Asian invasive earthworms in the urban and suburban forests in Baltimore: species identities, competition, and the effects on C dynamics

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Invasion of Asian earthworms of the genus Amynthas into deciduous forests in eastern US has raised concerns related to the potential impacts on soil and litter-living fauna, native plant communities, and soil C dynamics. In the last five years, four species, including A. agrestis, A. gracilis, A. hupeiensis, and A. tokioensis, were confirmed or recorded for the first time in Baltimore, increasing the numbers of Asian species in the region from two to six. Three of these species, A. hilgendorfi, A. agrestis and A. tokioensis, have become dominant and in many cases co-occurring in urban and suburban forests. Observations indicated that the Asian species may outcompete the incumbent earthworms in the region and their high rates of leaf litter consumption may have potential impacts on soil C biogeochemistry. The objective of this study is to test the hypothesis of interspecific competition between A. hilgendorfi and three incumbent earthworm species in the region, one native and two European, and to further understand the processes through which species-specific responses and interspecific interactions affect soil respiration. We conducted a lab mesocosm experiment using ¹³C and ¹⁵N double-enriched leaf litter, and traced litter-derived C and N into earthworm tissue, soil and, in the former case, CO₂ efflux. We documented the first direct evidence of competition in earthworms and confirmed that food is the limiting resource. Isotope data indicated that A. hilgendorfi led to a reduction of litter-derived C and N assimilation in the two European species (26.6 ± 6.8 and 23.9 ± 9.5 % in Lumbricus rubellus and 31.0 ± 3.0 and 49.2 ± 2.85 % in Octolasion lacteum, respectively; P < 0.001 in all cases), suggesting that A. hilgendorfi is the superior competitor for food. Earthworms generally had non-significant effects on soil respiration. However, interspecific interactions between the two European species had significant negative effects, potentially through affecting anaerobic microsites in the soil. Litter C-derived soil respiration was reduced by A. hilgendorfi and by the native species Eesenoides lonnbergi, but not by O. lacteum. Structural equation modeling indicated that the effects of earthworms on litter C-derived soil respiration were positively mediated by microbial biomass, and negatively mediated, presumably, by microbial/extracellular enzyme activities, but the effects are stronger in the latter and strongest in the Asian earthworm A. hilgendorfi.