Microbial diversity in three floodplain soils at the Elbe River (Germany)

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Abstract

Microbial communities in floodplain soils are exposed to periodical flooding. A long-term submerged Eutric Gleysol (GLe), an intermediate flooded Eutric Fluvisol (FLe), and a short-time flooded Mollic Fluvisol (FLm) at the Elbe River (Germany) with similar organic carbon contents (Corg) between 8.1% and 8.9% were selected to test the quality of phospholipid fatty acids (PLFA), soil microbial carbon (Cmic), basal respiration (BR), metabolic quotient (qCO2), and Cmic/Corg ratio to characterize and discriminate these soils with microbial parameters.

The three floodplain soils can be differentiated by Cmic and by total PLFA-biomass. Due to the different flooding durations and the time since the soils were last flooded Cmic and PLFA-biomass increase in the order GLe<FLe<FLm. Both parameters correlate significantly (r=0.999;p<0.05). The Cmic/Corg ratios are low in comparison to terrestrial soils and revealed the same ranking over the three soils like Cmic. Contrary, qCO2 and BR are highest in GLe and lowest in FLm according to inundation regime. The diminished Cmic, high BR, and high qCO2 values in GLe seem to be an unspecific response of aerobic soil microorganisms on the long flooding period and the resulting short time for developing after last flooding as well as the low pH value. Different plant communities and their residues may influence the microbial diversity additionally.

The PLFA profiles were dominated by the group of saturated fatty acids that together constituted almost 62–72% of the total fatty acids identified in the soils. In GLe all groups of PLFA, inclusive monounsaturated fatty acids, are lowest and in FLm highest, while in FLe the PLFA fractions show an intermediary amount of the three soils. The FLm had most of the time aerobic conditions and revealed therefore the highest Cmic, PLFA-biomass, especially monounsaturated fatty acids, Cmic/Corg ratio as well as relatively low BR and qCO2 value. These indicate that microorganisms in FLm are more efficiently in using carbon sources than those in GLe and FLe.

All 26 identified PLFA were found in FLe and FLm, while the polyunsaturated fungi biomarker 18:2ω6,9c could not be detected in GLe. In this long-time submerged soil the environmental conditions which microorganisms are exposed might be disadvantageous for fungi.

Introduction

Wetlands function as important links between terrestrial and aquatic systems. Past studies paid little attention to soils in semi-terrestrial ecosystems, their properties and ecological processes (Reddy and Patrick, 1993). Consequently, semi-terrestrial soils, especially floodplain soils have been researched relatively little in comparison to terrestrial soils.

Floodplain soils are underlying largely fluctuations in water table and therefore various soil moisture conditions. Frequently, they have an oxidized soil body that allows aerobic processes to occur. However, they can become reduced when submerged. Floodplain soils are often characterized through high carbon contents due to high intensity of accumulation of organic material (Rinklebe, 2004).

It is well known that water and its biological availability is an important factor influencing soil microbial diversity. The soil microbial biomass is the living portion of soil organic matter and a principle component of the decomposer subsystem regulating nutrient cycling, energy flow and ecosystem productivity (Wardle, 1998). The substrate-induced respiration (SIR) method (Anderson and Domsch, 1978) utilizes the physiological respiration response of soil organisms to substrate amendment to provide an estimate of soil microbial biomass carbon. It has frequently been shown that soil microbial biomass decline upon drying and increase upon rewetting (Orchard and Cook, 1983; West et al., 1988, West et al., 1992; Wardle, 1998), whereas drying and rewetting of soils caused a decrease in microbial biomass (Van Gestel et al., 1993). Nevertheless, positive relationships between soil moisture and microbial biomass are not universal, and negative relations between these variables, mostly in conditions of high soil moisture, have sometimes been detected (Wardle, 1998; Rinklebe, 2004).

Generally, soil microbial biomass can be limited by soil moisture under both dry and wet conditions. Exposure of microbial communities to fluctuating moisture may lead to selection for organisms better adapted to these conditions. Although some researchers studied the effect of dry/wet cycles on soil microbial communities under different farming systems (Lundquist et al., 1999a) or on available carbon sources in the laboratory (Lundquist et al., 1999b), there is a gap to quantify impacts of flooding duration and type of soil on microbial diversity in floodplain soils, both in the laboratory and in the field.

Phospholipid fatty acids (PLFAs) are major constituents of the cell membranes of all microorganisms (Vestal and White, 1989). The large variety of PLFAs present in living organisms and extracted from soil may provide a unique fingerprint of the viable microbial community of a given soil, at a given time (Bossio and Scow, 1998; Lundquist et al., 1999a). Using multivariate statistical analyses, this variation in fatty acid (FA) composition between microorganisms can be exploited, revealing differences between microbial communities (Bossio et al., 1998; Macalady et al., 2000; Pennanen, 2001). Certain PLFAs are related to functional groups of organisms and appear promising in their use as biomarkers for these organism groups (Vestal and White, 1989). Zelles (1999) proposed that the classification of PLFAs into a number of fractions should simplify the evaluating procedure and improve the assessment of soil microbial communities. The PLFA method can provide information on a variety of microbial characteristics, such as biomass, physiology, taxonomic and functional identity, and overall community composition (Green and Scow, 2000).

In the research of terrestrial soils (e.g. Frostegård et al., 1993a; Zelles, 1999; Pennanen, 2001), sediment microbiology (e.g. Rajendran et al., 1992; Macalady et al., 2000) or groundwater microbiology (e.g. Green and Scow, 2000) the use of lipid-based techniques is well established. Several studies determined the soil microbial biomass (Gaunt, 1993; Witt, 1997), the metabolic diversity of microbial communities (Bossio and Scow, 1995) or PLFA (Bossio and Scow, 1998; Bai et al., 2000; Nakamura et al., 2003) in paddy rice fields. Only a few reports investigated soil respiration (Megonigal et al., 1996) or the impact of inundation dynamics and different management systems on soil respiration, and diverse enzyme activities (Emmerling, 1993) in floodplain soils. However, the microbial diversity in floodplain soils is poorly understood.

We hypothesized that the environmental conditions to which floodplain soils have been exposed (mainly inundation regime) may affect soil microbial biomass, function and community composition. To examine and to quantify these impacts we determined soil microbial carbon (Cmic), basal respiration (BR), metabolic quotient (qCO2), Cmic/Corg ratio, and PLFA in three selected floodplain soils at the Elbe River (Germany). Our objectives were (i) to characterize microbial diversity in three floodplain soils, and (ii) to discriminate these soils with microbial parameters.

Section snippets

Site and soils

The study site is located at the Elbe River (stream kilometre 417), 102 km north-westerly of Berlin, Germany (52°48′00″N, 12°02′15″E). The long-term annual precipitation is 449 mm and the long-term annual mean air temperature is 8.0 °C. The study site is periodically flooded. High flood events are closely related to the hydrometeorological conditions in the catchment area, which are fed predominantly by snow melting (winter and spring) and heavy rainfall (spring and summer).

After 8 years research

Results

Cmic and PLFA-biomass increased in the order GLe<FLe<FLm (Table 2, Fig. 1) according to the elevational and hydrological gradient (Table 1). Cmic/Corg ratios ranged between 0.41 and 1.16 and revealed the same ranking over the three soils like Cmic (Table 2). Contrary, qCO2 and BR are highest in GLe and lowest in FLm (Table 2) according to flooding duration (Table 1).

PLFA patterns of 26 FAs were analysed for characterization of the microbial community structures. The identified FAs, the

Discussion

Differences in the topography of floodplains are usually slight but cause mostly in important hydrological differences, which generally correspond with type of soil. FLe, FLm and GLe are commonly found along the Elbe River (Rinklebe, 2004). The floodplains in the Elbe area are dominated of fine-grained sediments. This so-called “Auenlehm” (meadow loam) covers the surface of large areas of today's meadows at the Elbe River. Dominating soils are FLe. Flood channels and depressions are embedded in

Conclusion

PLFAs, Cmic, BR, qCO2, and Cmic/Corg ratio in the three floodplain soils were strongly influenced by the gradient in elevation, type of soil, flooding duration, and time since the soils were last flooded. Different plant communities and their residues as well as pH value may influence the microbial diversity additionally. The soil microbial properties chosen here are suitable to estimate microbial diversity in floodplain soils at the Elbe River. Additional data will be necessary to determine

Acknowledgements

We thank Mrs. A. Gurenowitz for laboratory assistance and the anonymous reviewers.

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