Rice (Oryza sativa L.) is one of the major staple food crops in
Soil microorganisms contribute to soil quality and regulate many ecosystem processes such as nutrient transformations and litter decomposition, as well as influence soil structural and bioremediation . Both changes in soil microbial communities resulting from ecosystem management and global change can have significant impacts on ecosystem dynamics , and microorganisms respond sensitively to changes and environmental stress because they have intimate relations with their surroundings . Therefore, interest in quantifying impacts on the biotic and abiotic component has increased with concern for the sustainability of agricultural ecosystem.
However, there is little information regarding the changes of organic matter content, bacterial community structure and water use efficiency under plastic film mulching with non-flooding conditions. In a 2-year study, Liu and Wu  showed the influence of PM on certain soil properties. In a rice-wheat cropping system, Liu et al.  reported the changes of nutrient uptake and nutrient balance in soil under plastic film mulching with non- flooding conditions. Hence, the objective of the present study is to investigate the effects of continuous plastic film mulching on organic matter content, bacterial community structure and water use efficiency. It is important for developing the water-saving agriculture.
2. Materials and Methods
2.1. Site Description
The field experiment was initiated in May 2001 at Duntou Town of Lanxi (29˚19'N, 119˚43'E and
2.2. Field Experiment Design and Management
The experiment was a randomized complete block design with three treatments and three replications. The three treatments were plastic film mulching with no flooding (PM), no plastic film mulching and no flooding (UM), and the traditional flooding management (TF). The plot size was 15 m2. The TF plots were isolated from the PM and UM plots by a
Table 1. Soil physical and chemical characteristics.
treatment, the plots were flood-irrigated every 3 - 5 days to maintain a
Fertilizer rates were
2.3. Soil Sampling and Analyses
Soil samples were collected from 0 to
2.4. Molecular Biology Analyses
The bacterial community structure was also studied by a polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) approach . Whole-community total DNA was extracted and PCR were performed according to M. Y. Wu et al. .
2.5. Water Use Efficiency (WUE) Analysis
WUE was calculated as the amount of grain yield per unit of water used. Water losses due to runoff and leaching were assumed to be negligible .
2.6. Data Analysis
The difference between different treatments was determined using the least significant difference (LSD) test in the data processing system SAS 8.02 software.
3. Results and Discussion
3.1. Bacterial Community Profile Determined by 16S rDNA Targeted DGGE
The DGGE fingerprinting of PCR-amplified 16S rDNA was shown in Figure 1(a) and Figure 1(b), which was extracted from the soils with different treatments at tiller and heading stages. Apparently, the band patterns of three replicate samples from PM, UM, and TF were highly similar (>95% similarity). The total number of bands in DGGE profiles of different treatments is shown in Table 2. At tiller stage, the total number of bands was 33.3, 23.7, 27.0 for PM, TF, and UM treatment, respectively (p < 0.05*). At heading stage, the total number of bands was 31.7, 20.0, 28.3 for PM, TF, and UM treatment, respectively (p < 0.05*). That is, these changes in the number of bands suggest that PM treatment favored the development of a more total bacterial community compared with TF management. One dominant band showed little change among three treatments, suggesting that it may be a predominant bacterial community. It is possible that low pH in TF management could be responsible for the low numbers of dominant bacterial bands as found on DGGE, because there was an increased pH value in PM management  .
3.2. Changes of Organic Matter
The decreased trend of organic matter content was presented with continuous plastic film mulching under
Figure 1. Denaturing gradient gel electrophoresis gel of bacteria 16S rDNA polymerase chain reaction products amplified from three replicate samples of PM, UM, TF treatment soil-DNA extracts. (a) Tiller stage; (b) Heading stage. PM: non- flooded plastic film mulching cultivation; TF: traditional flooding cultivation; UM: non-flooded cultivation without plastic film mulching; 1, 2, 3: three replicate samples from PM, UM, TF, respectively.
Table 2. Total number of bands in DGGE profiles from PM, TF, UM treatments.
Standard error is indicated in parentheses (n = 3).
non-flooding condition. At Duntou sites, organic matter content were declined by 36.7%, 28.5% and 33.9% under PM, UM and TF, respectively, compared with the initiation (Figure 2). At Dingqiao sites, organic matter content under PM, UM and TF were decreased by 51.4%, 51.5% and 40.3% after 6 years, respectively (Figure 3). The reason for these results is that plastic film mulching increased soil temperature during the entire growing seasons, and resulted of the improving of bacterial community activities and diversities, which promoted the decomposition of organic matter. This was similar to the previous studies  .
3.3. Grain Yield and Water Use Efficiency (WUE)
No significant trend existed in rice yields among the different treatments (Table 3). PM produced similar rice grain yield as TF at Duntou and Dingqiao sites, the average was 7924 kg∙ha−1 and 7015 kg∙ha−1 for PM and 8150 kg∙ha−1 and 6990 kg∙ha−1 for TF. Grain yields in PM had significantly higher than UM plots for two years at two sites, 2.7%, 6.1% at Duntou and 0.8%, 8.1% at Dingqiao, respectively.
Plastic film mulching reduced rice water consumption. During rice growing seasons in different years, the total water consumption in PM plots was 723 - 890 mm, much lower than that in TF plots, which was 1388 - 1591 mm. Irrigation water used in PM was 103 - 253 mm and in UM 196 - 310 mm, which were much lower than those in TF (718 - 947 mm). The irrigation water use reduced by 72.8% - 85.6% in PM and 66.9 - 72.8% in UM compared to TF treatment. Water use efficiency (WUE) in terms of grain yield per unit consumed water increased by 70.2% - 80.4% for PM and 47.9% - 63.8% for UM. Irrigation water use efficiency in terms of grain yield per unit irrigation water improved by 273.7% - 1300.0% for PM and 171.8% - 585.4% for UM, compared with TF respectively. PM reduced water consumption and increased WUE significantly compared to TF. These results were strongly supported by other studies conducted in the same region or elsewhere  .
Figure 2. Dynamics of soil organic matter content in different years at Duntou site.
Figure 3. Dynamics of soil organic matter content in different years at Dingqiao site.
Table 3. Effect of plastic film mulching on grain yield and water use efficiency.
This study shows that 6-year PM treatment in rice field stimulated the development of total bacterial communities by DGGE, which resulted of the decreased organic matter content in two study areas, and PM also produced similar or higher rice grain yield compared to TF and improved water use efficiency.
We are grateful to Prof. Yongguan Zhu, Research Center for Eco-Environmental Science, Beijing, China, for providing the testing instrument for the experiment.
 Fan, X.L., Zhang, J.P. and Wu, P. (2002) Water and Nitrogen Use Efficiency of Lowland Rice in Ground Covering Rice Production System in South China. Journal of Plant Nutrition, 25, 1855-1862. http://dx.doi.org/10.1081/PLN-120013279
 Li, Y.S., Wu, L.H., Zhao, L.M., Lu, X.H., Fan ,Q.L. and Zhang, F.S. (2007) Influence of Continuous Plastic Film Mulching on Yield, Water Use Efficiency and Soil Properties of Rice Fields under Non-Flooding Condition. Soil and Tillage Research, 93, 370-378. http://dx.doi.org/10.1016/j.still.2006.05.010
 Clasen, A.T., Boyle, S.I., Haskins, K.E., Overby, S.T. and Hart, S.C. (2003) Community-Level Physiological Profiles of Bacteria and Fungi: Plate Type and Incubation Temper-ature Influences on Contrasting Soils. FEMS Microbiology Ecology, 44, 319-328. http://dx.doi.org/10.1016/S0168-6496(03)00068-0
 Winding, A., Hund-Rinke, K. and Rutgers, M. (2005) The Use of Microorganisms in Ecological Soil Classification and Assessment Concepts. Ecotoxicology and Environment Safety, 62, 230-248. http://dx.doi.org/10.1016/j.ecoenv.2005.03.026
 Liu, M. and Wu, L.H. (2004) Changes of Available Fe, Mn, Zn and Cu Contents among Different Layers of Non-?ooded Paddy Soils with Ground Mulching. Journal of Zhejiang Uni-versity (Agricultural and Life Science), 30, 646-649. (In Chinese)
 Liu, X.J., Wang, J.C., Lu, S.H., Zhang, F.S., Zeng, X.Z. and Ai, Y.W. (2003) Effects of Non-?ooded Mulching Cultivation on Crop Yield, Nutrient Uptake and Nutrient Balance in Rice-Wheat Cropping Systems. Field Crops Research, 83, 297-311. http://dx.doi.org/10.1016/S0378-4290(03)00079-0
 Nakatsu, C.H., Torsvik, V. and ?vre?s, L. (2000) Soil Community Analysis Using DGGE of 16S rDNA Polymerase Chain Reaction Products. Soil Science Society America Journal, 64, 1382-1388. http://dx.doi.org/10.2136/sssaj2000.6441382x
 Wu, M.Y., Wu, L.H., Zhao, L.M. and Chen, M.M. (2009) Effects of Continuous Plastic Film Mulching on Paddy Soil Bacterial Diversity. Acta Agriculturae Scandinavica Section B—Soil and Plant Science, 59, 286-294. http://dx.doi.org/10.1080/09064710802095024
 Cantero, M., Angás, P. and Lampurlanés, J. (2003C) Growth, Yield and Water Productivity of Barley (Hordeum vulgare, L.) Affected by Tillage and N Fertilization in Mediterra-nean Semiarid, Rained Conditions of Spain. Field Crops Research, 84, 342-357.
 Laverman, A.M., Braster, M., R?ling ,W.F.M. and Van Verseveld, H.W. (2005) Bacterial Community Structure and Metabolic Profiles in a Forest Soil Exhibiting Spatially Variable Net Nitrate Production. Soil Biology and Biochemistry, 37, 1581-1588. http://dx.doi.org/10.1016/j.soilbio.2005.01.019
 Wu, L.H., Zhu, Z.R., Liang, Y.C., Shi, W.Y. and Zhang, L.M. (1999) A High Yielding, Water-Saving and Fertilizer- Saving Cultivation Technique for Rice Mulched by Plastic Film under Dry Land Condition. Journal of Zhejiang Agricultural University, 25, 41-42. (In Chinese)
 Liu, X.J., Ai, Y.W., Zhang, F.S., Lu, S.H., Zeng, X.Z. and Fan, M.S. (2005) Crop Production, Nitrogen Recovery and Water Use Efficiency in Rice-Wheat Rotation as Affected by Non-Flooded Mulching Cultivation (NFMC). Nutrient Cycling in Agroecosystems, 71, 289-299. http://dx.doi.org/10.1007/s10705-004-6801-4