Assessment of soil fauna footprints at a rehabilitated coal mine using micromorphology and near infrared spectroscopy (NIRS)

Abstract

Soil micromorphology in thin section and near infrared reflectance spectroscopy (NIRS) are useful techniques for assessing the participation of soil macrofauna in the formation of aggregates and soil structure. The purpose of this study was to use micromorphological analysis and NIRS techniques to assess the role of soil fauna in the recovery of soil aggregates and in the modification of soil microstructure in a chronosequence of rehabilitated areas at Cerrejon coal mine (La Guajira, Colombia). 64 soil samples were taken from rehabilitated areas (from 1 to 20 years ago) and from natural dry tropical forests Soil macroaggregates were subdivided into three categories: biogenic (BA), physical (PA), and non-aggregated soil (NAS). 32 samples were used for NIRS analysis, while ten thin sections of resin-impregnated soil blocks were micromorphologically analyzed. Principal component analysis of NIRS spectra showed a clear separation between BA, PA, and NAS. Likewise, an increase in BA was observed in the intermediate and advanced stages of rehabilitation. Respect to the micromorphological features, there was a clear change from a matrix of silt-sized quartz, unaccomodated peds, and unstructured materials with non-existent biogenic activity at the 2-year site to the formation of consolidated aggregates, more homogenized soil and increased biological activity at the 20-year site. Soil biological activity, principally the footprint of macrofauna, was recognized using the two techniques as well as at both the micro- and macro-morphology scales. These results reveal how the rehabilitation program being undertaken at Cerrejón mine is promoting the soil macrofauna population and associated bioturbation.