Manuel Aira

How earthworm density affects microbial biomas and activity in pig manure

We studied the influence of the earthworm, Eisenia fetida, on the microbial populations during the vermicomposting of pig manure. Fresh pig manure was placed in replicated boxes with (two densities, 25 and 50) and without earthworms for a period of 16 d. Samples were destructively collected periodically and analyzed for microbial biomass nitrogen, microbial respiration, substrate-induced respiration (SIR) and substrate dehydrogenase activity. Microbial biomass N, microbial respiration, SIR and substrate dehydrogenase activity were significantly lower in the earthworm treatments after 8 d.

Eisenia fetida (Oligochaeta: Lumbricidae) Modifies the Structure and Physiological Capabilities of Microbial Communities Improving Carbon Mineralization During Vermicomposting of Pig Manure

Although microorganisms are largely responsible for organic matter decomposition, earthworms may also affect the rates of decomposition directly by feeding on and digesting organic matter and microorganisms, or indirectly affect them through their interactions with the microorganisms, basically involving stimulation or depression of the microbial populations. We tested the general hypothesis that microbial populations, and especially fungi, are enhanced by earthworm activity, and also whether earthworms are able to modify the biodiversity of microbial populations, and its relation to the function of the system. In addition, we examined the metabolic quotient and the effect of labile organic C to assess the relationships between earthworm and microbes. We found that decomposition of pig manure has two stages characterized by the presence or absence of earthworms. Thus, the presence of earthworms was related with increases in overall microbial biomass and activity, which decreased when earthworms left the substrate; the same pattern was observed for fungi. Furthermore, earthworms modified the physiological profiles of microbial communities of pig manure, increasing the diversity of substrates utilized. In addition, earthworms promoted a more efficient use of energy of microbial communities, as the metabolic quotient showed. The rate of carbon loss was almost twice where earthworms were present, revealing faster decomposition. Our data match with the recent findings that to maintain essential processes the functional properties of present species are at least as important as the number of species per se. This is in accordance with the “insurance hypothesis,” which states that a large number of species is probably essential for maintaining stable processes in changing environments, as the presence of earthworms would have promoted in pig manure.

Eisenia fetida (Oligochaeta, Lumbricidae) Activates Fungal Growth, Triggering Cellulose Decomposition During Vermicomposting

Cellulose is the most abundant polymer in nature and constitutes a large pool of carbon for microorganisms, the main agents responsible for soil organic matter decomposition. Cellulolysis occurs as the result of the combined action of fungi and bacteria with different requirements. Earthworms influence decomposition indirectly by affecting microbial population structure and dynamics and also directly because the guts of some species possess cellulolytic activity. Here we assess whether the earthworm Eisenia fetida (Savigny 1826) digests cellulose directly (i.e., with its associated gut microbiota) and also whether the effects of E. fetida on microbial biomass and activity lead to a change in the equilibrium between fungi and bacteria. By enhancing fungal communities, E. fetida would presumably trigger more efficient cellulose decomposition. To evaluate the role of E. fetida in cellulose decomposition, we carried out an experiment in which pig slurry, a microbial-rich substrate, was treated in small-scale vermireactors with and without earthworms. The presence of earthworms in vermireactors significantly increased the rate of cellulose decomposition (0.43 and 0.26% cellulose loss day−1, with and without earthworms, respectively). However, the direct contribution of E. fetida to degradation of cellulose was not significant, although its presence increased microbial biomass (Cmic) and enzyme activity (cellulase and β-glucosidase). Surprisingly, as fungi may be part of the diet of earthworms, the activity of E. fetida triggered fungal growth during vermicomposting. We suggest that this activation is a key step leading to more intense and efficient cellulolysis during vermicomposting of organic wastes.

Vermicomposting: Earthworms Enhance the Work of Microbes

Vermicomposting, a very efficient method of converting solid organic waste into an environmentally-friendly, useful and valuable resource, is an accelerated process that involves bio-oxidation and stabilization of the waste as a result of the interactions between some species of earthworms and microorganisms. Although microorganisms are the main agents for biochemical decomposition of organic matter, earthworms are critical in the process of vermicomposting. Complex interactions among the organic matter, microorganisms, earthworms and other soil invertebrates result in the fragmentation, bio-oxidation and stabilization of the organic matter.

Changes in microbial biomass and microbial activity of pig slurry after the transit through the gut of the earthworm Eudrilus eugeniae (Kinberg, 1867)

Here we studied the effects of gut transit through the earthworm Eudrilus eugeniae, on the physicochemical, biochemical, and microbial characteristics of pig slurry, by analyzing fresh casts. The reduction in the dissolved organic C contents in casts we recorded suggests that during digestion, earthworms assimilated labile organic C preferentially, which is a limiting growth factor for them. Furthermore, both microbial biomass and activity in pig slurry were significantly decreased by earthworm gut transit. It appears that E. eugeniae is able to digest microorganisms, although the addition of glucose to the food increased respiration, indicating that growth of microorganisms in casts could be limited by depletion of labile C. Despite reduced microbial biomass and activity, the metabolic diversity of microbial communities was greater in casts than in original pig slurry. Community level physiological profiles obtained from Biolog Ecoplate data revealed that, after earthworm gut transit, the microbial communities in casts and pig slurry were clearly differentiated by their physiological profiles. The results indicate that first stage in vermicomposting of pig slurry by E. eugeniae, i.e., casting, produced changes that will influence the dynamics of the organic matter degradation by reducing forms of N and C available to microorganisms, hence restricting their growth and multiplication. Nevertheless, the reduced microflora of casts was characterized by an increased catabolic potential that might lead to thorough degradation of pig slurry.

Changes in microbial community structure and function during vermicomposting of pig slurry

Most studies investigating the effects of earthworms on microorganisms have focused on the changes before and after vermicomposting rather than those that occur throughout the process. In the present study, we designed continuous feeding reactors in which new layers of pig slurry (1.5 and 3 kg) were added sequentially to form an age gradient inside the reactors in order to evaluate the impact of the earthworm species Eisenia fetida on microbial community structure and function. The activity of earthworms greatly reduced the bacterial and fungal biomass and microbial diversity relative to the control values. However, the pronounced presence of earthworms in the younger layers stimulated microbial activity and as such increased carbon mineralization probably due to the fact that the microorganisms may have been less resource-limited as a result of earthworm activity, as indicated by the ratio of monounsaturated to saturated PLFAs.

Microbial and nutrient stabilization of two animal manures after the transit through the gut of the earthworm Eisenia fetida (Savigny, 1826)

Here we studied how the transit through the gut of the earthworm Eisenia fetida affects the microbial and nutrient stabilization of pig and cow manure, by analyzing fresh casts. Earthworms reduced the pools of dissolved organic C and N in casts from both types of manure, as wells as mineral N. Microbial biomass was enhanced only in casts from pig manure and did not change in casts from cow manure, and fungal populations only raised in casts from cow manure. Earthworms reduced microbial activity in casts from cow manure and did not modify in casts from pig slurry. Enzyme activities in casts also depended on the manure ingested; there were no changes in dehydrogenase and beta-glucosidase activities, whereas acid and alkaline phosphatases increased. The results indicate that the first stage in vermicomposting of pig and cow manure by E. fetida, i.e. casting, produced a microbial stabilization decreasing the activity of microorganisms; this stabilization occurred despite of the increase in microbial biomass. The strong reduction in nutrient pools of manures may be the responsible of this contradiction. These changes will influence the dynamics of the organic matter degradation by reducing forms of C and N available to microorganisms and hence restricting their growth and multiplication. Nevertheless, casts were also characterized by an increased enzyme potential that might lead to a further thorough degradation of pig and cow manure.

Effects of two species of earthworms (Allolobophora spp.) on soil systems: a microfaunal and biochemical analysis

Summary In this work we studied how earthworm digestion affects both soil biochemical and microfaunal characteristics. We sampled paired fresh casts and bulk soil of two earthworm species, Allolobophora caliginosa and A. molleri, inhabiting a pasture and a riverside, respectively. We focused on the differences in faunal composition and biochemical characteristics between cast and bulk soil. Ammonium and carbohydrate contents were higher in casts than in the surrounding soil. DON content was lower in the casts than in the surrounding soil. Microbial biomass (measured as microbial biomass-N, arginine ammonification and SIR) and microbial activity (measured as basal respiration, β-glucosidase, alkaline phosphatase, dehydrogenase and protease) were also higher in casts in the surrounding soil than. Flagellate protozoa and nematode numbers were lower in A. caliginosa casts and higher in A. molleri casts than in the surrounding soil.

Epigeic Earthworms Exert a Bottleneck Effect on Microbial Communities through Gut Associated Processes

Background Earthworms play a critical role in organic matter decomposition because of the interactions they establish with microorganisms. The ingestion, digestion, assimilation of organic material in the gut and then casting is the first step in earthworm-microorganism interactions. The current knowledge of these direct effects is still limited for epigeic earthworm species, mainly those living in man-made environments. Here we tested whether and to what extent the earthworm Eisenia andrei is capable of altering the microbiological properties of fresh organic matter through gut associated processes; and if these direct effects are related to the earthworm diet. Methodology To address these questions we determined the microbial community structure (phospholipid fatty acid profiles) and microbial activity (fluorescein diacetate hydrolysis) in the earthworm casts derived from three types of animal manure (cow, horse and pig manure), which differed in microbial composition. Principal Findings The passage of the organic material through the gut of E. andrei reduced the total microbial biomass irrespective of the type of manure, and resulted in a decrease in bacterial biomass in all the manures; whilst leaving the fungi unaffected in the egested materials. However, unlike the microbial biomass, no such reduction was detected in the total microbial activity of cast samples derived from the pig manure. Moreover, no differences were found between cast samples derived from the different types of manure with regards to microbial community structure, which provides strong evidence for a bottleneck effect of worm digestion on microbial populations of the original material consumed. Conclusions/Significance Our data reveal that earthworm gut is a major shaper of microbial communities, thereby favouring the existence of a reduced but more active microbial population in the egested materials, which is of great importance to understand how biotic interactions within the decomposer food web influence on nutrient cycling.

Selective reduction of the pathogenic load of cow manure in an industrial-scale continuous-feeding vermireactor

Vermicomposting is a suitable technology for processing different wastes, to produce a valuable end product (vermicompost). However, the pathogenic load of the waste must be greatly reduced in order to prevent risks to human health. Although Eisenia andrei may reduce the levels of several pathogens, the feasibility of vermicomposting, with regard to pathogen reduction, has not been tested on an industrial scale. This work studied whether vermicomposting in a continuous feeding vermireactor, is able to reduce the pathogenic load of cow manure. The effect of E. andrei on pathogens depended on the type of pathogen; thus, levels of Clostridium, total coliforms and Enterobacteria were not modified, but levels of faecal enterococci, faecal coliforms and Escherichia coli were reduced to acceptable levels. Pathogens could have maintained their levels in continuous feeding vermireactors, as fresh layers of manure are added to the top, which allows the vertical spread of pathogens through leaching.