Nitrogen Storage Change and δ15N Transitions of Peat Columns in Undrained and Forestry-drained Boreal Mires

Abstract

Peatland drying, driven by global warming and human impact alters nitrogen (N) cycling, potentially affecting gaseous and fluvial N fluxes, N stores in peat and in the underlying mineral subsoil, as well as the peat and subsoil N content and stable isotope (15N/14N) proportions (δ15N). Complete peat columns may reveal effects of past climatic periods and drainage for forestry on N stratigraphy within the peat. We measured N concentrations and δ15N values in peat and subsoil at undrained and drained peatlands in Finland. On three peatlands having paired natural and drained sites, we quantified changes in N, 15N and 14N storage, and calculated δ15N values of the lost or gained N for bog having three synchronous layers on undrained and drained sites. There was preferential downward transfer of 15N in the uppermost peat of drained bog. Also, the subsoil beneath the drained fen had increased δ15N values compared to undrained site. δ15N values were generally below zero, except in one bog and in the subsoil under a fen. In peat layers dating to the Mid-Holocene, high dry bulk density, C%, N%, and humification index, coupled with low C/N ratio, were connected to the highest δ15N values in profile, similarly to those observed at the surface peat δ15N maxima. Drainage decreased total N content and transferred N from upper peat layers to deeper strata. In opposite to expectations, on peatlands having parallel undrained and drained sites, δ15N value increase or decrease was not connected clearly to decreased N mass.