ABSTRACT
Paddy rice fields are unique environments for soil microorganisms, as the soils are maintained under waterlogged conditions during the main stages of crop growth. Waterlogging the soil results in several advantages for rice production: (1) it provides a continuous water supply to paddy rice; (2) it changes the adverse pH of alkaline and acidic soils toward a neutral or slightly acidic pH, which is favorable for the growth of rice; (3) it diminishes the incidence of soil sickness and the outbreaks of soil-borne diseases often observed under continuous monoculture in upland fields; (4) it depresses weed growth, especially of C4-type grasses; (5) it favors biological N2 fixation, resulting in paddy fields that are more fertile than upland cultivated fields; (6) irrigation water supplies nutrients, such as Ca, Mg, Fe, Si, and S; and (7) paddy rice fields act as water reservoirs and prevent soil erosion [1].Soil ecosystems in a waterlogged paddy field are roughly divided into five subsystems during the waterlogged period of rice growth: floodwater, surface-oxidized and reduced layers in the Apg horizon, plow-pan layer, and the subsoil underneath. The development of a reduced layer in the Apg horizon results from the slower (1/10,000 times slower) diffusion rate of 0 2 in the water phase than in the air phase, a condition that cannot meet the 0 2 demand of microorganisms growing in paddy fields. Floodwater, therefore, prepares a relevant and unique soil environment in paddy fields.Organic matter production occurs in the floodwater as the result of photosynthesis by floating plants and algae. The pH of floodwater shows remarkable 35
diurnal variations because of the active photosynthesis of algae (often reaches pH 9.5-10 at midday) during the day and algal and microbial respiration (lower than pH 7) during the night [2].The surface oxidized layer is a thin aerobic layer (a few mm in thickness) that is influenced by the overlying floodwater. This layer is generally defined by the brown ferric color. Although it is not certain whether all of the layer is aerobic (oxic), it is the layer in which aerobic microorganisms proliferate. The reduced layer in the Apg horizon is the most characteristic layer in waterlogged paddy fields wherein anaerobic microorganisms are active supplied with organic/ inorganic fertilizers, plant residues, and rhizodeposition from rice roots.The plow-pan layer is the transitional layer with a compact soil structure that separates the Apg horizon from the subsoil underneath. The subsoil is a weakly reduced layer as the result of the low content of available microbial substrates that are transported from the plow layer by percolating water.Thus, aerobic, aerobic, strongly reduced, reduced, and weakly reduced soil subsystems are arranged sequentially from the floodwater to the subsoil in paddy field ecosystems during the waterlogged period of rice growth. This is in contrast to ecosystems in upland fields where surface aerobic and inner anaerobic environments develop in a soil aggregate [3].Ishizawa and Toyoda [4] compared the numbers of microorganisms enumerated in soil samples from 21 paddy fields (fallow season under upland conditions) and 27 upland crop fields in Japan (Table 1). Aerobic bacteria and sulfate (S 0 42-) reducers were more numerous and actinomycetes, fungi, and nitrifiers were less numerous in paddy fields than in upland crop fields. The numbers of anaerobic bacteria and denitrifiers in paddy soils were not significantly different from those in upland crop soils, probably because of the seasons when the samples of paddy soils were collected (fallow season under upland conditions). In addition, the composition of the microbiota in paddy fields tended to shift gradually toward the composition in upland crop fields during the drained period after harvest. However, the characteristics unique to paddy fields (predominance of microorganisms adapted to anaerobic conditions) were maintained until the next season of rice cultivation [4].Araragi and Ishizawa [5] studied the actinomycete flora in 15 paddy fields in Japan twice, in autumn (after drainage) and in spring (before flooding). Seasonal variation in the actinomycete biota was generally small and consisted of 79% Streptomyces, 14% Micromonospora, 4.3% Streptosporangium, and 2.5% Nocardia in autumn and 75%, 14%, 7.9%, and 2.6% in spring, respectively. Streptomyces, Micromonospora, and Streptosporangium forming scant or nonaerial mycelia were noticeable in the paddy fields compared with the upland crop fields. In addition, the ratios of actinomycetes showing amylase, cellulase, and protease activities to the total actinomycete isolates were different between paddy field and upland crop soils. Generally, the ratios of actinomycetes antagonistic
Table 1 Comparison of Numbers of Microorganisms Between Paddy Soils and Upland Crop Soils in Japan (g 1 soil) [4] MicroorganismsLayersMean depth (cm)
Paddy soils Upland crop soils Plow layer 0-14 2nd14-23 3rd23-37 Plow layer 0-14 2nd14-29 3rd29-43
Aerobes (X106) 29.5 ± 14.8a 11.4 ± 10.4C 7.6 ± 7.0°“ 23.1 ± 12.9b 6.3 ± 5.2* 1.6 ± 1.4dActinomycetes (X 105) 23.2 ± 22.3b 7.9 ± 9.1cd 3.6 ± 4.5d 47.6 ± 31.3a 17.1 ± B .S1* 3.5 ± 2.8dFungi (X104) 7.75 ± 4.76b 1.31 ± 1.31c 0.68 ± 1.12c 2.47 ± 1.42a 4.66 ± 3.23* 1.08 ± 0.9 l cAnaerobes (X105) 21.8 ± 10.6a 9.7 ± 9.0b 1.8 ± 1.6C 16.6 ± 13.3a 5.9 ± 5.8^ 1.5 ± 2.5CS 0 42-reducers (X103) 43.6 ± 39.3a 14.3 ± 15.1b 3.9 ± 5.1b 2.9 ± 4.5b 2.2 ± 3.9b 0.0 l bDenitrifiers (X104) 18.9 ± 28.0a 6.9 ± 6.2b 6.1 ± 5.3b 14.6 ± 13.9* 5.7 ± 7.6* —Nitrifiers (X103) 10.9 ± 20.2b — — 70.4 ± 74. l a 95.4 ± 125.1s — Paddy soils sampled during fallow seasons under upland conditions. Values are means ± standard deviation, and those not followed by the same letter differ significantly at p < 0.05 from the analysis o f variance.
