Contribution of deeper soil horizons to N and C cycling during the snow-free season in alpine tundra, NW Italy

- Inorganic N forms were similar in the topsoil and in the subsoil at all sites.
- N and C forms were similar between topsoil and subsoil in the less evoluted soil.
- Soil moisture was the driving factor of N and C forms in topsoil and subsoil.
- The soil N-NO3− concentration significantly increased at the end of the snow-free season.
- Topsoil and subsoil N-NO3− were hydrologically connected to water N-NO3−.

In alpine tundra the contribution of subsurface soil horizons to N and C cycling, their intraseasonal variability and soil/water interaction in the snow-free season have been poorly studied. The hypothesis that subsoil pedoclimatic factors (soil moisture and soil temperature) and nutrients (extractable N-NH4+, N-NO3−, DON, DOC, Nmicr and Cmicr) can differ significantly from those of the topsoil was tested for 3 snow-free seasons at 3 study sites (site 1, 3 and 5) in the alpine tundra of the NW Italian Alps. In addition, the intraseasonal variability of both topsoil and subsoil extractable N and C forms was checked monthly from July to October, and they were related to those measured in the surface water of an alpine lake (Cimalegna Lake). The soil moisture did not show significant differences between topsoil and subsoil, with the exception of site 5, and was strictly correlated with the N and C forms studied at both soil depths, except for N-NO3−. The soil temperature was always slightly higher in the topsoil than in the subsoil, due to the incident solar irradiance, and was positively correlated with topsoil DON and Cmicr. At all study sites, N-NH4+ and N-NO3− showed no significant differences between topsoil and subsoil, while subsoil DON, DOC, Nmicr and Cmicr significantly differed from those in the topsoil, especially at site 5. Only N-NO3− had a significant intraseasonal variability reaching the highest values in October at both soil depths, mainly due to the end of the plants growing season. The temporal variation of N-NO3− concentration observed in the lake strictly reflects the temporal changes occurred in the soils underling the fundamental role of soil biocenosis in limiting leaching losses of nitrates.