TY - JOUR

T1 - Carbon-nitrogen interactions in European forests and semi-natural vegetation - Part 1

T2 - Fluxes and budgets of carbon, nitrogen and greenhouse gases from ecosystem monitoring and modelling

AU - Flechard, Chris R.

AU - Ibrom, Andreas

AU - Skiba, Ute M.

AU - de Vries, Wim

AU - van Oijen, Marcel

AU - Cameron, David R.

AU - Dise, Nancy B.

AU - Korhonen, Janne F. J.

AU - Buchmann, Nina

AU - Legout, Arnaud

AU - Simpson, David

AU - Sanz, Maria J.

AU - Aubinet, Marc

AU - Loustau, Denis

AU - Montagnani, Leonardo

AU - Neirynck, Johan

AU - Janssens, Ivan A.

AU - Pihlatie, Mari

AU - Kiese, Ralf

AU - Siemens, Jan

AU - Francez, Andre-Jean

AU - Augustin, Juergen

AU - Varlagin, Andrej

AU - Olejnik, Janusz

AU - Juszczak, Radoslaw

AU - Aurela, Mika

AU - Berveiller, Daniel

AU - Chojnicki, Bogdan H.

AU - Dammgen, Ulrich

AU - Delpierre, Nicolas

AU - Djuricic, Vesna

AU - Drewer, Julia

AU - Dufrene, Eric

AU - Eugster, Werner

AU - Fauvel, Yannick

AU - Fowler, David

AU - Frumau, Arnoud

AU - Granier, Andre

AU - Gross, Patrick

AU - Hamon, Yannick

AU - Helfter, Carole

AU - Hensen, Arjan

AU - Horvath, Laszlo

AU - Kitzler, Barbara

AU - Kruijt, Bart

AU - Kutsch, Werner L.

AU - Lobo-do-Vale, Raquel

AU - Lohila, Annalea

AU - Longdoz, Bernard

AU - Marek, Michal

AU - Matteucci, Giorgio

AU - Mitosinkova, Marta

AU - Moreaux, Virginie

AU - Neftel, Albrecht

AU - Ourcival, Jean-Marc

AU - Pilegaard, Kim

AU - Pita, Gabriel

AU - Sanz, Francisco

AU - Schjørring, Jan K.

AU - Sebastia, Maria-Teresa

AU - Tang, Y. Sim

AU - Uggerud, Hilde

AU - Urbaniak, Marek

AU - van Dijk, Netty

AU - Vesala, Timo

AU - Vidic, Sonja

AU - Vincke, Caroline

AU - Weidinger, Tamas

AU - Zechmeister-Boltenstern, Sophie

AU - Butterbach-Bah, Klaus

AU - Nemitz, Eiko

AU - Sutton, Mark A.

PY - 2020

Y1 - 2020

N2 - 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, 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 (BAS-FOR) 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 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 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, N2O and especially NO3- 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.

AB - 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, 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 (BAS-FOR) 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 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 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, N2O and especially NO3- 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.

KW - LONG-TERM IMPACTS

KW - EDDY-COVARIANCE

KW - REACTIVE NITROGEN

KW - DRY DEPOSITION

KW - ORGANIC NITROGEN

KW - TROPICAL FORESTS

KW - QUALITY-CONTROL

KW - BOREAL FORESTS

KW - OXIDE FLUXES

KW - TREE GROWTH

U2 - 10.5194/bg-17-1583-2020

DO - 10.5194/bg-17-1583-2020

M3 - Journal article

VL - 17

SP - 1583

EP - 1620

JO - Biogeosciences

JF - Biogeosciences

SN - 1726-4170

IS - 6

ER -