Influence of different forms of acidities on soil microbiological properties and enzyme activities at an acid mine drainage contaminated site
Introduction
Oxidation of sulfides through complex biogeochemical process is one of the most common and single most important causes of acidity from coal mining activities. Progressive oxidation leads to lowering of pH and increase in concentrations of dissolved sulfates and metals including Al, Fe in leaching waters impacting surrounding soils and terrestrial ecosystems [1]. These waters, known as acid mine drainage (AMD) increase the acidity of the soil; affect vegetation and soil biology [2]. Low pH is also toxic to algal and fungal organisms [3]. At low pH, dissolved metals cross the membrane adding toxicity and killing some organisms by destroying ionic balance [4].
The mining in Jaintia coalfield started in early 1970 and even today coal is being excavated through primitive sub-surfaces mining. Presence of high concentration of sulfur coupled with unscientific mining (without proper plan and management) generates low pH AMD in this area affecting the soil, water quality and the aquatic life causing diverse biological effects including change in the abundance, biomass, and diversity of invertebrates [5].
Soluble, exchangeable, organically complexed, and sorbed Al in the soil are responsible for the acidic condition [6], potentially toxic to microorganisms [7]. Soil pH affects organic C solubility and increases the availability of biologically toxic Al with decreasing pH [7]. This, in turn, affects microbial community structure and changes the microbial activity [8]. In addition to microbial and biochemical properties, the nutrient turnover in the soil is significantly reduced under acidic pH condition due to combined effect of H+ and Al3+ [9]. Increased KCl-extractable Al in the soil causes distinct reduction in microbial biomass and their activities in acidic tea garden soil [7].
The central role played by the soil microorganisms in nutrient cycling and energy-flow relations in natural and anthropogenically affected environment necessitates determining easily assessable biological indicators pertaining to ecosystem disturbances. Since microorganisms form a vital part of the soil food web, the total soil microbial biomass is a key parameter of ecological stress [10]. Hence, the drop in soil microbial biomass often leads to declining of rate of nutrient recycling and degree of labile nutrient pool. Soil microbial biomass responds readily to disturbance effects; and, therefore provides early notice with regard to the deterioration in soil quality [11]. Moreover, microbial biomass study does not reveal the impact on different microbial components and communities; and, hence, does not reflect the metabolic state of the microorganisms. The qCO2 being the ratio of basal soil respiration to microbial biomass, has been proposed as a measure of microbial response to stress [12]. The status of the microbial community can be precisely judged by QR, the ratio of basal soil respiration to substrate-induced respiration [13]. Soil enzymes are also used to estimate the adverse effects of acidity on soil quality [14], [15], [16]. The fluorescein diacetate hydrolyzing activity (FDA) reflects the overall microbial activity in soil [17]. β-Glucosidase, urease and phosphatases are few of the key enzymes in the transformation of soil C, N and P, respectively. β-Glucosidase is an important enzyme involved in the mineralization of soil organic matter (SOM) (i.e. polysaccharides) while urease and phosphatase are essential for the release of mineral N and P from SOM; and, therefore, in the mobilization of essential nutrients for plant growth. Moreover, all these enzymes are known to be affected by acidity as well as the SOM content [18].
An understanding of the impact of acidity on microbial community is a prerequisite for effective soil management practices. The literatures on microbial biomass, respiration and enzyme activities in acid mine drainage soil under tropical conditions are scanty. Further, there is lack of study on the combined effect of all these parameters on soils around AMD affected soil. Therefore, understanding the relationship of different forms of acidity with the microbiological properties in acid mine drainage contaminated soils is important.
Section snippets
Study area and site description
The study area lies within the Jaintia Hills (latitude 25°15′N to 25°30′N and longitude 92°15′E to 92°30′E) situated about 70 km from Shillong, the capital of Meghalaya State in India (Fig. 1). The climate is subtropical monsoonal with an average rain fall of 2500 mm distributed over 7 months of a year. Based on the atmospheric conditions, the wet seasons extend from April to October, followed by a dry period from November to March. During wet seasons, the monthly rainfall ranges from 154 to 785
Physicochemical properties of soil
Physicochemical properties of soils are shown in Fig. 2. The pH (water) of the baseline soil samples ranged from 4.7 to 5.3 (mean 4.95); while in contaminated soils between 2.5 and 3.8 (mean 3.11), respectively. The pH (KCl) of the baseline soil samples ranged from 4.4 to 5.1 (mean 4.70); while in contaminated soils were between 2.2 and 3.5 (mean 2.80), respectively. pH (KCl) of soil is lower than the pH (water) due to the presence of exchangeable Al [6]. pH (water, KCl) of the contaminated
Conclusions
Acid mine drainage affected soils are highly acidic and contain high organic matter. The significant and positive correlations between the different forms of acidity suggest that loss in one form of acidity is replenished by the other. The high acidity due to the mine drainage did not apparently inhibit microbial parameters of soils. Among the different forms of acidities, exchangeable acidity is the major factor for the decrease of the microbial biomass and enzyme activities. There are two
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Department of Mining Engineering, National Institute of Technology, Rourkela, Orissa 769008, India.