Fertilization Effects of Solid Digestate Treatments on Earthworm Community Parameters and Selected Soil Attributes

An increasing number of soils, including those in EU countries, are affected by organic matter deficiency and the deterioration of nutrients, and using mineral fertilizers is often associated with negative environmental impacts. One of the basic recommendations for sustainable agriculture is to increase the proportion of organic fertilizers in crop production and preserve soil biodiversity. An increasingly common organic fertilizer is biogas plant digestate, the physical and chemical properties of which depend primarily on the waste material used in biogas production. However, the fertilizer value of this additive and its effects on the soil environment, including beneficial organisms, remain insufficiently studied. Soil macrofauna, particularly earthworms, play a crucial role in soil ecosystems, because they significantly impact the presence of plant nutrients, actively participate in forming soil structures, and strongly influence organic matter dynamics. The present study was undertaken to determine the effects of fertilizing a silt loam soil with the solid fraction of digestate in monoculture crop production on earthworm community characteristics and the resulting changes in selected soil physicochemical properties. The research was conducted at a single site, so the original soil characteristics across the experimental plots were identical. Plots were treated annually (for 3 years; 2021–2023) with different levels of digestate: DG100 (100% of the recommended rate; 30 t ha −1 ), DG75 (75% of the recommended rate; 22.5 t ha −1 ), DG50 (15 t ha −1 ), DG25 (7.5 t ha −1 ), and CL (a control plot without fertilizer). An electrical method was used to extract earthworms. Those found at the study site belonged to seven species representing three ecological groups: Dendrodrilus rubidus (Sav.), Lumbricus rubellus (Hoff.), and Dendrobaena octaedra (Sav.) (epigeics); Aporrectodea caliginosa (Sav.), Aporrectodea rosea (Sav.), and Octolasion lacteum (Örley) (endogeics); and Lumbricus terrestris (L.) (anecics). Significant differences in the abundance and biomass of earthworms were found between the higher level treatments (DG100, DG75, and DG50), and the lowest level of fertilization and the control plot (DG25 and CL). The DG25 and CL plots showed an average of 24.7% lower earthworm abundance and 22.8% lower biomass than the other plots. There were no significant differences in the earthworm metrics between the plots within each of the two groups (DG100, DG75, and DG50; and DG25 and CL). The most significant influence on the average abundance and average biomass of Lumbricidae was probably exerted by soil moisture and the annual dosage of digestate. A significant increase in the abundance and biomass of Lumbricidae was shown at plots DG100, DG75, and DG50 in the three successive years of the experiment. The different fertilizer treatments were found to have different effects on selected soil parameters. No significant differences were found among the values of the analyzed soil traits within each plot in the successive years of the study.