Reactive Nitrogen Fluxes over Peatland in Response to the Atmospheric Burden of Intensive Agriculture

Abstract:

During a 10-month field campaign, we used a total reactive atmospheric nitrogen (ΣN_r) converter (TRANC) coupled to a fast-response chemiluminescence detector to measure ΣN_r concentrations and fluxes between a peatland ecosystem and the atmosphere. Oxidized N_r species such as NO_x, HONO, HNO₂, aerosol NO₃ and reduced N_r species such as NH₃, aerosol NH₄ were converted to NO with the total exchange flux being determined using the eddy-covariance technique. The field site was part of a natural park with a very small remaining protected zone of less than 2 km x 2 km. The park was surrounded by an area of intensive agricultural land use. Total N_r concentrations were mainly varying between 20 and 60 ppb with peak values of up to 120 ppb coinciding with the main fertilization periods on the neighboring agricultural land in early spring and fall. The trend in weekly averaged ΣN_r concentrations from TRANC measurements was in good agreement with results from KAPS denuder filter systems when the latter were combined with the missing and apparently highly variable NO_x contribution. Wind direction and land use in the closer vicinity clearly regulated whether total N_r concentrations were NH₃ or NO_x-dominated. Values of monthly averaged diurnal flux courses ranged between -40 and +20 ng N m^-2 s^-1 with the majority of fluxes showing net deposition of ΣN_r to the land surface. The cumulative net exchange of ΣN_r at the end of the observation period resulted in an uptake of the ecosystem of only ~1 kg N ha^-1 with intermittent periods showing net ΣN_r release. This is an indicator for an oversaturation of this N-limited ecosystem caused by agricultural practices such as livestock production and fertilization. Bog plants were temporarily not capable of taking up the surplus nitrogen from the atmosphere, which highlights the importance for a reformulation of protection guidelines for vulnerable ecosystems such as peatlands.