Fate of blue-green algae in the food web of flooded rice-field ecosystems

 As part of an integrated pest management project to study the role blue-green algae (BGA) may play in the food web of rice-field ecosystems, ¹⁴C-labelled filamentous and monocellular BGA were used as food for fish, zooplankton and benthic fauna in artificial rice fields in the form of three aquaria. ¹⁴C present in the organisms was then traced by liquid scintillation to follow the manner in which the labelled BGA were consumed by different organisms. In this study the grazing rate of fish (mud carp) was compared to that of benthic organisms and zooplankton. It was found that fish consumed the BGA at the fastest rates and in the largest amounts, followed by the benthic species and zooplankton. It was also found that filamentous BGA were consumed in higher amounts than monocellular BGA. The importance of grazing in nutrient recycling is emphasized. Received: 22 December 1997     

Effect of Free-Air CO2 Enrichment (FACE) on the Community Structure of Aquatic Organisms in Floodwater of a Japanese Paddy Field during the Flooding Period of Rice Cultivation

In the present study, the effect of free-air CO₂ enrichment (FACE) on the diversity and the community structure of aquatic organisms in the floodwater of a paddy field in northern Japan was evaluated. The varieties and the number of aquatic organisms ranging from 30 μm to 2 cm were compared four times at intervals of 20 d between the FACE (599 ppm CO₂) and ambient air (384 ppm CO₂) plots during the flooding period of rice cultivation. Aquatic organisms were classified mainly at the order level. Twenty-eight taxonomical groups were detected, and the number of groups in the FACE plots was significantly lower than that in the ambient plots. Zygnematales and Chaetonotida were less frequently detected in the FACE plots. The population densities of Zygnematales, and Chaetonotida also tended to be lower in the FACE plots than in the ambient plots in the late flooding period (after 50 d of flooding). No groups showed a significantly higher population density and frequency of presence in the FACE plots than in the ambient plots. The seasonal variations in the number of taxonomical groups and the community structure of aquatic organisms were less conspicuous in the FACE plots than in the ambient plots. We concluded that the FACE conditions adversely affected the communities of aquatic organisms in paddy fields.     

Effect of grazer regulation and algal inoculation on photodependent nitrogen fixation in a wetland rice field

Summary A dry season field experiment conducted for two consecutive years highlighted problems of achieving increased populations of N₂-fixing blue-green algae (BGA) in wetland rice fields. Inoculation of non-indigenous BGA strains, either dried or as fresh viable inocula even at high levels of application, was unsuccessful. A limiting effect of grazing invertebrate populations on BGA establishment was evident, but other factors were involved. Reducing grazer pressure did not permit establishment of inoculated BGA; interspecific competition and environmental factors may explain the inoculation failure. Grazer regulation permitted the establishment of a fast-growing indigenous N₂-fixing Anabaena and the doubling of N₂-fixing activity over a control. Neither inoculation nor grazer control affected grain yields significantly.     

Frequency of phage-infected bacterial cells in the floodwater of a Japanese paddy field

In aquatic environments, viruses play an important role in the microbial food web through microbial mortality from viral lysis. Bacteriophages (phages) compose the majority of viral communities in the floodwater of paddy fields. The present study evaluated bacterial mortality from phage lysis in the floodwater of a Japanese paddy field based on the frequency of phage-infected bacterial cells by transmission electron microscopy. Floodwater was sampled five times during the rice cultivation period from two plots of NPK plus lime (no-compost plot) and NPK plus lime and compost (compost plot) under a long-term and ongoing fertilizer trial that began in 1925. The frequency of visibly infected cells in the compost plot was larger, ranging from 2.4 to 3.6% (average 3.0%), than that in the no-compost plot, which ranged from 1.6 to 2.9% (average 2.0%). The fractions of bacterial mortality from phage lysis in the floodwater samples were estimated to range from 12.8 to 27.3% (average 17.2%) and from 21.7 to 35.0% (average 27.9%) for the samples collected from the no-compost plot and the compost plot, respectively. This is the first study to estimate bacterial mortality from phage lysis in the paddy field ecosystem, and the frequency of phage lysis in floodwater was found to be within the frequency ranges observed in other aquatic environments.     

Beneficial effects of blue-green algae and Azolla, excluding supplying nitrogen, on wetland rice fields: a review

 The role of blue-green algae (BGA) and Azolla in supplying N to rice fields is well documented. In addition, they also bring about, directly or indirectly, a number of changes in the physical, chemical and biological properties of the soil and soil-water interface in rice fields. For example, BGA liberate extracellular organic compounds and photosynthetic O₂ during their growth, while Azolla prevent a rise in the pH, reduce water temperature, curb NH₃ volatilisation and suppress weeds; and both of them contribute biomass. On decomposing, they influence the redox activity and result in the formation of different organic acids in soil. All such changes brought about by BGA and Azolla in soil may ultimately influence plant-available nutrients and also soil characteristics. An attempt has been made in this review to highlight these effects of BGA and Azolla in rice fields and discuss their possible implications relating to management and productivity of rice-field systems. Received: 4 February 1998     

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

Abstract

Viruses are the most abundant biological entities in marine and freshwater environments. Many studies have shown the ecological importance of viruses in the primary production and microbial food web in aquatic environments. However, no studies have examined viral abundance in the floodwater of paddy fields. The present study surveyed the abundance of virus-like particles (VLPs) and bacteria in the floodwater of a Japanese paddy field under a long-term fertilizer trial since 1925 during the rice cultivation period. Virus-like particles and bacterial abundances in the floodwater ranged from 5.6 × 10⁶ to 1.2 × 10⁹ VLPs mL^?1 and from 9.2 × 10⁵ to 4.3 × 10⁸ cells mL^?1 with mean abundances of 1.5 × 10⁸ VLPs mL^?1 and 5.1 × 10⁷ cells mL^?1, respectively, and increased with an increase in the turbidity of the floodwater with suspended particles. The magnitude of seasonal variation was more than 50-fold for VLP abundance and 100-fold for bacterial abundance. The virus-to-bacterium ratios fluctuated over the rice cultivation period, ranging from 0.11 to 72 and their increase correlated with the decrease in bacterial abundance. Our results suggest that viral abundance in the floodwater of paddy fields is larger than in natural marine and freshwater environments.     

Chemical composition of cultures and natural samples of N2-fixing blue-green algae from rice fields

Summary Laboratory cultures, soil cultures, and natural samples of N₂-fixing blue-green algae (BGA) from rice fields were analyzed for dry matter, ash, N, C, P, and a few other constituents.Results show a very large variability of the composition. Dry matter contents ranged from 0.28% to 13.6% (average 3.3%). Ash contents ranged from 15.6% to 71.3%. Nitrogen contents ranged from 1.9% to 11.8% on an ash-free basis (average 6%). Carbon content was less variable, ranging from 37% to 72% and averaging 43.7%.A decrease in N and pigment contents, and an increase in reducing sugars, was observed in aging laboratory cultures.Large differences in composition were observed between field samples and material grown in artificial medium. Soil-grown BGA and field samples were characterized by very high ash contents, N contents lower than those in laboratory cultures, and P deficiency.Extrapolation from (1) average dry matter, ash, and N contents and (2) records of BGA biomass in rice fields indicates that an algal bloom has a potentiality of about 15–25 kg N per hectare and that a BGA biomass of agronomic significance is visible to the naked eye.     

Comparison of bacterial community structures at main habitats in paddy field ecosystem based on DGGE analysis

Bacterial communities at different habitats in a Japanese paddy field ecosystem were compared to understand the bacterial world in the ecosystem as a whole by analyzing data of the denaturing gradient gel electrophoresis (DGGE) band patterns and the sequenced DGGE bands. The habitats were floodwater, percolating water, microcrustacean inhabiting in floodwater, plow layer soil, rice roots, rice straw and rice straw compost incorporated in soil, rice straw placed on the soil surface, plant residues in paddy fields, and rice straw under composting process. Phylotype (band) richness, diversity, evenness, and stability of the bacterial communities at the respective habitats were evaluated based on the DGGE profile data. Phylotype richness was greater near plant residues, rice straw buried in soil and rice straw placed on soil surface, while it was smaller at microcrustacean and rice straw compost buried in soil. The samples from plow layer soil and rice straw compost buried in soil showed considerably higher index values for diversity, evenness, and stability, while those from rice straw placed on soil surface and microcrustacean had lower values of the indices than other habitats. Sequences of totally 250 DGGE bands were assigned to phyla or classes. Distribution of bacterial members to phylogenetic taxa was different among the respective habitats. Inhabitants in plow layer soil were most widely distributed among the groups (nine phyla: Proteobacteria, Chloroflexi, Chlorobi, Verrucomicrobia, Acidobacteria, Nitrospira, candidate division OP10, Cyanobacteria, and Actinobacteria), while those in floodwater and microcrustacean were restricted to only three phyla (Proteobacteria, Bacteroidetes, and Actinobacteria). Proteobacteria and Bacteroidetes were found at all the habitats and the habitats except for plow layer soil, respectively, whereas abundant members belonged to Chloroflexi and Actinobacteria in plow layer soil. “Comprehensive mapping” of DGGE fragments was conducted by principal component analysis based on evolutionary distances of the fragments to 202 reference bacterial strains to overview phylogenetic relationships of bacterial members among the respective habitats. The score plots with the first and second principal components distinctly characterized bacterial members at the respective habitats, and the similarity between the respective communities was clearly demonstrated. Overall, bacterial communities at the respective habitats were distinct and different in the diversity and stability to each other, which may have contributed to the diversity of overall bacterial communities in the paddy field ecosystem.     

淹灌和间歇灌溉对晚稻田杂草群落多样性的影响

为探明节水灌溉对稻田杂草群落组成及物种多样性的影响,运用群落生态学方法,开展了淹灌与间歇灌溉2种灌溉模式下水稻不同生育阶段稻田杂草群落动态变化研究。结果表明:间歇灌溉和淹灌稻田杂草种类分别有10科13属14种、10科11属11种;间歇灌溉稻田各生育期杂草密度平均降低27.8%;优势杂草种类虽与淹灌基本一致,但阔叶类杂草矮慈姑重要值降低9.76%,单子叶杂草浮萍重要值降低65.38%;间歇灌溉稻田以旱生杂草生长为主,总体各生育期旱生杂草群落密度增加277.51%,水生杂草群落密度降低81.44%,而淹灌稻田以水生杂草为主;间歇灌溉稻田Margalef物种丰富度指数、Shannon-Wienner多样性指数、Simpson优势度指数和Pielou均匀度指数,平均提高27.01%、14.31%、9.09%和3.48%,乳熟期差异尤为显著。间歇灌溉提高了稻田杂草多样性,有效地抑制了优势种杂草生长,有利于改善稻田生态保护。该研究可为节水灌溉稻田杂草控制及多样性保护提供科学依据。     

Unique viral capsid assembly protein gene (g20) of cyanophages in the floodwater of a Japanese paddy field

In order to evaluate the genetic diversity of cyanophage communities of rice fields, viral capsid assembly protein gene (g20) was amplified with primers CPS1 and CPS8. The DNA was extracted three times from viral concentrates obtained from floodwater samples collected in each of four different plots (no fertilizer; P and K chemical fertilizers; N, P, and K chemical fertilizers; and chemical fertilizers with compost). Denaturing gradient gel electrophoresis (DGGE) gave different g20 clones. The sequencing of DGGE bands revealed that the g20 genes of the floodwater were divergent and that the majority of clones formed several unique groups. However, they were more closely related to g20 sequences from freshwaters than to those from marine waters, suggesting that g20 genes in terrestrial aquatic environments are different from those in marine environments.     

淹水稻田氮的损失

A field microplot experiment was conducted during the tillering stage of paddy rice to investigate nitrogen(N) Iosses from flooded rice fields following fertilizer application. After application of ammonium bicarbonate, most of nitrogen in the flood water was present as NH₄-N and its concentration varied widely with time. Concentrations of both NO₃-N and NO₂-N in the floodwater were low due to the weakened nitrification. Under flooded anaerobic reducing conditions, soil solution concentrations of NO₃-N and NH₄-N were nothigh, ranging from 0.6 mg L^-1 to 4.8 mg L^-1, and decreased with soil depth. However, the ground water wasstill contaminated with NO₃-N and NH₄-N. Rainfall simulation tests showed that the N losses via runoff inrice fields were closely related to the time intervals between fertilizer applications and rainfall events. Whena large rain fell for a short period after fertilizer application, the N losses via runoff could be large, which could have a considerable effect on surface water quality. Both irrigation and N fertilizer application must be controlled and managed with great care to minimize N losses via runoff from agricultural land.     

Comparison of community structures of microbiota at main habitats in rice field ecosystems based on phospholipid fatty acid analysis

The present study compares the community structures of microbiota at different habitats in Japanese rice fields by comparing their phospholipid fatty acid (PLFA) compositions to understand the contribution of different habitats to microbiological diversity. The data were collected from four neighboring rice fields. Comparison was made for the PLFA compositions extracted from the floodwater, percolating water, rice soils under flooded and drained conditions, rice straw (RS) placed in flooded and drained rice soils, RS in the composting process, and RS compost placed in a flooded rice field. Average amounts of PLFAs were 33 μg L⁻¹ in the floodwater, 17.1 μg L⁻¹ in the percolating water from plow layers, 34.6 μg L⁻¹ in the percolating water from subsoil layers, 108 μg g⁻¹ dry weight basis (dw) in flooded rice soils, 382 μg g⁻¹ dw in RS materials, 2,510 μg g⁻¹ dw in RS composts, 2,850 μg g⁻¹ dw in RS composts after application to a flooded rice soil, 222 μg g⁻¹ wet weight basis (ww) in RS in drained rice soils, and 284 μg g⁻¹ ww in RS in flooded rice soils. The total amount of PLFAs to the soil depth of 10 cm was estimated to be about 12 g m⁻². The PLFA compositions were different from each other depending on the habitats. Rice soils were characterized by the predominance of actinomycetes and Gram-positive bacteria in comparison with the other habitats. In contrast, the microbial communities in the floodwater and percolating water were characterized by the predominance of Gram-negative bacteria and eukaryotes (presumably algae), and Gram-negative bacteria, respectively. The microbial community of RS materials was dominated by fungi. Gram-positive bacteria became predominant in RS after application to flooded rice soils, while RS placed in a drained rice field after harvesting rice was characterized by the predominance of Gram-negative bacteria and fungi. The community structures at respective habitats were stable and specific, irrespective of the season of sampling and the duration of decomposition of RS.     

Assessing conservation trade-offs: identifying the effects of flooding rice fields for waterbirds on non-target bird species

I examined how winter flooding of post-harvest rice fields—a management practice used to benefit waterbirds—affects field use by other birds. In addition to waterbirds previously studied, I recorded 56 bird species in rice fields. Of these, five were more abundant in flooded fields, ten were more abundant in unflooded fields, no difference was detected for 19, and the remainder were too rare to draw any conclusions. Species that were more common in unflooded fields were all carnivorous or granivorous in winter, whereas species that were more common in flooded fields were mostly insectivores commonly associated with aquatic habitats. The net effects of the responses by individual species were fewer raptor species in flooded fields, but no difference in the species richness of other landbirds. Winter flooding potentially has negative effects for some birds, but has no discernible effects on most species studied and may benefit some passerines.     

Effects of nitrogen fertiliser and pesticide management on floodwater ecology in a wetland ricefield

We investigated the effects of N fertiliser and pesticide applications on the population dynamics of benthic molluscs in a tropical wetland rice field. Populations were monitored for two consecutive dry seasons in selected treatments during a study on the effects of agricultural practices on the floodwater ecology of tropical rice fields. The most abundant species recorded in the ricefields were the snailsMelanoides tuberculata andMelanoides granifera. Population densities and biomass values in planted plots ranged between 0 and 1530 individuals m^-2 and 0 and 1060 kg ha^-1, respectively. Snails were more abundant in unplanted than planted plots (1991: 170–2040 versus 0–1040 individuals m^-2, respectively). Populations in planted plots declined as the crop season progressed. Snail populations were significantly reduced by the broadcast application of mineral N fertiliser at 110 kg N ha^-1. There was little evidence that snails were affected by carbofuran or butachlor applications.     

Phylogenetic study on a bacterial community in the floodwater of a Japanese paddy field estimated by sequencing 16S rDNA fragments after denaturing gradient gel electrophoresis

Phylogenetic positions of characteristic bands of 16S rDNA that were obtained from the floodwater of a Japanese paddy field by denaturing gradient gel electrophoresis (DGGE) analysis in a previous work (Biol Fertil Soils 36:306–312, 2002) were determined to identify dominant bacterial members in the floodwater. Sequences of DGGE bands were affiliated with the Cytophaga–Flavobacterium–Bacteroides group, β-Proteobacteria, and Actinobacteria and showed phylogenetically close relationships with species inhabiting other aquatic environments, although most of their closest relatives were uncultured bacterial clones.     

Effect of Application of Rice Straw and Compost on the Bacterial Communities Associated with Moina sp. in the Floodwater of a Paddy Soil Microcosm: Estimation Based on DGGE Pattern and Sequence Analyses

Bacterial communities associated with Moina sp. in the floodwater of a paddy field microcosm were examined by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rDNA. Eighteen out of 20 eubacterial DGGE bands were sequenced. The associated eubacterial communities mainly consisted of the Cytophaga-Flavobacterium-Bacteroides group and α-, β-, and γ-Proteobacterial groups, irrespective of the application of rice straw and rice straw compost. The effect of the application of rice straw and compost on the communities was not appreciable, compared with host specificity. An uncultured Cytophagales bacterium was estimated to be specifically associated with Moina sp. Presence of bacteria that are specific to rice straw treatment was also estimated.     

Excessive application of nitrogen and phosphorus fertilizers induces soil acidification and phosphorus enrichment during vegetable production in Yangtze River Delta,China

Intensive vegetable cultivation has developed very rapidly in China, and investigation of current soil nutrient problems in vegetable fields and their potential environmental risk is important for local soil nutrient management strategies. Three hundred and sixty‐six soil samples were collected from greenhouse vegetable fields, open vegetable fields and rice/wheat rotation fields in southern Jiangsu Province, the most intensive vegetable‐producing areas in Yangtze River Delta, China, for the analysis of their soil fertility status. Soil acidification and P enrichment were the main problems identified in this area of vegetable production, with about 20 and 17% of the open and greenhouse vegetable field soils, respectively, being extremely acid with soil pH values below 5.0. In contrast, no soils under rice/wheat rotation fields were as acidic. Percentages of sites with Olsen‐P concentrations < 90 mg/kg were 61, 85 and 0% for soils growing greenhouse vegetable, field vegetable and rice/wheat, respectively. The nitrogen (N) surplus for vegetable fields exceeded 170 kg/ha/crop, and the phosphorus (P) surplus exceeded 40 kg/ha/crop. Thus, current vegetable production leads to potential environmental risks of N and P pollution of nearby aquatic bodies. Insufficient supplementation with potassium fertilizers was found in some vegetable fields. Several ameliorative measures are proposed.     

脲酶抑制剂与硝化抑制剂对稻田氨挥发的影响

采用密闭室间歇通气法和15N标记技术研究了尿素施入稻田后氨挥发损失特征以及脲酶抑制剂(N-丁基硫代磷酰三胺,NBPT)和硝化抑制剂(3, 4-二甲基吡唑磷酸盐,DMPP)对稻田氨挥发损失的影响。结果表明,稻田施用尿素后第4天氨挥发速率达到峰值,氨挥发损失主要发生在施肥后21天内。与单施尿素处理相比,添加NBPT处理的氨挥发速率峰值降低27.04%,累积氨挥发损失量降低21.65%;NBPT与DMPP配施时,氨挥发速率峰值降低12.95%,累积氨挥发损失量降低13.58%;而添加DMPP时,氨挥发速率峰值增加23.61%,累积氨挥发损失量与单施尿素的差异不显著。相关性分析表明,地表水中铵态氮浓度和pH值与氨挥发速率均达极显著正相关,说明二者是影响氨挥发速率的主要因素,而气温、 地温和水温与氨挥发速率的相关性不显著。与单施尿素相比,添加脲酶抑制剂可显著增加稻谷产量。脲酶抑制剂与硝化抑制剂配合施用可更有效地提高氮肥的回收率。综合降低氨挥发、 提高水稻产量及地上部氮肥回收率的效果,添加脲酶抑制剂以及脲酶抑制剂与硝化抑制剂配施的两个处理效果较为理想,硝化抑制剂不宜单独添加。     

Eukaryotic communities associated with the decomposition of rice straw compost in a Japanese rice paddy field estimated by DGGE analysis

To estimate the succession and phylogenetic composition of the eukaryotic communities responsible for the decomposition of rice straw compost under flooded conditions during the cultivation period of paddy rice, denaturing gradient gel electrophoresis (DGGE) analysis targeting 18S rDNA followed by sequencing was conducted in a Japanese paddy field. The eukaryotic communities in rice straw compost incorporated into the flooded paddy field were influenced by the mid-season drainage and mainly composed of fungi (Ascomycota, Zygomycota, and Chytridiomycota) and protozoa (Ciliophora, Euglyphida, and Dactylopodida), most of which existed continuously during the cultivation period of paddy rice. The results indicated that these eukaryotic members were associated with the decomposition of rice straw compost in paddy field soil directly or indirectly.     

Effects of Inoculation of Phototrophic Purple Bacteria on Grain Yield of Rice and Nitrogenase Activity of Paddy Soil in a Pot Experiment

Phototrophic purple bacteria (PPB) are one group of dinitrogen (N₂)-fixing bacteria often found in the anoxic and photic zone of paddy soil. In the present study, the effects of inoculation of PPB on grain yield of rice and changes in their populations and nitrogenase activity were investigated with and without surface application of rice straw in a pot experiment. As an inoculant, Rhodopseudomonas palustris strain KN122 isolated from a paddy soil was selected, and the cell suspension was inoculated into the floodwater once or three times during the cultivation. As a result, the inoculation of PPB increased the grain yield of rice. Compared to the control, the grain yield was 9% higher in the inoculated plot without rice straw application. The treatment was more effective in combination with rice straw application. In the plots where PPB were inoculated with rice straw, grain yields were 21% (single inoculation) and 29% (triple inoculation) higher. Populations of PPB in the plots without rice straw application significantly increased by the inoculation, unlike those in the plots with rice straw application, except for the bulk soil. Phototrophic and heterotrophic nitrogenase activities (acetylene-reducing activity) associated with the soils or residues of the rice straw were not affected by PPB inoculation throughout the experiment. This indicates that the inoculation of PPB into floodwater may not be effective for enhancing N₂ fixation in paddy soils and that the beneficial effect of PPB inoculation on the grain yield of rice may be due to unidentified functions of PPB other than biological N₂ fixation.