Elucidating the role of earthworms on the fate of fertilizer N with synthetic and organic fertilizer application

Earthworms’ activities not only increase soil nitrogen (N) uptake by crops but also lead to N losses to environment. However, it remains unclear whether earthworms’ impact on the fate of fertilizer N differs based on the type of fertilizer application. Therefore, the present pot experiment examined the transformation and fate of two types of ¹⁵N-labeled fertilizer (synthetic fertilizer urea and organic fertilizer compost applied at rate of 400 mg N/pot) with and without earthworms (Amynthas corticis) in a soil-lettuce system over three seasons of cultivation. Results showed that earthworms increased the fresh biomass of lettuce in all three seasons, regardless of the type of fertilizer used. However, the effect of earthworms on fertilizer N uptake varied depending on the type of fertilizer. With earthworms present, lettuce took up an additional 20.97 mg/pot of synthetic fertilizer N in the first season, which sharply decreased to 2.72 mg/pot and 4.63 mg/pot in the second and third seasons, respectively. In contrast, the uptake of organic fertilizer N by lettuce increased by 10.08–11.24 mg/pot throughout the entire experiment when earthworms were present. The presence of earthworms increased the percentage of synthetic fertilizer N lost to the environment by 0.8 %, due to increased N leaching, N₂O emission, NH₃ volatilization, etc. In contrast, earthworms decreased the percentage of organic fertilizer N lost to the environment by 1.9 %, primarily through reduced NH₃ volatilization, etc. This study underscores the pivotal role of earthworms in modulating fertilizer N dynamics, with organic fertilizer offering superior ecosystem services compared to synthetic fertilizer. Given that only one earthworm species was studied and nearly half of the organic fertilizer remained in the soil, future long-term experiments incorporating diverse earthworm species and changes in the soil’s native N pool are essential to fully understand the role of earthworms in agro-ecosystem N cycling.