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

[摘要] 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. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from − 70 to 826 g C m −2  yr −1 at total wet  +  dry inorganic N r deposition rates ( N dep ) of 0.3 to 4.3 g N m −2  yr −1 and from − 4 to 361 g C m −2  yr −1 at N dep 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 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  3 g N m −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 g N m −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 g N m −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 .

[学科分类] 大气科学