## 1 Introduction  
The ubiquity of bacteria and the diverse roles they play in natural environments have led to growing interest in harnessing bacterial activities for various anthropogenic purposes. From a physical point of view, soil is regarded as an inorganic multiphase system comprising solids, fluids, and gases. However, soil is also a living system, being one of the largest terrestrial carbon pools, constituting about 33% of the total terrestrial carbon [1]. The organic carbon in the top 1 m constitutes more than 50% of the total soil carbon. Prokaryotes comprise up to 17% of the soil organic carbon [2]. These unicellular organisms, mostly bacteria 0.5-5.0 \(\times\) 10\({}^{-6}\) m in size, are about three orders of magnitude smaller than the pore throat size of sand and about the D10 size of kaolinite [3]. Soil bacteria, either motile or fixed to mineral surfaces (grains), may change the chemical and physical properties of their surroundings depending on their metabolism. Microbial biomass and biodiversity often exhibit exponential decreases with depth [4]; nevertheless, there are still active cells in deeper soil horizons [5]. Many bacteria are capable of inducing mineral precipitation through various metabolic paths in both oxic and anoxic environments. [PERSON] [6] showed that most heterotrophic bacteria can induce precipitation of calcium carbonate (CaCO\({}_{3}\)), a common natural cementing agent, by various metabolic pathways. Microbial-induced calcite precipitation (MICP) is an emerging technique aiming to mitigate different environmental and engineering challenges, including soil erosion, soil liquefaction, fracture sealing, restoration of stone monuments, and others [7; 8; 9; 10; 11; 12; 13]. Hydrolysis of urea, catalyzed by the microbial enzyme urease (urea amidohydrolase, EC 3.5.1.5), is considered the most efficient microbial pathway for MICP [14; 15]. The hydrolysis of urea produces ammonium and carbonate, seen in Equation (1), thus increasing the saturation for calcium carbonate, which could lead to its precipitation, usually as calcite, in Equation (2). \[CO(NH_{2})_{2}+2H_{2}O\xrightarrow{urase}2 NH_{4}^{+}+CO_{3}^{2-} \tag{1}\]  
\[CO_{3}^{2-}+Ca^{2+}\leftrightarrow CaCO_{3}\downarrow \tag{2}\]  
Two alternative approaches to facilitate in situ MICP in soils are biostimulation and bioaugmentation. Biostimulation encourages indigenous urea-hydrolyzing bacteria by providing appropriate enrichment and precipitation media; it relies on the natural ubiquity of ureolytic soil bacteria and bacterial spatial distribution [15; 16]. Bioaugmentation introduces large volumes of bacterial cultures into the treated soil along with a growth and precipitation medium; therefore, it requires large volumes of pure-cultured specializing ureolytic bacteria, e.g., _Sporosarcina pasteurii_. Producing and transporting large volumes of these cultures is an expensive and delicate procedure; their injection and homogeneous distribution throughout the treated site are difficult to achieve and might encounter regulatory hindrances [17]. Moreover, the introduced bacteria are likely to decline in numbers due to low compatibility with the environment as well as competition and predation by indigenous bacteria [18]. As the two methods involve the introduction of large quantities of urea along with a source of organic carbon, both biostimulation and bioaugmentation are likely to affect and change the indigenous microbial population. MICP soil improvement was successfully demonstrated at a variety of scales [10; 19; 20; 21; 22]. However, reliance on bioaugmentation has restricted the technology from becoming a cost-competitive alternative to more traditional ground amelioration techniques. Biostimulation, the use of selective substrates and environmental factors to stimulate the growth of native microorganisms with desirable metabolic capabilities, has been researched extensively in the field of bioremediation [23; 24] with success in several notable field-scale applications [25; 26].