Does earthworm density change the quality of degraded volcanic soil?

Abstract

Earthworms, as ecosystem engineers, are known to inoculate organic residues with microorganisms, consuming and processing mineral soil and its organic matter, which may lead to changes in soil quality. We conducted a field experiment, in randomized complete blocks, using soil mesocosms incubated with 4 different densities of earthworms: L1 = 100 earthworms m−2, L2 = 500 earthworms m−2, L3 = 1000 earthworms m−2, and C0 = control without earthworms. All earthworm populations consisted of epigeic, endogeic and anecic earthworms in a ratio of 2:3:3, respectively. The earthworms used were Lumbricus rubellus, Octolasium cyaneum, Aporrectodea calliginosa, and Lumbricus friendi. Physical (aggregate stability and structure heterogeneity), chemical (pH CaCl2, Mineral-N, Olsen-P, exchangeable K+, Ca+2, Mg+2, Na+ and Al) and biological indicators (microbial biomass) were assessed to evaluate the changes in soil quality. After applying an equivalent of 18 tons ha−1 of aged horse manure to each mesocosm, soils were sampled at two depths: 0–25 cm and 25–50 cm and on two occasions: prior to inoculating the soil with earthworms, T0, and 96 days after the soil was inoculated with earthworms, T1. Earthworms were found to increase soil mineral nitrogen (NO3 + NH4) and exchangeable calcium (Ca+2) and decrease exchangeable aluminum (Al+3) and soil microbial biomass (g C g−1). These results could imply that earthworms promote the mineralization of organic nitrogen, increase the cation content, and decrease the solubility of the principal toxic elements in soil (Al+3). Finally, the incorporation of earthworms at the 0–25 cm depth proved to increase the quality of the degraded soil, which was reflected in the chemical quality parameters.