Littoral enchytraeids and Eisenia fetida earthworms facilitate utilization of marine macroalgae as biofertilizers

Marine macroalgae are often used as biofertilizers in the coastal regions across the world. However, the effects of soil fauna in macroalgae decomposition are not fully understood. We conducted a microcosm experiment to assess the possibility of using a mixture of native Enchytraeidae and the model earthworm species Eisenia fetida to accelerate decomposition of dominant algae taxa from White Sea (Fucus spp.) and Black Sea (Cystoseira spp.) after their amendment to soil with simultaneous mitigation of the associated carbon dioxide (CO2) emissions. During 45 days of the microcosm experiment, we measured CO2 emission from the soil and evaluated changes in macroalgal biomass, δ13C and δ15N isotopic signatures of soil, macroalgae, earthworm faeces and enchytraeid tissues as well as bulk carbon and nitrogen content in soil. The mass of Cystoseira spp. residues, unlike that of Fucus spp., significantly decreased during the experiment (by approximately 73 % of the initial level). This effect was amplified by the co-action of E. fetida and Enchytraeidae (by 65 % of the initial). The addition of Enchytraeidae into microcosms led to the more than two-fold reduction of CO2 emission in all treatments. Differential enrichment of soil and earthworm casts with macroalgae-derived carbon and nitrogen, as suggested by stable isotope analysis, indicated that macroalgae-originating carbon pathway is rather driven by a synergistic activity of enchytraeids and earthworms (δ13C enrichment up to 4.8 ‰). Nitrogen transfer was dependent on the microbial activity and could be modulated and channelized by enchytraeids, whose δ15N values strictly corresponded to algal taxa. This highlights the importance of annelids in increasing the value of macroalgae as biofertilizers.