This study investigated the effects of biogas digestate application to soil with rice straw on nitrate leaching potential and nematicidal activity toward root-knot nematodes Meloidogyne incognita. The following seven treatments were set up: (i) control (CONT); (ii) chemical fertilizer (CF); (iii) wet biogas digestate derived from pig manure (WBD); (iv) and (v) dry biogas digestate derived from a mixture of pig manure and rice straw at an initial C/N ratio of 20 and 30 (DBD20 and DBD30); (vi) and (vii) DBD20 mixed with rice straw to adjust the C/N ratio to 16 (Mix1) and 30 (Mix2), respectively. The application rate of CF and digestates was adjusted to 200 mg N kg−1 soil based on the inorganic ammonium nitrogen contents. Nitrate contents readily increased in all the treatments with incubation, except for Mix2, and those at day 90 were decreased with increasing initial labile C contents. Garden balsam was grown as a test plant for root-knot nematodes using the soils at day 90 and the results showed that the gall index was significantly lower in Mix2 and Mix1 than in CF. These results suggest that dry digestate mixed with rice straw might have potential for lower nitrate leaching and nematicidal properties.

We previously reported that incorporation of bean sprout residue into soil stimulated hatching of the soybean cyst nematode (SCN), Heterodera glycines. The objective of this study was to evaluate the effects of short-term growth of mung bean (Vigna radiata), the raw material of bean sprout, and its incorporation into the soil on the density of SCN in soil. When mung bean was grown in pots for 2 and 4 weeks in a SCN-infested andosol, incorporated into the soil and further incubated for 2 weeks, the SCN density decreased markedly. Mung bean was also grown in pots under different moisture conditions. The SCN density decreased in the soils with 48-68 g water, but not with 39 g water (100 g dry soil)−1 in which mung bean grew the poorest. The present study suggests that short-term growth of mung bean may be useful to decrease the density of SCN in soil.

A field experiment was carried out to evaluate the effect of two whole-crop rice (Oryza sativa L.) cultivars, TULT and Takanari, on methane (CH 4) emission in a paddy field fertilized with biogas slurry (BS) at rates of 0 (NF), 100 (BS100) and 300 (BS300) kg nitrogen (N) ha −1 , in comparison with chemical fertilizer CF100 (100 kg N ha −1). Takanari produced significantly higher biomass (P < 0.001) than TULT and showed significantly (P < 0.01) lower CH 4 emission than TULT. BS applications caused higher CH 4 emission (52 ± 27 and 80 ± 19 g m −2 in BS100 and BS300, respectively) than did CF100 (42 ± 18 g m −2) and NF (28 ± 10 g m −2) in TULT. In contrast, there was no significant difference in CH 4 emission in Takanari among the treatments (26 ± 2, 26 ± 2, 32 ± 4, 29 ± 8 g m −2 in NF, CF100, BS100 and BS300, respectively). Methane oxidizing bacteria (MOB) showed significantly (P < 0.05) higher populations in Takanari than in TULT at harvest, which might be due to the higher root biomass (10.3 ± 2.2 g hill −1) in Takanari than in TULT (8.9 ± 1.8 g hill −1). MOB was significantly correlated with tiller number (R 2 = 0.176*) and plant biomass (R 2 = 0.242*). BS application showed higher copper (Cu) uptake in Takanari while it was not high in TULT. In contrast, it showed no difference in zinc (Zn) uptake in both varieties. Uptake of Cu was not different between the two varieties, while uptake of Zn in the grain was higher in TULT than in Takanari. The present study suggests that CH 4 emission deriving from BS application in paddy field can be mitigated by selecting an appropriate cultivar, like Takanari. However, care should be taken for heavy metal uptake in selecting cultivars.

Summary Our previous study using pots reported that short-term growth of mung bean (Vigna radiata) may be useful to decrease the density of the soybean cyst nematode (SCN), Heterodera glycines, in soil. The objective of this study was to determine whether short-term growth of mung bean and its incorporation by ploughing decreased SCN density in infested fields. Firstly, we did pot experiments to evaluate the optimum temperature and moisture for hatching in soil. SCN hatching was stimulated at 25 and 30°C and not at 20°C; however, it was stimulated at alternating temperature conditions between 20 and 25°C. Soil moisture levels with pF 2.76 or less were required to stimulate SCN hatch in soil. Field experiments were done in Saitama, Kanagawa and Nara Prefectures, Japan. SCN density was reduced by nearly half even in control plots, in which mung bean was not cultivated and ploughed, in Saitama and Nara Prefectures. However, SCN density was reduced by nearly 80% or more in the three Prefectures, except for one plot in Kanagawa, and the soil temperature and moisture conditions were kept at around 20-30°C and at <pF 2.8. Increase in yield of green soybean by SCN control was estimated at 350 kg (1000 m)−2. Overall, the present study revealed that short-term field cultivation of mung bean and ploughing was a profitable method to decrease SCN density in infested fields and thereby to increase yield of green soybean.

The effects of a microbial inoculant (Thervelics ® : a mixture of cells of Bacillus subtilis C-3102 and carrier materials) on rice (Oryza sativa cv. Milkyprincess) and barley (Hordeum vulgare cv. Sachiho Golden) were evaluated in four pot experiments. In the first and second experiments, the dry matter production of rice and barley increased significantly by 10-20% with the inoculation of the mixture at a rate of 10 7 cfu ⋅ g -1 soil compared with the non-inoculated control. In the third experiment, the growth promoting effects of the mixture, the autoclaved mixture and the carrier materials were compared. The dry mater production of rice grains was the highest in the mixture, and it was significantly higher in the three treatments than in the control, suggesting that the carrier materials may also have a plant growth promoting effect and the living cells might have an additional stimulatory effect. To confirm the efficacy of the living cells in the mixture, only B. subtilis C-3102 cells were used in the fourth experiment. In addition, to estimate the mechanisms in growth promotion by B. subtilis C-3102, three B. subtilis strains with similar or different properties in the production of indole-3-acetic acid (IAA), protease and siderophore and phosphatesolubilizing ability were used as reference strains. Only B. subtilis C-3102 significantly increased the dry matter production of rice grains and the soil protease activity was consistently higher in the soil inoculated with B. subtilis C-3102 throughout the growing period. These results indicate that the microbial inoculant including live B. subtilis C-3102 may have growth promoting effects on rice and barley.