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Microbial Community Dynamics of an Aquifer Biostimulated to Precipitate Calcite
Microbial-induced calcite (CaCO₃) precipitation (MICP) is a well-known natural phenomenon where microbes precipitate calcite in their environment as a result of metabolic activity. It has recently been of interest as a bioengineered technique to stabilize soils for construction applications. A known metabolic pathway to induce MICP is ureolysis, where introduced urea can be cleaved to ammonia by the microbial enzyme urease, increasing local pH and alkalinity. In the presence of calcium ions, calcium carbonate precipitation is promoted as the bicarbonate equilibrium shifts. MICP holds the promise as a non-invasive and environmentally benign technique to selectively stabilize the soil mass by reducing porosity and strengthening the mineral fabric. In-situ biostimulation of indigenous microbes was performed by injecting dilute molasses and urea over the course of 16 days in an oligotrophic aquifer in Rifle, CO.
Molasses was injected three times at increasing concentrations to stimulate overall microbial growth, and urea was injected throughout the period to spur ureolysis. Daily geochemical measurements and microbiological samples were taken from groundwater, and artificial sediment cores (ASCs) were suspended from monitoring wells throughout the injection period to serve as proxies for aquifer sediment. Conductivity increased and pH generally decreased over the course of the injection period, suggesting alternative processes such as fermentation. Microbial community structure (by sequencing the 16S rRNA gene) showed significant changes in community structure and diversity, with Proteobacteria dominating both groundwater and ASC end-stage communities, and an overall decrease in alpha diversity in all samples over time. Geochemistry showed evidence of nitrification, denitrification, sulfate reduction, and ureolysis and iron reduction toward the end of the study. Significant differences in representative phyla were also indicative of these processes, indicating community shifts with geochemistry. A quantitative PCR assay (droplet digital polymerase chain reaction or ddPCR) for the bacterial ureC gene peaked on day 14, the only day that well-known ureolytic phylum Firmicutes were dominant in the study. Overall results suggest some successful induction of ureolysis and calcite precipitation in the aquifer environment, albeit with alternative biogeochemical processes occurring simultaneously. Future investigations of this sort will help elucidate the viability and efficacy of MICP treatment in changing microbial community structure and ultimately physically strengthening the substrate mass by microbial action
Short-term effects of subsoil management by strip-wise loosening and incorporation of organic material
Agricultural production in Central Europe increasingly suffers from extreme drought events. Improving root access to nutrient and water resources in the subsoil below the plow layer is a potential option to maintain productivity during dry summers. Here, we tested a strip-wise subsoil amelioration method that combines subsoil loosening with organic matter incorporation into the subsoil (biowaste or green waste compost) and compared it with a treatment of only subsoil loosening and a non-ameliorated control. A field experiment with randomized block design was conducted on a Luvisol with an argic horizon (Bt), with a rotation of spring barley and winter wheat. In the first two years after amelioration, we monitored soil physico-chemical parameters, microbial biomass, and shoot and root growth at anthesis as well as harvested grain yield and quality. Subsoil loosening with organic matter incorporation significantly decreased soil bulk density at the depth of compost incorporation when biowaste compost was used, but not when green waste compost had been incorporated. Nutrient stocks, nutrient availability and microbial biomass were not consistently affected by the subsoil amelioration. Never- theless, the incorporation of organic material, especially biowaste compost, significantly increased root growth into the subsoil and subsequently significantly enhanced crop nutrient uptake, biomass and grain yield pro- duction. Green waste compost incorporation had less pronounced effects, with an increase in grain yield only in the second year after amelioration. Differences in crop development could not be explained by any single soil parameter, suggesting that it was rather a combined effect of loosened subsoil and better supply of subsoil re- sources that resulted in an increase in subsoil root length density and subsequently led to better crop performance