Invasive earthworms Amynthas tokioensis and Amynthas agrestis alter macronutrients (Ca, Mg, K, P) in field and laboratory forest soils

Pheretimoid earthworms of the family Megascolecidae are invading deciduous forests across the northern and northeastern United States with unknown consequences on soil and nutrient dynamics. This study investigated plant macronutrients (Ca, Mg, K, and P) at partially invaded forest sites in Madison, WI, USA, and in forest soil incubated with Amynthas agrestis or Amynthas tokioensis. Exchangeable, organic-bound, and/or inorganic Ca, Mg, K, and P concentrations and pools were typically greater inside than outside the invasion front. Exchangeable K and organic K were significantly increased by Amynthas earthworms. These changes in nutrient status were associated with an increased pH and greater soil organic matter (SOM), suggesting litter decomposition by Amynthas was likely responsible for nutrient additions. Under laboratory soil incubations, increases in Ca, Mg, K, and P occurred for A. tokioensis but this was less consistent for A. agrestis. Soil incubations also demonstrated that A. tokioensis stabilized more nutrients and SOM than A. agrestis despite being fed one-third less oak litter. Thus, while Amynthas earthworms alter forest soils due to differences in feeding, digestion, and mineralization among species, whether these differences facilitate co-invasion or bring about long-term changes in ecosystem soil pools remains unclear.