JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

3-27
The Effect of γ-Sterilization of Carrier Materials on
the Shelf Life of Biofertilizer
K. Tejima a) , T. Yokoyama b) , K. Satoh a) and I. Narumi a)
a) Radiation-Applied Biology Division, QuBS, JAEA,
b) Institute of Agriculture, Tokyo University of Agriculture and Technology
Numerous studies on the inoculation technique of
beneficial microorganisms to plants at agricultural fields
have been carried out worldwide. However, except in a
case of a low competition with native microbes, the practical
technique that shows remarkable effect on the plant growth
has not been established. Biofertilizer is a substance that
holds beneficial microorganisms for plant growth (e.g.
Rhizobia) in the carrier such as peat, perlite, charcoal and
soil 1) . To use biofertilizer more efficiently, it is necessary
to keep the amount of viable inoculants in the biofertilizer
for a definite period of time. The inoculants survival in the
biofertilizer could be affected by the physical and chemical
properties of carrier materials and by the competition with
native microbes in the carrier. Gamma-irradiation is
expected to sterilize the carrier materials without changes in
physical and chemical properties. In an effort to demonstrate
the advantage of γ-sterilization, the survivals of inoculants
were monitored to assess the shelf life of biofertilizer.
Two kinds of peat (commercial sterilized peat moss from
Canada and Japanese peat soil: Named “P1” and “P2”,
respectively), Japanese forest subsoil of the Kanto Loam
Formation (Named “L”), and three kind of Japanese
cultivated soil (topsoil of the Andosol, Gray Paddy soils and
Brown Fluvic soils: Named “S1”, “S2” and “S3”,
respectively) were prepared. Except P1, the soil samples
were air-dried and passed through a 2 mm sieve. All soil
samples were stored at room temperature in the dark. In
order to optimize the γ-ray dose to sterilize soils for the
purpose of use as a biofertilizer’s carrier material, the
survival rate of native microbes in the prepared soils were
determined. P2, L, S1 and S2 were packed in plastic bags
and irradiated with 1 k, 5 k, 10 k, 20 k, 30 k and 50 kGy of
γ-rays. As a result, 50 kGy was determined to be suitable
to sterilize soil (Fig. 1). This result was confirmed by
additional experiments where P1 and S3 were also sterilized
completely by 50 kGy γ-irradiation.
As carrier materials, P1, P2, S1 and S3 were chosen. To
make the soil-based artificial carriers, the S1 and S3 were
mixed with charcoal powder (3:1) (Named “CS1” and
“CS3”, respectively). Following the addition of 20 mL of
sterilized water per 100 g of the carriers (P1, P2, CS1 and
CS3), they were sterilized by 50 kGy γ-irradiation or
autoclaving at 121 °C for 40 min. After that, water
suspension (20 mL) of Bradyrhizobium japonicum strain
USDA110 was inoculated to sterilized or non-sterilized
carriers. The final density of inoculants in the biofertilizer
was 6.6 × 10 7 cells g –1 . The biofertilizer was packed in
plastic bags and stored for one month in the 4 °C or 30 °C.
JAEA-Review 2010-065
- 83 -
Before and after storage, viable inoculants in the
biofertilizer were enumerated by the dilution plate method.
Relative ratios of viable inoculants in the biofertilizer
stored for one month are shown in Table 1. The relative
ratio at pre-storage is defined as 1. As a result of storage at
30 °C, the relative ratio increased in the biofertilizer made
from γ-sterilized or autoclave-sterilized carrier, though it
tended to decrease in the biofertilizer made from
non-sterilized carrier. The γ-sterilization was superior to
the autoclave-sterilization in enhancing the inoculants
survival. For P1, the relative ratio was high irrespective of
sterilization treatment in this study because P1 had already
been sterilized during manufacturing process. On the other
hand, only a small change in the relative ratio was detected
in the biofertilizer stored at 4 °C.
Reference
1) K. Senoo and I. Narumi, FNCA Biofertilizer Manual
(2006) 41.
Fig. 1 Survival rates of native microbes in the different
kinds of soils after γ-irradiation. Survival microbes
were not detected at 50 kGy irradiation.
Table 1 Relative ratios of the viable inoculants in the
biofertilizer.