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. The impact of atmospheric reactive nitrogen (Nr)deposition on carbon (C) sequestration in soils and biomass of unfertilized,natural, semi-natural and forest ecosystems has been much debated. Manyprevious results of this dC∕dN response were based on changes in carbonstocks from periodical soil and ecosystem inventories, associated withestimates of Nr deposition obtained from large-scale chemical transportmodels. This study and a companion paper (Flechard et al., 2020) strive toreduce uncertainties of N effects on C sequestration by linking multi-annualgross and net ecosystem productivity estimates from 40 eddy covariance fluxtowers across Europe to local measurement-based estimates of dry and wetNr deposition from a dedicated collocated monitoring network. Toidentify possible ecological drivers and processes affecting the interplaybetween C and Nr inputs and losses, these data were also combined within situ flux measurements of NO, N2O and CH4 fluxes; soilNO3- leaching sampling; and results of soil incubationexperiments for N and greenhouse gas (GHG) emissions, as well as surveys of availabledata from online databases and from the literature, together with forestecosystem (BASFOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests rangedfrom −70 to 826 g C m−2 yr−1 at total wet + dry inorganicNr deposition rates (Ndep) of 0.3 to 4.3 g N m−2 yr−1and from −4 to 361 g C m−2 yr−1 at Ndep rates of 0.1 to 3.1 g N m−2 yr−1 in short semi-natural vegetation (moorlands,wetlands and unfertilized extensively managed grasslands). The GHG budgetsof the forests were strongly dominated by CO2 exchange, while CH4and N2O exchange comprised a larger proportion of the GHG balance inshort semi-natural vegetation. Uncertainties in elemental budgets were muchlarger for nitrogen than carbon, especially at sites with elevated Ndepwhere Nr leaching losses were also very large, and compounded by thelack of reliable data on organic nitrogen and N2 losses bydenitrification. Nitrogen losses in the form of NO, N2O and especiallyNO3- were on average 27 % (range 6 %–54 %) of Ndep at siteswith Ndep &lt; 1 g N m−2 yr−1 versus 65 % (range35 %–85 %) for Ndep &gt; 3 g N m−2 yr−1. Such largelevels of Nr loss likely indicate that different stages of N saturationoccurred at a number of sites. The joint analysis of the C and N budgetsprovided further hints that N saturation could be detected in alteredpatterns of forest growth. Net ecosystem productivity increased with Nrdeposition up to 2–2.5 g N m−2 yr−1, with large scatterassociated with a wide range in carbon sequestration efficiency (CSE,defined as the NEP ∕ GPP ratio). At elevated Ndep levels (&gt; 2.5 g N m−2 yr−1), where inorganic Nr losses were alsoincreasingly large, NEP levelled off and then decreased. The apparentincrease in NEP at low to intermediate Ndep levels was partly theresult of geographical cross-correlations between Ndep and climate,indicating that the actual mean dC∕dN response at individual sites wassignificantly lower than would be suggested by a simple, straightforwardregression of NEP vs. Ndep.

biogeosciences

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