Relationship Between Soil Moisture Regime And Microbial Biomass, Activity, Diversity In Paddy Soils

Paddy soils play an important role in food production and environmental quality. With the increasing population pressure and decreasing water availability for agriculture, great attention has been paid to improving irrigation water management of paddy soils. Due to permanent or periodic prolonged saturation, the physical, chemical and microbial properties of paddy soils are different from wetland soils and aerobic soils. However, the measurements of soil properties are mainly originated from aerobic soils. In present study, a series of laboratory and pot experiments were conducted to investigate microbial biomass, basal respiration, enzyme activities, and microbial community diversity in relation to soil moisture regime and paddy growth stage. The results were summarized as follows:(1) The effects of pretreatment and storage of flooded soil on microbial characteristics were investigated. Five treatments including flooded (contrast), flooded-air-drying, flooded-freezing, flooded-air-drying-freezing, flooded-air-drying-refrigeration were designed with pretreatment and different storage methods. Flooded and flooded-air-drying-freezing treatments were followed by freeze-drying before analyzing the total Phospholipid fatty acid (PLFA) and PLFA profile. The results showed that flooded-air-drying-refrigeration significantly decreased total PLFA, while other treatments were independent of air-drying or freezing. The contents of some PLFA groups and biomarkers were changed in response to air-drying pretreatment and different storage methods. Statistical analysis with correspondence analysis showed that air-drying and storage methods shifted the microbial community structure, but the effect of air-drying pretreatment followed by freezing and refrigenration on soil microbial community structure was more pronounced than direct flooded-freezing. These results indicated that deep freezing followed by freeze-drying may be the most recommendable procedure before soil biochemical analysis in flooded paddy soils.(2) Based chloroform-fumigation method, a chloroform-fumigaiton extraction-water bath method was developed for measuring microbial biomass carbon in flooded soils. Liquid chloroform was directly added to the flooded soil to be tested, which was fumigated for 24h at room temperature under normal atmospheric pressure and in darknesss, and then microbial biomass was extracted with 0.5 mol L^-1 K₂SO₄ solution. At the same time, non-fumigation control was extracted directly with 0.5 mol L^-1 K₂SO₄ solution. After a series of analyses with water bath time, chloroform concentration, and soluble carbon, the results showed that water bath under 100℃for 45-60 min with extraction did not affect content of soluble carbon, and this condition was enough to remove chloroform residue before measuring TOC. The developed method was tested on seven paddy soils which were incubated aerobically or anarobically and compared with the standard chloroform fumigation extraction. The results of the tests were reliable and reproducible, suggesting that chloroform-fumigaiton extraction-water bath method is a rapid and effective method for measuring microbial biomass carbon in flooded soil.(3) Influence of soil moisture regime (non-flooding, flooding-drying, and flooding) on microbial community diversity and activity was investigated by determining Biolog sole carbon source utilization pattern, Phospholipid fatty acid (PLFA) profiles and enzyme activity indices. Increased dehydrogenase and invertase activities were observed in Treatment Flooding as compared to what in the other two treatments, which apparently led to enhanced soil basal respiration. The average well colour development (AWCD) of the carbon sources on Biolog plates was significantly lower in Treatment Non-flooding than in the other two treatments. The fungal indicator (18:2w6,9c) and the proportion of fungal biomarker to bacterial biomarkers (15:0i+15:0a+16:0i+16:1w5c+17:0i+17:0a+17:0cy+17:0+18:1w7c+19:0cy) were lower under flooding conditions. Cluster analyses of the sole carbon source utilization and PLFA data demonstrated that Treatment Non-flooding differed from the other two treatments in soil microbial community. Although certain similarity was found between Treatment Flooding and Treatment Flooding-drying in structure and functional diversity of soil microbial community, the findings of the study suggested that there were some drastic changes in microbial community composition and activity associated with variation of soil moisture regime.(4) The effects of flooded-air-drying, paddy cropping, and their interaction on soil microbial properties were investigated. Flooded-air-drying decreased soil basal respiration and dehydrogenase activity, paddy cropping decreased soil dehydrogenase activity, and their interaction decreased soil basal respiration, metabolic quotient, dehydrogenase activity. Flooded-air-drying, paddy cropping and their interaction significantly increased the contents of aerobic bacterial, G-, G+ indicated by Phospholipid fatty acid (PLFA) in 105 days after transplating. Flooded-air-drying and interaction increased the contents of bacterial, actinomycetes, aerobic bacterial, G-, and G+. The content of type I methanotrophs indicator was significantly higher than control in the whole paddy growth stage. The cluster analysis of PLFA data showed that flooded-air-drying had lower effect on soil microbial community than paddy cropping.(5) Soil microbial genetic diversity of bacteria, ammonia-oxidizing archaea (AOA) and bacteria (AOB) were investigated using polymerase chain reaction-denaturing gradient gel electroporesis (PCR-DGGE). Flooded-air-drying, paddy cropping, and their interaction treatment had significant effect on soil bacterial genetic diversity. The genetic diversities of AOA and AOB were also changed by the above treatments. However, the shift of microbial diversity was more variable in AOB than that in AOA. The content of AOA was more abundance in flooded-air-drying, paddy cropping and their interaction than that in control. The changes in bacterial, AOA and AOB genetic diversities may be due to the differences in soil water, oxygen, carbon dioxide, and nutrient contents induced by irrigation and paddy cropping.