Utilization of microbial abundance and diversity as indicators of the origin of soil aggregates produced by earthworms

This study aimed at testing the capability of microbial community structure and abundance to be used as bioindicators of the origin of earthworm cast aggregates. Compact surface casts produced by Amynthas khami and surrounding aggregates lacking visible signs of biological activity (control) were left to disaggregate by natural rainfall and separated into four size classes (5–2, 2–0.5, 0.5–0.25 and <0.25 mm). The genetic structure and the abundance of the bacterial and fungal communities were characterized using B- and F-ARISA fingerprinting approach and quantitative PCR directly from DNA extracted from soil. Bacteria and to a lesser extent fungi were more abundant in casts than in control aggregates for all the size fractions. In addition, PCA carried out from B- and F-ARISA confirmed the different microbial properties between cast and control aggregates for all the aggregate size fractions. In conclusion, this study confirms the cryptic properties of earthworm casts when fragmented by the rain and the relevance of bacterial and fungal abundance and diversity as biological indicators of the origin of soil aggregates.