Seasonal variations in the abundance of virus-like particles and bacteria in the floodwater of a Japanese paddy field

Figures & data

Table 1 Total C and N contents, cation exchange capacity and pH (H2O) in the soil

Site	Total C (g kg^−1)	Total N (g kg^−1)	Cation exchange capacity (cmolc kg^−1)	pH
NF	8.5	0.84	8.3	6.1
CF	9.8	1.02	9.0	5.2
CFCa	10.1	1.02	9.0	5.8
CM	23.6	2.07	13.4	5.2
CF, plot with chemical fertilizers; CFCa, plot with chemical fertilizers and lime; CM, plot amended with chemical fertilizers, lime and compost; NF, plot without fertilizer.

Table 2 Calendar of field management in the study paddy field in 2005

Flooding	7 June	First topdressing^§	29 July
Puddling	8 June	Second top dressing^§	8 August
Basal fertilization^†	8 June	Drainage	28 September
Transplanting	9 June	Harvesting	26 October
Herbicides^‡	20 June
Midseason drainage	15–25 July
^†60 kg N, 86 kg P2O5 and 56 kg K2O in the CF plot, 60 kg N, 86 kg P2O5, 56 kg K2O and 1.1 t CaO in the CFCa plot and 60 kg N, 86 kg P2O5 and 56 kg K2O, 1.1 t CaO and 22.5 t of rice straw compost in the CM plot. N, P, K and Ca were applied as (NH4)2SO4, calcium superphosphate, KCl and lime, respectively.
^‡1.5 kg bentiocarb, 51 g bensulfuron-methyl and 300 g ha^−1 mefenacet.
^§20 kg N ha^−1 in the plots with fertilization. CF, plot with chemical fertilizers; CFCa, plot with chemical fertilizers and lime; CM, plot amended with chemical fertilizers, lime and compost.

Figure 1  Seasonal variations in (a) temperature, (b) pH and (c) turbidity determined by absorbance at 660 nm wavelength in the floodwater samples in a plot without fertilizer (○), a plot with chemical fertilizers (□), a plot with chemical fertilizers and lime (⧫) and a plot amended with chemical fertilizers, lime and compost (▴) during the rice cultivation period. Shaded columns show the period of midseason drainage. Values are means ± standard deviation; n = 4.

Figure 2  Comparison of viral counts using epifluorescence microscopy (EFM) and transmission electron microscopy (TEM). The line indicates the linear regression (EFM = 1.27 × TEM −13.80, R2 = 0.824).

Figure 3  Seasonal variations in the abundance of (a) virus-like particles (VLPs) and (b) bacteria in the water samples in a plot without fertilizer (○), a plot with chemical fertilizers (□), a plot with chemical fertilizers and lime (⧫), a plot amended with chemical fertilizers, lime and compost (▴), and the irrigation water (★) during the rice cultivation period. Shaded columns show the period of midseason drainage. Values are means ± standard deviation; n = 4.

Figure 4  Relationships between (a) virus-like particles (VLPs) and (b) bacterial abundance and floodwater turbidity in a plot without fertilizer (○), a plot with chemical fertilizers (□), a plot with chemical fertilizers and lime (⧫) and a plot amended with chemical fertilizers, lime and compost (▴) during the rice cultivation period. Correlation coefficients were 0.688 (significant at the 1% level) between the abundance of VLPs and the floodwater turbidity and 0.691 (significant at the 1% level) between the abundance of bacteria and the floodwater turbidity.

Figure 5  Percentage of planktonic virus-like particles (VLPs) in the total viral populations. The figure shows the average values with a half difference between duplication. (□), 15 June; (▓), 7 July; (▒), 2 August; (░), 18 August; (▀), 13 September. CF, plot with chemical fertilizers; CFCa, plot with chemical fertilizers and lime; CM, plot amended with chemical fertilizers, lime and compost; NF, plot without fertilizer.

Figure 6  Seasonal variations in virus-to-bacterium ratios (VBRs) in the floodwater in a plot without fertilizer (○), a plot with chemical fertilizers (□), a plot with chemical fertilizers and lime (◊) and a plot amended with chemical fertilizers, lime and compost (▴) during the rice cultivation period. The correlation coefficient between the abundance of VLPs and bacteria was 0.608 (significant at the 1% level).

Figure 7  Relationships between virus-like particles (VLPs) and bacterial abundance and virus-to-bacterium ratios (VBRs) in the floodwater in a plot without fertilizer (○), a plot with chemical fertilizers (□), a plot with chemical fertilizers and lime (⧫) and a plot amended with chemical fertilizers, lime and compost (▴) during the rice cultivation period. The correlation coefficients were 0.041 (not significant) between VLPs and VBRs and −0.296 (significant at the 1% level) between bacteria and VBRs.