Riverine nitrogen export in Swedish catchments dominated by atmospheric inputs
(English)Manuscript (preprint) (Other academic)
The net anthropogenic nitrogen input (NANI) approach addressing N loading from agricultural sources and the atmosphere were applied to 36 major Swedish catchments. We tested three alternatives of the empirical relationship between NANI and riverine N export using different ways of estimating atmospheric N deposition as the major N input (31-94 %) in these catchments. The relationship between riverine N export and NANI was strongest for the NANI calculation using NOy (r2 linear =0.704, r2 exponential =0.723) compared to NOy + net NHx (r2 linear =0.623, r2 exponential =0.670) and total NOy + NHx deposition (r2 linear =0.615, r2 exponential =0.658). The y-intercept (NANI= 0) of the linear and exponential regression models were between 40-160 kg N km-2 year-1 indicating a natural background flux from the catchment without anthropogenic inputs of some 100 kg N km-2 year-1. This agrees with similar results from North American boreal catchments. The slope of the three linear regressions varies from 0.24 (NOy + Net NHx) to 0.25 (NOy and NOy+ NHx), suggesting that in average 25% of the human inputs of nitrogen are exported by the rivers to the Baltic Sea. Agricultural catchments, in the middle and southern Sweden, have increased their riverine N export up to an order of magnitude compared to the inferred natural background flux as an effect of anthropogenic loading. Although, the relatively unperturbed northernmost catchments receive significant N loads from atmospheric deposition these catchments do not show any significant elevated riverine N export. The fact that nitrogen export in Swedish catchments appears to be higher in proportion to NANI at higher loads suggests that N retention could be saturating as loading rates increase. Such nonlinear or threshold responses have significant implications for nutrient management.
Research subject Applied Environmental Science
IdentifiersURN: urn:nbn:se:su:diva-31484OAI: oai:DiVA.org:su-31484DiVA: diva2:280731