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Data from Soil chemistry turned upside down: a meta-analysis of invasive earthworm effects on soil chemical properties

Citation

Ferlian, Olga et al. (2020), Data from Soil chemistry turned upside down: a meta-analysis of invasive earthworm effects on soil chemical properties, Dryad, Dataset, https://doi.org/10.5061/dryad.59zw3r23d

Abstract

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.

Methods

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 vice versa; (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.

Funding

H2020 European Research Council, Award: 677232

Deutsche Forschungsgemeinschaft, Award: FZT 118