1.5 to 150-270 nmol C2H4 culture −1 h −1 . The consortium of hemicellulose-degrading Agrobacterium sp. strain HMC1 or Flexibacter sp. strain HMC2 and NFB also showed enhanced NFA, ranging from 0-0.1 to 16-38 nmol. In contrast, no enhanced NFA was detected in the consortium of pectin-degrading Burkholderia sp. strain BT1 or Paenibacillus sp. strain P1 and NFB. These results may indicate that beneficial microbial interactions occur in sago palm to enhance nitrogen-fixing activity through collaborative utilizations of starch, hemicellulose and their degradation products.

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The effect of organic amendment on the resistance and resilience of the organic matter decomposing activity was compared between soils amended with compost and with chemical fertilizers. The impact of metam sodium disinfection on cellulose‐decomposing activity and on the number of nematodes in three types of soils was periodically measured. In an andosol, cellulose‐decomposing activity was significantly suppressed by soil disinfection only in the chemically fertilized soil (CF‐soil) and not in the soils to which cow manure compost and okara (the residue in tofu production)/coffee compost was added. In a brown lowland soil, cellulose‐decomposing activity was significantly suppressed by soil disinfection in the CF‐soil, but not in the soils to which higher amounts of cow manure compost and pig manure compost had been added. In a red‐yellow soil, cellulose‐decomposing activity was significantly suppressed by soil disinfection in all soils, but its resilience was higher in the soils to which cow manure compost or coffee compost was added compared with the CF‐soil. Total numbers of nematodes were markedly decreased by soil disinfection in all soils. These results may suggest that the resistance and resilience of cellulose‐decomposing activity against soil disinfection were enhanced by organic amendments, while disinfection had fatal effects on soil nematodes. In most of the organically amended soils, the mean weight diameters of aggregates were larger compared with the CF‐soils, suggesting that highly structured soil pore networks may provide shelters for the soil microbes responsible for cellulose decomposition against disinfection. This hypothesis was supported by the result that the resistance of cellulose‐decomposing activity against soil disinfection decreased when the soil structure was destroyed by grinding in a mortal and pestle.

The objective of this study was to compare the economic threshold (ET) of Pratylenchus penetrans in radish fields of andosols using the Baermann method and the combination method of soil compaction and real-time polymerase chain reaction (PCR). Soil samples were collected from 26 plots at depths of 0 to 30 and 30 to 60 cm before seeding of radishes. The number of P. penetrans in each sample was estimated by the Baermann method and the combination method. No P. penetrans was detected in 13 plots by the Baermann method, while the number of plots in which no P. penetrans was detected was only two by the combination method. The number of spots caused by P. penetrans on radishes harvested from the plots was also counted. It was difficult to determine the ET of P. penetrans estimated by the Baermann method, based on whether severe damage (more than 10 spots on average per radish) was seen. However, the ET of P. penetrans estimated by the combination method was determined at 5.3 J2 equivalents per 20 g dry soil. In plots with P. penetrans densities lower than the ET as evaluated by the combination method, the ratio of plots with no damage was 87%. The results suggest that the combination method has an advantage in the estimation of damage to radish by nematode.

Inoculation with arbuscular mycorrhizal fungi (AMF) can increase the growth of host plants, especially under condition of low phosphate (P) availability. Although this effect is shown relatively easily in simplified systems such as pot experiments, it is often hard to show in the field because of complicating factors such as competition with indigenous AMF. We conducted an AMF inoculation experiment with three Japanese soybean cultivars (Enrei, Misuzudaizu, and Akishirome) in an allophanic (Umbric Silandic) Andosol field under the long-term selective application of major nutrients (NPK and-P) and bare fallow. In the inoculation plots, introduced AMF were well colonized in soybean roots at flowering stage. In the-P plots, inoculation tended to increase the shoot dry weight of all the three soybean cultivars; this effect remained until harvest. Although a significant difference is not recognized, there was a tendency of residual effect on Enrei in the following year. In the NPK plots, inoculation did not significantly increase the shoot dry weight. We thought that in the-P plots, the long-term selective application of N and K and the long-term maintenance of bare fallow created the soil conditions of low P availability and poor native AMF. Thus, introduced AMF can benefit soybean growth and yield in the soil with low competitor AMF density and low phosphate availability.