Earthworms and long-term straw management practices interactively affect soil carbon and nitrogen forms across soil depths

The earthworm functions in agriculture are determined by soil managements, including straw management practices (e.g., straw mulched on soil surface and incorporated into soil), which affect the spatial distribution of resources for earthworms as well as other organisms. However, it remains unclear that how earthworms and straw management practices jointly affect soil C and N forms and their spatial distributions. Based on a decade-long field experiment, we explored the effects of endogeic earthworms (Metaphire guillelmi) on contents of different soil C and N forms across soil depths (0–7.5 cm, 7.5–15 cm and 15–30 cm) under straw surface mulching and incorporation. Under straw mulching, earthworms did not affect the soil organic carbon (SOC) content but reduced the ammonium nitrogen and nitrate nitrogen (NH4+-N and NO3--N) content regardless of soil depths. And earthworms decreased the contents of total nitrogen (TN), microbial biomass carbon and nitrogen (MBC and MBN) at 0–7.5 cm layer, while increased those of MBC and MBN at 7.5–15 cm layer. In contrast, with straw incorporation, SOC, TN, MBC and MBN contents increased in the presence of earthworms. The most apparent positive effects of endogeic earthworms on the above total and microbial measurements occurred at 7.5–15 cm, with increases ranging from 25.5% to 65.7% under straw incorporation. Earthworms also tended to increase soil NH4+-N and NO3--N at 7.5–15 and 15–30 cm layers. Overall, the effects of endogeic earthworms depended on the straw management practices and soil depths. The interaction between earthworms and organic straw incorporation causes nutrients accumulated towards subsurface layer.