[{"type":"article-journal","title":"Carbon-nitrogen interactions in European forests and semi-natural vegetation - Part 1: Fluxes and budgets of carbon, nitrogen and greenhouse gases from ecosystem monitoring and modelling","issued":{"date-parts":[["2020"]]},"volume":"17","issue":"6","page":"1583-1620","container-title":"Biogeosciences","DOI":"10.5194\/bg-17-1583-2020","author":[{"family":"Sutton","given":"M. A."},{"family":"Flechard","given":"C. R."},{"family":"Ibrom","given":"A."},{"family":"Skiba","given":"U. M."},{"family":"De Vries","given":"W."},{"family":"Van Oijen","given":"M."},{"family":"Cameron","given":"D. R."},{"family":"DIse","given":"N. B."},{"family":"Korhonen","given":"J. F. J."},{"family":"Buchmann","given":"N."},{"family":"Legout","given":"A."},{"family":"Simpson","given":"D."},{"family":"Sanz","given":"M. J."},{"family":"Aubinet","given":"M."},{"family":"Loustau","given":"D."},{"family":"Montagnani","given":"L."},{"family":"Neirynck","given":"J."},{"family":"Janssens","given":"I. A."},{"family":"Pihlatie","given":"M."},{"family":"Kiese","given":"R."},{"family":"Siemens","given":"J."},{"family":"Francez","given":"A.-J."},{"family":"Augustin","given":"J."},{"family":"Varlagin","given":"A."},{"family":"Olejnik","given":"J."},{"family":"Juszczak","given":"R."},{"family":"Aurela","given":"M."},{"family":"Berveiller","given":"D."},{"family":"Chojnicki","given":"B. H."},{"family":"D\u00e4mmgen","given":"U."},{"family":"Delpierre","given":"N."},{"family":"Djuricic","given":"V."},{"family":"Drewer","given":"J."},{"family":"Dufr\u00eane","given":"E."},{"family":"Eugster","given":"W."},{"family":"Fauvel","given":"Y."},{"family":"Fowler","given":"D."},{"family":"Frumau","given":"A."},{"family":"Granier","given":"A."},{"family":"Gross","given":"P."},{"family":"Hamon","given":"Y."},{"family":"Helfter","given":"C."},{"family":"Hensen","given":"A."},{"family":"Horvath","given":"L."},{"family":"Kitzler","given":"B."},{"family":"Kruijt","given":"B."},{"family":"Kutsch","given":"W. L."},{"family":"Lobo-Do-Vale","given":"R."},{"family":"Lohila","given":"A."},{"family":"Longdoz","given":"B."},{"family":"Marek","given":"M. V."},{"family":"Matteucci","given":"G."},{"family":"Mitosinkova","given":"M."},{"family":"Moreaux","given":"V."},{"family":"Neftel","given":"A."},{"family":"Ourcival","given":"J.-M."},{"family":"Pilegaard","given":"K."},{"family":"Pita","given":"G."},{"family":"Sanz","given":"F."},{"family":"Schjoerring","given":"J. K."},{"family":"Sebasti\u00e0","given":"M.-T."},{"family":"Sim Tang","given":"Y."},{"family":"Uggerud","given":"H."},{"family":"Urbaniak","given":"M."},{"family":"Van DIjk","given":"N."},{"family":"Vesala","given":"T."},{"family":"Vidic","given":"S."},{"family":"Vincke","given":"C."},{"family":"Weidinger","given":"T."},{"family":"Zechmeister-Boltenstern","given":"S."},{"family":"Butterbach-Bahl","given":"K."},{"family":"Nemitz","given":"E."}],"publisher":"Copernicus Publications","ISSN":"1726-4170, 1726-4189","abstract":"The impact of atmospheric reactive nitrogen (N$_{r}$) deposition on carbon (C) sequestration in soils and biomass of unfertilized, natural, semi-natural and forest ecosystems has been much debated. Many previous results of this dC\/dN response were based on changes in carbon stocks from periodical soil and ecosystem inventories, associated with estimates of N$_{r}$ deposition obtained from large-scale chemical transport models. This study and a companion paper (Flechard et al., 2020) strive to reduce uncertainties of N effects on C sequestration by linking multi-annual gross and net ecosystem productivity estimates from 40 eddy covariance flux towers across Europe to local measurement-based estimates of dry and wet N$_{r}$ deposition from a dedicated collocated monitoring network. To identify possible ecological drivers and processes affecting the interplay between C and N$_{r}$ inputs and losses, these data were also combined with in situ flux measurements of NO, N$_{2}$O and CH$_{4}$ fluxes; soil NO$_{3}$\u0305 leaching sampling; and results of soil incubation experiments for N and greenhouse gas (GHG) emissions, as well as surveys of available data from online databases and from the literature, together with forest ecosystem (BASFOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from -70 to 826 gCm$^{-2}$ yr$^{-1}$ at total wet+dry inorganic N$_{r}$ deposition rates (N$_{dep}$) of 0.3 to 4.3 gNm$^{-2}$ yr$^{-1}$ and from -4 to 361 g Cm$^{-2}$ yr$^{-1}$ at N$_{dep}$ rates of 0.1 to 3.1 gNm$^{-2}$ yr$^{-1}$ in short semi-natural vegetation (moorlands, wetlands and unfertilized extensively managed grasslands). The GHG budgets of the forests were strongly dominated by CO$_{2}$ exchange, while CH$_{4}$ and N$_{2}$O exchange comprised a larger proportion of the GHG balance in short semi-natural vegetation. Uncertainties in elemental budgets were much larger for nitrogen than carbon, especially at sites with elevated N$_{dep}$ where N$_{r}$ leaching losses were also very large, and compounded by the lack of reliable data on organic nitrogen and N$_{2}$ losses by denitrification. Nitrogen losses in the form of NO, N$_{2}$O and especially NO$_{3}$\u0305 were on average 27%(range 6 %\u201354 %) of N$_{dep}$ at sites with N$_{dep}$ < 1 gNm$^{-2}$ yr$^{-1}$ versus 65% (range 35 %\u201385 %) for N$_{dep}$ > 3 gNm$^{-2}$ yr$^{-1}$. Such large levels of N$_{r}$ loss likely indicate that different stages of N saturation occurred at a number of sites. The joint analysis of the C and N budgets provided further hints that N saturation could be detected in altered patterns of forest growth. Net ecosystem productivity increased with N$_{r}$ deposition up to 2\u20132.5 gNm$^{-2}$ yr$^{-1}$, with large scatter associated with a wide range in carbon sequestration efficiency (CSE, defined as the NEP = GPP ratio). At elevated N$_{dep}$ levels (> 2.5 gNm$^{-2}$ yr$^{-1}$), where inorganic N$_{r}$ losses were also increasingly large, NEP levelled off and then decreased. The apparent increase in NEP at low to intermediate N$_{dep}$ levels was partly the result of geographical cross-correlations between N$_{dep}$ and climate, indicating that the actual mean dC\/dN response at individual sites was significantly lower than would be suggested by a simple, straightforward regression of NEP vs. N$_{dep}$.","kit-publication-id":"1000118975"}]