Substantial net emissions of reactive nitrogen from intensively managed grassland on bog peat soil

Drained agriculturally used peatlands are hotspots of greenhouse gas emissions. Their exchange of reactive nitrogen (N_r) with the atmosphere, however, has been less investigated, mainly due to challenges in accurate and feasible flux measurements. In this study, we present data from a two-year field campaign on intensively managed grassland on bog peat soil. We used a modified custom-built converter called TRANC with fast time response connected to a dual-channel chemiluminescence detector (CLD) for eddy flux measurements of total reactive nitrogen (∑N_r) and total odd nitrogen (NO_y). The difference of the two channels was taken for an estimation of reduced nitrogen (NH_x). We found good agreement between time-integrated N_r concentrations from TRANC and a DELTA denuder sampler system. Over the two-year observation period, roughly two thirds of all recorded half-hourly ∑N_r and NH_x fluxes were positive, i.e. indicating ecosystem N_r loss to the atmosphere. Emission peaks occurred after fertilization events and mainly during the warmer months. Monthly median ∑N_r fluxes were ranging between -8 to 57 ng N m^-2 s^-1. We further found an enhancement of emissions under dry conditions and clear diurnal patterns in all N_r fluxes with peaks occurring around noon and close-to-neutral exchange during nighttime. The net loss of ∑N_r to the atmosphere was calculated to reach 9.3 kg N ha^-1 in 2020 and 6.7 kg N ha^-1 in 2021. Our setup allowed for an estimation of NH_x emission factors, at least for the organic inputs. Taking one week after each fertilization into account by summing up all recorded and gap-filled fluxes, emission factors for NH_x were in the range of 1.2 to 2.5% of added fertilizer nitrogen. Our study demonstrates the applicability of the modified TRANC converter for eddy flux measurements and provides useful data for understanding N cycling in agroecosystems to derive sustainable management options for farmers and conservationists.