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

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${}_3$̅ 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. ... mehrMulti-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${}_3$̅ were on average 27%(range 6 %–54 %) of N${}_{dep}$ at sites with N${}_{dep}$ < 1 gNm${}^{-2}$ yr${}^{-1}$ versus 65% (range 35 %–85 %) 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 2–2.5 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${}_{dep}$.