Earthworm effects on soil biogeochemistry in temperate forests focusing on stable isotope tracing: a review

Earthworms (Oligochaeta) are globally distributed soil-dwelling invertebrates that alter soil properties through feeding, casting, and burrowing behaviors. Soil physicochemical modification, which may directly influence the availability and dynamics of organic and inorganic nutrients in the soil, such as carbon and nitrogen, includes soil texture, porosity, and pH. Temperate forests produce year-round plant litter, the primary food source for earthworms, and litter processed by earthworms significantly contributes to soil organic material storage. In recent decades, studies on temperate forest ecosystems have attempted to elucidate and quantify the earthworm impact on soil organic material dynamics, mainly targeting carbon and nitrogen, using isotope analysis methods. This paper summarizes studies on the following topics: (1) effect of earthworm modification on soil property to understand these alterations’ interaction with carbon and nitrogen dynamics, and (2) isotope tracing method, used to elucidate the earthworm effect on carbon and nitrogen transformation and movements in temperate forests. The particular emphasis on the isotope method is based on its capability of time-adjusted quantification of organic materials in the ecosystem compartments. Also, isotopic labeling in biomass has a broad range of applications, such as tracing assimilated food sources, identifying trophic interactions in soil food webs, and addressing material dynamics in complex linkages between earthworms and their environment. In addition, we provide perspectives on other methodologies, such as chronology and population ecology, as feasible options to further assist the isotope tracing of earthworms’ impact on soil nutrient dynamics.