DOI: 10.5061/dryad.59zw3r23d
title: Data from Soil chemistry turned upside down: a meta-analysis of invasive earthworm effects on soil chemical properties

description: Recent studies have shown that invasive earthworms can dramatically reduce
 native biodiversity, both above and below the ground. However, we still
 lack a synthetic understanding of the underlying mechanisms behind these
 changes, such as whether earthworm effects on soil chemical properties
 drive such relationships. Here, we investigated the effects of invasive
 earthworms on soil chemical properties (pH, water content, and the stocks
 and fluxes of carbon, nitrogen, and phosphorus) by conducting a
 meta-analysis. Invasive earthworms generally increased soil pH, indicating
 that the removal of organic layers and the upward transport of more
 base-rich mineral soil caused a shift in soil pH. Moreover, earthworms
 significantly decreased soil water content, suggesting that the burrowing
 activities of earthworms may have increased water infiltration of and/or
 increased evapo-transpiration from soil. Notably, invasive earthworms had
 opposing effects on organic and mineral soil for carbon and nitrogen
 stocks, with decreases in organic, and increases in mineral soil. Nitrogen
 fluxes were higher in mineral soil, whereas fluxes in organic soil were
 not significantly affected by the presence of invasive earthworms,
 indicating that earthworms mobilize and redistribute nutrients among soil
 layers and increase overall nitrogen loss from the soil. Invasive
 earthworm effects on element stocks increased with ecological group
 richness only in organic soil. Earthworms further decreased ammonium
 stocks with negligible effects on nitrate stocks in organic soil, whereas
 they increased nitrate stocks but not ammonium stocks in mineral soil.
 Notably, all of these results were consistent across forest and grassland
 ecosystems underlining the generality of our findings. However, we found
 some significant differences between studies that were conducted in the
 field (observational and experimental settings) and in the lab, such as
 that the effects on soil pH decreased from field to lab settings, calling
 for a careful interpretation of lab findings. Our meta-analysis provides
 strong empirical evidence that earthworm invasion may lead to substantial
 changes in soil chemical properties and element cycling in soil.
 Furthermore, our results can help explain the dramatic effects of invasive
 earthworms on native biodiversity, e.g., shifts towards the dominance of
 grass species over herbaceous ones, as shown by recent meta-analyses.
description: We compiled a dataset of published data to investigate the
 effects of exotic earthworms on eight soil chemical properties: pH, water
 content, and the stocks and fluxes of C, N, and P. We conducted a search
 in Web of Science on September 27, 2018, using literature published
 between 1945 and September 2018, applying the following search string:
 (“lumbric*“ OR “earthworm*“) AND (“invasi*“ OR “exotic“ OR “non-native“ OR
 “peregrine“ OR “alien“ OR “introduce*“) AND (“soil NEAR/2 carbon“ OR
 “*organic carbon” OR “soil NEAR/2 nitr*“ OR “soil NEAR/2 ammoni*” OR “soil
 NEAR/2 phosph*” OR “soil water“ OR “soil moisture“ OR “soil humidity“ OR
 “pH“). In addition, unpublished studies from doctoral theses were included
 in the dataset. All studies were screened for the following inclusion
 criteria and included in the dataset when applicable: (1) studies that
 tested the effects of exotic earthworms using an earthworm
 treatment/control data or regression data (earthworm biomass or
 abundance), if the probability was high that earthworm presence influenced
 the respective soil property but not <i>vice versa</i>; (2)
 studies that reported at least one of the following soil chemical
 properties: pH, water content, stocks or fluxes of C, N, or P; and (3)
 studies where control soils had been devoid of native or exotic earthworms
 (for studies with treatment/control data). Review, opinion, and
 perspectives papers were excluded from the list. We
 extracted means, variances, and sample sizes of treatments with
 (treatment) and without (control) earthworms as well as correlation
 coefficients of regressions between earthworm biomass/abundance and soil
 chemical properties and sample sizes from regression studies. In addition,
 from each study, we extracted information on earthworm species studied,
 study type (field observation vs. field experiment vs. lab study),
 ecosystem/continent (continent: North America vs. Australia/Oceania;
 ecosystem: forest vs. grassland), soil layer (organic vs. mineral), and
 the specific target response variable that was measured. We created
 additional variables for each of the datasets by assigning ecological
 groups to the earthworm species used in the studies, such as the presence
 of epigeic, endogeic, and anecic earthworm species, and ecological group
 richness. We further included a variable on earthworm species
 richness. For earthworm treatment/control data, we
 calculated effect sizes for the effects of earthworm invasion on soil
 chemical properties using log-response ratio. We ran standard
 meta-analyses and tested for total heterogeneity of effect sizes within
 each model. We explored potential publication bias using funnel plots for
 visual inspection. We, additionally, used fail-safe-numbers (Rosenberg’s
 weighted method) for statistical inspection. Moreover, we investigated how
 much of the heterogeneity between studies is explained by the covariates
 ‘study type’, ‘soil layer’, and the earthworm-species-related covariates
 in a multi-level meta-analysis. We used study ID as random factor in each
 of the models.