Role of Earthworms on C and N Biogeochemical Cycles and Potential Links to Greenhouse Gas Emissions

Earthworms play a critical role in many essential soil processes (e.g., soil organic matter (SOM) decomposition and nutrient cycling, improved soil structure, better drainage and aeration, etc.), which have important implications for the sequestration/emissions of carbon (C) and nitrogen (N) in soils. This is the result of a wide array of activities, including feeding, casting, burrowing, and interacting with other above- and below-ground organisms. The soil mixing, the incorporation of fresh organic residues into the soil, the formation of micro and macroaggregates, and the changes in soil structure promote both SOM mineralization and stabilization. Therefore, several studies have tried to elucidate whether the positive effects of earthworms on mineralization and nitrification/denitrification processes would make them responsible for increased greenhouse emissions (mainly CO2 and N2O) or to be the mediators for trapping nutrients and thus helping to reduce C and N emissions instead. Part of the problem is the difficulties in measuring the “direct” earthworm contribution to the dynamic transformations (mobilization vs. stabilization) of SOM and their separation from their “indirect” effects (via plant and microbial interactions). Proving that earthworms are strengthening the greenhouse effect requires a more complete understanding of the functional dissimilarity among species and the modulating effects of the soil abiotic conditions. Here, I will synthesize the current knowledge on the role of earthworms on biogeochemical cycles and how earthworm biogenic structures (casts, burrows, and pores) could act as potential hotspots for C and N mineralization/retention. The final implications for C and N stocks will also need to take into account experimental biases and research gaps to fully integrate all controlling factors so that the role of earthworms on C and N cycling and dynamics can be unambiguously assessed.