# Acidic Neutralization by Indigenous Bacteria Isolated from Abandoned Mine Areas
## 1 Introduction  
Soil acidification, defined as a decrease in the soil pH to below 5.5, is a serious problem that damages ecosystems, making it difficult to maintain nutrient balance and biodiversity [1]. Acidified soils reduce soil productivity and plant growth yield through the loss of basic cations (i.e., Ca\({}^{2+}\), Mg\({}^{2+}\), K\({}^{+}\), and Na\({}^{+}\)) and dissolution of acidic cations (i.e., Al\({}^{3+}\) and Mn\({}^{2+}\)). Moreover, soil acidification causes the leaching of toxic heavy metals, which contributes to the severe pollution of the neighboring environments [2; 3; 4; 5]. In particular, high concentrations of metals associated with fine precipitates, including metals adsorbed to colloidal hydrous ferric oxides [6] and metals [7] in the receiving waters can pose a hazard to some aquatic organisms, even if the acidic precursors (e.g., acid mine drainage) are mitigated [8]. Acid deposition from atmospheric acid rain and fertilizer overuse contributes intensively to soil acidification [9; 10; 11]. Moreover, most of the abandoned mine areas have been left unattended for a long time without any other action, so the acidification has been aggravated by the discharge of mine wastes, such as acid mine drainage and mine ore [12; 13; 14]. The problem is worsening in Korea because of the recent acceleration of forest soil acidification. Furthermore, most soils in Korea are derived from granite and granite genesis as the parent materials, which are more likely to be acidic than calcareous shale or limestone [15]. Therefore, it is essential to develop sustainable acid soil restoration technology. Conventional technologies using alkaline chemicals, such as calcium carbonate (CaCO\({}_{3}\)), lime (CaO) [16], and phosphogypsum (CaSO\({}_{4}\) 2H\({}_{2}\)O) [17], have been applied to ameliorate acidic soil. However, repeated and excessive use of these chemical ameliorants could cause serious problems, including re-acidification and hardening of the soils, nutrient imbalance caused by the accumulation of salts, and an increase of mineral nutrients [16; 17; 18; 19]. Biological soil remediation is a methodology for promoting economically environmental conservation. Some microorganisms in soil have adapted to be tolerant to acidic conditions through various mechanisms that may play an important role in changing the soil pH [20; 21; 22]. For example, several bacteria could increase the pH of the surrounding environment by producing ammonia by the deamination and decarboxylation of nitrogenous substrates that are broken down to urea and amino acid to maintain the cellular pH homeostasis [23; 24; 25]. For instance, acid-tolerant bacteria of _Bacillus cereus_ isolated from the acidic soil of a tea plantation secrete urease to neutralize the pH [26]. _Lactobacillus acidophilus_ has been shown to respond to acid stress through well-organized metabolisms associated with decarboxylation reactions in the biomolecular analysis [27]. In addition, _Klebsiella_ sp. and _Raoultella_ sp. remediated the soil spiked by acid mine drainage derived from pyrite oxidation, increasing the concentration of NH\({}_{4}\)\({}^{+}\) while simultaneously raising the pH [28]. It has been reported that sulfate-reducing bacteria that are tolerant to low pH and high concentrations of arsenic and ferrous can neutralize pH to promote sulfate reduction while producing alkalinity [29]. In biological soil restoration approaches, biostimulation promotes the growth of indigenous microorganisms by supplying rate-limiting nutrients (e.g., nitrogen, phosphorous, oxygen, and other electron donors) in contaminated soil [30]. It is widely used to enhance the decomposition efficiency of various organic contaminants [31; 32; 33]. Nitrogen fertilizer has been utilized to restore soils from acidification [34] and facilitate the circulation of nitrogen [35].