Effects of endogeic earthworms on the soil organic matter dynamics and the soil structure in urban and alluvial soil materials

Earthworms are considered as key actors of soil processes at different spatial and temporal scales and provide essential ecosystem services linked to climate regulation or primary production. However, little is known about their basic functional roles (e.g. organic matter decomposition, soil structuring processes) in perturbed systems such as urban or alluvial soils. Alluvial soils are characterized by regular physical perturbation through flooding and associated erosion/sedimentation processes which are rather similar to perturbations (e.g. temporal instability, spatial heterogeneity) affecting urban soils. Due to their close soil characteristics, we hypothesized that in both cases, soil functioning is similar with respect to soil fauna activity. Under controlled conditions, our objective was to investigate the effects of two endogeic earthworm species, Allolobophora chlorotica (pink morph) and Aporrectodea rosea (the two most abundant species found in the studied urban site), on soil organic matter (SOM) dynamics and soil structure (network of earthworm burrows) comparing an urban and an alluvial soil. We investigated the growth of individuals (weight gain and reproduction success) and assessed their effects on SOM decomposition (cumulative C–CO2 emission, nitrogen and phosphorus mineralization) and soil structure (macroporosity, total length and connectivity of segments) after one and three months of incubation. Our results showed higher growth of A. rosea in the alluvial soil compared to the urban soil. However, the total length of burrows, carbon and nitrogen mineralization were often higher in the urban soil especially when the two species were combined. This trend can be mainly explained by lower organic matter content found in the urban soil which may influence positively the burrowing activity and negatively the growth of earthworms. Endogeic earthworms appear a key feature of the soil functioning in the urban context through their roles on organic matter transformation, the formation and maintenance of the soil structure

Impact of two root systems, earthworms and mycorrhizae on the physical properties of an unstable silt loam Luvisol and plant production

Background and aims: Soil organisms are known to engineer the soil physical properties, but their impact is difficult to assess and poorly documented. Shrinkage analysis has a good potential for such assessment. This study analyses the effects of mycorrhizae (Glomus intraradices), earthworms (Allolobophora chlorotica) and two plants, Allium porrum (leek) and Petunia hybrida (petunia), on the physical properties of an unstable loamy Luvisol, as well as the biological interactions between the soil organisms. Methods: In addition to soil organism biomass, shrinkage analysis and soil aggregate stability analysis were used to characterize the soil physical properties. Results: The soil aggregate stability, specific volume and structural pores volumes were increased with plant roots compared to control. The drilling effect of roots could not explain the pore volume increase, which was several orders of magnitude larger than the volume of the roots. Leek had larger impact on volumes while petunia mostly increased soil aggregate stability. Mycorrhizae increased the soil stability and the soil volume. Earthworms alone decreased the pore volumes at any pore size, and plant roots mitigated this. Conclusions: Our results highlight (1) the large impact of soil biota on soil physical properties, (2) that their separated effects can either combine or mitigate each other and (3) that the observed changes are varying in intensity according to soil type and plant typ

Bacterial communities in trace metal contaminated lake sediments are dominated by endospore-forming bacteria

Lake sediments in areas close to the outlet of wastewater treatment plants are sinks for pollutants. Bacterial communities in sediments are likely affected by the released effluents, but in turn they might modify the distribution and bioavailability of pollutants. On the shore of Lake Geneva, Switzerland, wastewater from the City of Lausanne is treated and discharged into the lake via an outlet pipe in the Vidy Bay. The objectives of this study were to assess (1) the impact of the treated wastewater release on the bacterial communities in the Vidy Bay sediments and (2) the potential link between bacterial communities and trace metal sediment content. Bacterial community composition and abundance were assessed in sediments collected in three areas with different levels of contamination. The main factors affecting bacterial communities were inferred by linking biological data with chemical analyses on these sediments. Near to the outlet pipe, large quantities of bacterial cells were detected in the three upper most cm (3.2 × 109 cells assessed by microscopy and 1.7 × 1010 copies of the 16S rRNA gene assessed by quantitative PCR, per gram of wet sediment), and the dominant bacterial groups were those typically found in activated sludge (e.g. Acidovorax defluivii and Hydrogenophaga caeni). Three samples in an area further away from the outlet and one sample close to it were characterized by 50 % of endospore-forming Firmicutes (Clostridium spp.) and a clear enrichment in trace metal content. These results highlight the potential role of endospore-forming Firmicutes on transport and deposition of trace metals in sediments

Bacterial communities in trace metal contaminated lake sediments are dominated by endospore-forming bacteria

Lake sediments in areas close to the outlet of wastewater treatment plants are sinks for pollutants. Bacterial communities in sediments are likely affected by the released effluents, but in turn they might modify the distribution and bioavailability of pollutants. On the shore of Lake Geneva, Switzerland, wastewater from the City of Lausanne is treated and discharged into the lake via an outlet pipe in the Vidy Bay. The objectives of this study were to assess (1) the impact of the treated wastewater release on the bacterial communities in the Vidy Bay sediments and (2) the potential link between bacterial communities and trace metal sediment content. Bacterial community composition and abundance were assessed in sediments collected in three areas with different levels of contamination. The main factors affecting bacterial communities were inferred by linking biological data with chemical analyses on these sediments. Near to the outlet pipe, large quantities of bacterial cells were detected in the three upper most cm (3.2×109 cells assessed by microscopy and 1.7×1010 copies of the 16S rRNA gene assessed by quantitative PCR, per gram of wet sediment), and the dominant bacterial groups were those typically found in activated sludge (e.g. Acidovorax defluivii and Hydrogenophaga caeni). Three samples in an area further away from the outlet and one sample close to it were characterized by 50% of endospore-forming Firmicutes (Clostridium spp.) and a clear enrichment in trace metal content. These results highlight the potential role of endospore-forming Firmicutes on transport and deposition of trace metals in sediments

Nicodrilus nocturnus and Allolobophora icterica drill compacted soils but do not decrease their bulk density – A laboratory experiment using two contrasted soils at two different compaction levels

Earthworms are known to play an important role in soil processes, especially in the regeneration of soil structure. However, quantitative studies about their role on soil physical properties are still scarce. In this study the effects of two earthworm species (Nicodrilus nocturnus as anecic, Allolobophora icterica as endogeic) following three treatments (N. nocturnus only, A. icterica only and both species with 80% weight of N. nocturnus and 20% of A. icterica) on soil specific volumes and pore properties are evaluated in mesocosms (30 cm height and 15 cm diameter) for a loamy Anthrosol and a silt loam Luvisol. The soils were repacked to bulk density observed in the field (1,15 and 1,25 g cm−3 respectively) and to compacted bulk density (1,4 and 1,5 g cm−3 respectively). Except earthworm-free controls, introduced earthworm biomass was close to 500 g.m−2. The experiment lasted 23 weeks, under constant temperature and soil matrix potential, and earthworms were fed with hay. The impact of earthworms on soil porosities and specific volumes was assessed using (i) computed tomography on mesocosm and (ii) shrinkage analysis on undisturbed cubic samples (150 cm3). Anecic surface cast bulk density was determined after wax coating. At mesocosm scale, the specific volume of compacted soils increased significantly with the anecic and mixed earthworm treatments (+1.9% for the Anthrosol and +2.6% for the Luvisol), while no change was observed with endogeics regardless of the initial level of compaction or the soil type. After subtracting the burrow volumes, the remaining soil matrix specific volume showed significant decrease with earthworms in case of loose soils, particularly with endogeics with 5.6% decrease of the specific soil matrix volume, while the compacted soil matrix was not decompacted. At undisturbed cubic sample scale, shrinkage analysis confirmed these observations with earthworms decreasing the larger structural pores and promoting a more rigid plasma. Anecic surface casts showed intermediate bulk density (0.82 cm3 g−1 for the Anthrosol and 0.73 cm3 g−1 for the Luvisol) between compacted (0.73 cm3 g−1 for the Anthrosol and 0.67 cm3 g−1 for the Luvisol) and loose (0.88 cm3 g−1 for the Anthrosol and 0.81 cm3 g−1 for the Luvisol) soil matrices. We concluded that the decompaction effect of earthworms was due to the opening of burrows at mesocosm soil scale, while the matrix volume was i) either compacted in case of loose soil especially with endogeics at the expense of the >150 µm equivalent radius structural pores or ii) unchanged in case of compacted soil. Our results support the conclusion that earthworms alone cannot regenerate the matrix of compacted soils and even compact the soil matrix in case of loose soils

Impact of two root systems, earthworms and mycorrhizae on the physical properties of an unstable silt loam luvisol and plant production

Background and aims : Soil organisms are known to engineer the soil physical properties, but their impact is difficult to assess and poorly documented. Shrinkage analysis has a good potential for such assessment. This study analyses the effects of mycorrhizae (Glomus intraradices), earthworms (Allolobophora chlorotica) and two plants, Allium porrum (leek) and Petunia hybrida (petunia), on the physical properties of an unstable loamy Luvisol, as well as the biological interactions between the soil organisms. Methods : In addition to soil organism biomass, shrinkage analysis and soil aggregate stability analysis were used to characterize the soil physical properties. Results : The soil aggregate stability, specific volume and structural pores volumes were increased with plant roots compared to control. The drilling effect of roots could not explain the pore volume increase, which was several orders of magnitude larger than the volume of the roots. Leek had larger impact on volumes while petunia mostly increased soil aggregate stability. Mycorrhizae increased the soil stability and the soil volume. Earthworms alone decreased the pore volumes at any pore size, and plant roots mitigated this. Conclusions : Our results highlight (1) the large impact of soil biota on soil physical properties, (2) that their separated effects can either combine or mitigate each other and (3) that the observed changes are varying in intensity according to soil type and plant type