Subsurface earthworm casts can be important soil microsites specifically influencing the growth of grassland plants

Abstract

Earthworms (Annelida: Oligochaeta) deposit several tons per hectare of casts enriched in nutrients and/or arbuscular mycorrhizal fungi (AMF) and create a spatial and temporal soil heterogeneity that can play a role in structuring plant communities. However, while we begin to understand the role of surface casts, it is still unclear to what extent plants utilize subsurface casts. We conducted a greenhouse experiment using large mesocosms (volume 45 l) to test whether (1) soil microsites consisting of earthworm casts with or without AMF (four Glomus taxa) affect the biomass production of 11 grassland plant species comprising the three functional groups grasses, forbs, and legumes, (2) different ecological groups of earthworms (soil dwellers—Aporrectodea caliginosa vs. vertical burrowers—Lumbricus terrestris) alter potential influences of soil microsites (i.e., four earthworms × two subsurface microsites × two AMF treatments). Soil microsites were artificially inserted in a 25-cm depth, and afterwards, plant species were sown in a regular pattern; the experiment ran for 6 months. Our results show that minute amounts of subsurface casts (0.89 g kg−1 soil) decreased the shoot and root production of forbs and legumes, but not that of grasses. The presence of earthworms reduced root biomass of grasses only. Our data also suggest that subsurface casts provide microsites from which root AMF colonization can start. Ecological groups of earthworms did not differ in their effects on plant production or AMF distribution. Taken together, these findings suggest that subsurface earthworm casts might play a role in structuring plant communities by specifically affecting the growth of certain functional groups of plants.

Herbivory of an invasive slug in a model grassland community can be affected by earthworms and mycorrhizal fungi

“…Hence, some of the reported earthworm effects could also result from nutrient input caused by decomposing worm tissue rather than from actual earthworm activity. In previous experiments with the same substrate type and earthworm species, we documented earthworm activity throughout several months (Zaller et al 2013b ) with earthworm recovering rates of up to 93 % (Zaller et al 2011c ).…”

“…We observed a decline of earthworm biomass over the course of the experiment irrespective of AMF treatment. Some decline in earthworm biomass is frequently observed in earthworm laboratory studies, especially when experiments lasted several months (e.g., Wurst et al 2004b ; Zaller et al 2013b ). In the current study, we observed regular earthworm activity by surface castings until the end of the experiment; however, we did not quantify this activity and did not count the number of earthworms present at harvest.…”

Herbicides in vineyards reduce grapevine root mycorrhization and alter soil microorganisms and the nutrient composition in grapevine roots, leaves, xylem sap and grape juice

“…Pelosi et al 2014 ; Zaller et al 2014 ; Gaupp-Berghausen et al 2015 ; Travlos et al 2017 ). We explain the contrasting effects of the current study as follows: first, we measured earthworm activity by their surface cast production; however, it might well be that earthworms responded by changing their subsurface casting behaviour instead (Zaller et al 2013 ). Second, the herbicides were applied within grapevine rows only but not between rows leaving two-thirds of the area untreated.…”

Glyphosate-based herbicides reduce the activity and reproduction of earthworms and lead to increased soil nutrient concentrations

“…Monitoring surface casting activity has recently been proposed as an ecotoxicity test better related to earthworms’ ecological role than standard laboratory tests 26 . Although the studied earthworm species differ in their feeding behavior, both have been shown to cast on the soil surface when foraging for leaf litter and other organic material 27 28 . The peak in surface casting activity observed after herbicide application was therefore likely the consequence of an increased availability of dead leaf material.…”