Microbial properties of soil aggregates created by earthworms and other factors: spherical and prismatic soil aggregates from unreclaimed post-mining sites

Abstract

Soil aggregates between 2 and 5 mm from 35- and 45-year-old unreclaimed post-mining sites near Sokolov (Czech Republic) were divided into two groups: spherical and prismatic. X-ray tomography indicated that prismatic aggregates consisted of fragments of claystone bonded together by amorphous clay and roots while spherical aggregates consisted of a clay matrix and organic fragments of various sizes. Prismatic aggregates were presumed to be formed by plant roots and physical processes during weathering of Tertiary mudstone, while earthworms were presumed to contribute to the formation of spherical aggregates. The effects of drying and rewetting and glucose addition on microbial respiration, microbial biomass, and counts of bacteria in these aggregates were determined. Spherical aggregates contained a greater percentage of C and N and a higher C-to-N ratio than prismatic ones. The C content of the particulate organic matter was also higher in the spherical than in the prismatic aggregates. Although spherical aggregates had a higher microbial respiration and biomass, the growth of microbial biomass in spherical aggregates was negatively correlated with initial microbial biomass, indicating competition between bacteria. Specific respiration was negatively correlated with microbial biomass. Direct counts of bacteria were higher in spherical than in prismatic aggregates. Bacterial numbers were more stable in the center than in the surface layers of the aggregates. Transmission electron microscopy indicated that bacteria often occurred as individual cells in prismatic aggregates but as small clusters of cells in spherical aggregates. Ratios of colony forming units (cultivatable bacteria) to direct counts were higher in spherical than in prismatic aggregates. Spherical aggregates also contained faster growing bacteria.

Humus accumulation, humification, and humic acid composition in soils of two post-mining chronosequences after coal mining

“…This content of fossil organic matter is highly variable. Frouz et al (2011a) reported a range between 2.5 and 11.2 % of fossil C. Variation in fossil organic matter can cause variation in content of aliphatic components observed between our sites.…”

“…Difference in spectra can be partly caused by presence of Fe+ ions derived from siderite, which was abundant in the spoil material and may have affected the spectra by affecting phase coherence. We suspect, however, that data from the youngest reclaimed site were unusual because most of the organic matter at that site was derived from fossil kerogen, which is characteristically dominated by its aliphatic component (Krıbek et al 1998;Frouz et al 2011a). This content of fossil organic matter is highly variable.…”

“…Based on a study of the soil microstructure performed at the same sites, most litter at the reclaimed sites was rapidly consumed by soil fauna and converted into earthworm casts and other forms of macrofauna feces while most litter at the unreclaimed sites accumulated on soil surface (Frouz et al 2007). As a consequence, most of the organic matter in reclaimed sites, but not in the unreclaimed sites, is bound in soil aggregates (Frouz et al 2007(Frouz et al , 2011a and is in close contact with the alkaline clay-rich mineral matrix. Earlier studies show that passage of litter through soil fauna and contact with clay minerals during gut passage may accelerate humic acid formation and increase the proportion of humic acids to fulvic acids (Frouz et al 2011b).…”

“…In unreclaimed sites, most humification occurs in the acidic accumulation of organic material in various stages of decay in the litter and fermentation layers on soil surface. In the reclaimed plots, however, most humification involves small fragments of organic matter linked to mineral soils (Frouz et al 2007(Frouz et al , 2011a. Attachment of partly oxidized organic matter to clays may stabilize the compounds and thereby reduce further decomposition (Zou and Bashkin 1998) and promote the accumulation of soil organic matter in the soil profile ).…”

“…Reclaimed sites were densely colonized by earthworms, which play important role in bioturbation even on young sites, while earthworms was found only in the oldest of the unreclaimed sites (Frouz et al 2007). No morphological or chemical differentiation of soils was evident in the carboniferous clays that were >20 cm deep (Frouz et al 2011a). …”