Aporrectodea earthworms respond to salt and organic matter levels in captive and free-choice mesocosms

As popular soil health indicators, earthworms play a role in soil assessments, especially when it comes to salinity. Salinity influences soil chemistry, structure, hydrology, and biological activity. To better understand the response of Aporrectodea earthworms to salinity, we conducted experiments in captive mesocosms that ranged in salinity (EC1:1 = 1 – 4.5 dS/m) and organic matter content (3.4% - 10%), and split-bin mesocosms that offered earthworms contrasting combinations of salt and organic matter levels. We observed that in captive situations, adult Aporrectodea earthworms survived in soils at all salinity and organic matter levels for 60 days. When mobility was allowed, adult Aporrectodea earthworms occurred in higher abundance in non-saline soils compared to saline soils, and elevated organic matter only alleviated the aversion to salinity when the alternative soil had less organic matter content. Based on these experiments, we conclude that earthworms prefer to reside in high organic matter, non-saline soils and prefer to avoid saline soils unless they are augmented with organic matter. The utility of earthworms as soil health indicators in a salinity context depends on their ability to select and move into more favorable environments, rather than their tolerance to salt ions.