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
Authors: Flechard Chris R. , Ibrom Andreas , Skiba Ute M. , de Vries Wim , van Oijen Marcel , Cameron David R. , Dise Nancy B. , Korhonen Janne F. J. , Buchmann Nina , Legout Arnaud , Simpson David , Sanz Maria J. , Aubinet Marc , Loustau Denis , Montagnani Leonardo , Neirynck Johan , Janssens Ivan A. , Pihlatie Mari , Kiese Ralf , Siemens Jan , Francez André-Jean , Augustin Jürgen , Varlagin Andrej , Olejnik Janusz , Juszczak Radosław , Aurela Mika , Berveiller Daniel , Chojnicki Bogdan H. , Dämmgen Ulrich , Delpierre Nicolas , Djuricic Vesna , Drewer Julia , Dufrêne Eric , Eugster Werner , Fauvel Yannick , Fowler David , Frumau Arnoud , Granier André , Gross Patrick , Hamon Yannick , Helfter Carole , Hensen Arjan , Horváth László , Kitzler Barbara , Kruijt Bart , Kutsch Werner L. , Lobo-do-Vale Raquel , Lohila Annalea , Longdoz Bernard , Marek Michal V. , Matteucci Giorgio , Mitosinkova Marta , Moreaux Virginie , Neftel Albrecht , Ourcival Jean-Marc , Pilegaard Kim , Pita Gabriel , Sanz Francisco , Schjoerring Jan K. , Sebastià Maria-Teresa , Tang Y. Sim , Uggerud Hilde , Urbaniak Marek , van Dijk Netty , Vesala Timo , Vidic Sonja , Vincke Caroline , Weidinger Tamás , Zechmeister-Boltenstern Sophie , Butterbach-Bahl Klaus , Nemitz Eiko , Sutton Mark A. 
Published: March 2020

Abstract. 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. 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 Nr 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
Nr deposition from a dedicated collocated monitoring network. To
identify possible ecological drivers and processes affecting the interplay
between C and Nr inputs and losses, these data were also combined with
in situ flux measurements of NO, N2O and CH4 fluxes; soil
NO3- 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
Nr deposition rates (Ndep) of 0.3 to 4.3 g N m−2 yr−1
and 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 budgets
of the forests were strongly dominated by CO2 exchange, while CH4
and N2O 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 Ndep
where Nr leaching losses were also very large, and compounded by the
lack of reliable data on organic nitrogen and N2 losses by
denitrification. Nitrogen losses in the form of NO, N2O and especially
NO3- were on average 27 % (range 6 %–54 %) of Ndep at sites
with Ndep < 1 g N m−2 yr−1 versus 65 % (range
35 %–85 %) for Ndep > 3 g N m−2 yr−1. Such large
levels of Nr 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 Nr
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 Ndep levels (> 2.5 g N m−2 yr−1), where inorganic Nr losses were also
increasingly large, NEP levelled off and then decreased. The apparent
increase in NEP at low to intermediate Ndep levels was partly the
result of geographical cross-correlations between Ndep 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. Ndep.