The effects of saline toxicity and food-based AD digestate on the earthworm Allolobophora chlorotica

Natalio, A.I.M., Back, M.A., Richards, A. and Jeffery, S. (2021) The effects of saline toxicity and food-based AD digestate on the earthworm Allolobophora chlorotica. Geoderma, 393.

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Anaerobic digestion (AD) is used to produce biogas and can offer a solution in waste management. Digestate, the AD by-product, can be applied to soil to improve fertility. However, the response of soil biological communities is not fully understood. There are mixed reports on its impact on earthworm survival. This study aimed to investigate digestate effects on earthworm mortality, and to elucidate potential mechanisms underlying it, if observed, after digestate application to soil. Juvenile and adult Allolobophora chlorotica were used as model organisms and added to microcosms prepared in a glasshouse trial. Five replicated treatments were: liquid Digestate; Osmotic-Stress (i.e. same salt concentration as digestate); Labile-C (i.e. same Biological Oxygen Demand as Digestate); Synthetic-Digestate a mixture of Osmotic-Stress and Labile-C (i.e. same salt concentration and BOD as digestate); and Water as the control. Treatments were applied at two different standardised rates equivalent to the digestate’s N content (i.e. 150 kg N ha−1 eq. or 300 kg N ha−1 eq.). The two development stages of A. chlorotica had different responses to treatments. Adult biomass was significantly greater in the Water control R150 treatment than in Digestate. Significantly lower juvenile biomass was observed in the Digestate R300 treatment than in the Labile-C and Water control treatments. The biomass of adults in the Labile-C R300 treatment was significantly greater than in the Digestate, Osmotic-Stress, Synthetic-Digestate and Water control treatments. Both life-stages exhibited a decline in biomass across all treatments, but the adults had higher mortality rates. The biomass of adults and juveniles declined, respectively, by 90% and 62% for Digestate applied at the lower rate, and by 96% and 90% at the higher rate. Whereas the abundance of adults and juveniles exhibited 80% and 24% drop at the lower rate, and a 90% and 84% drop at the higher rate. This study demonstrates that digestate can have negative impact on earthworm morbidity and mortality when applied to soil at 60% water filed pore space, with most of the total weight loss per pot due to reduced earthworm abundance. A likely hypothesis could be the osmotic stress induced by salts present in the digestate. However, there are other factors that interact with this effect, including possibly anaerobic impacts caused by high water content soils, as well as other mechanisms that have not been fully elucidated through this experimental design. Nevertheless, this work provides the basis for further ecotoxicology studies on the impact of digestate applied to soil. Further, while this works has shown that digestate can negatively impact A. chlorotica survival, whether the same is true for other earthworm species, ecotypes and life-cycle stages warrants further investigation. Considering the important role that worms play in soil health, field scale studies are also required to monitor the impacts of repeated digestate application on earthworm communities.

Item Type: Article
Keywords: Lumbricidae, Ecological category, Toxicology, Soil health, Soil amendments, Bioindicators
Divisions: Agriculture and Environment (from 1.08.20)
Depositing User: Mrs Rachael Giles
Date Deposited: 04 Mar 2021 16:19
Last Modified: 27 Feb 2022 04:30

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