Human activities have rapidly accelerated the global nitrogen (N) cycling since the late nineteenth century. This acceleration has manifold impacts on ecosystem N and carbon (C) cycles and thus on the emissions of the greenhouse gases nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) and their effect on global warming.
First, elevated N use in agriculture leads to increased direct N2O emissions from agricultural systems. Second, N use in agriculture leads to emissions of ammonia (NH3), nitric oxide (NO) and nitrogen dioxide (NO2) and leaching of nitrate (NO3-) from agricultural systems, which cause indirect N2O emissions from soils and waterbodies due N inputs to ecosystems downwind or downstream of its origin. N use in agriculture may, however, also cause changes in CO2 exchange (emission or uptake) in agricultural soils due to N fertilization (direct effect) and in non- agricultural soils due to NHx (NH3+NH4) deposition (indirect effect). Furthermore, NOx (NO+NO2) emissions from combustion processes, as well as from fertilized soils, lead to elevated NOy (NOx+ other oxidized N) deposition, further affecting CO2 exchange. Due to N limitation in most (semi-) natural terrestrial ecosystems and marine systems, increased N deposition usually increases net primary production (NPP), thus stimulating C sequestration in those systems. NOx emissions, however, also induce tropospheric ozone (O3) formation, with elevated O3 leading to a reduction of NPP and plant C sequestration. The impacts of human N fixation on soil CH4 exchange are insignificant compared to the impacts on N2O and CO2 exchange (emissions or uptake). Ignoring shorter lived components and related feedbacks, the net impact of human N fixation on climate thus mainly depends on the magnitude of the cooling effect of CO2 uptake as compared to the magnitude of the warming effect of (direct and indirect) N2O emissions.
The estimated impact of human N fixation on N2O emission is 8.0 (7.0-9.0) Tg N2O- N yr−1, which is equal 1.02 (0.89-1.15) Pg CO2-C eq. yr-1. The estimated CO2 uptake due to N inputs to terrestrial and aquatic ecosystems equals -0.75 (-0.56 - -0.97) Pg CO2-C eq. yr-1. At present, the impact of human N fixation on increased CO2 sequestration in terrestrial and marine ecosystems thus largely (on average near 75%) compensates the stimulating effect on N2O emissions from those systems. On the long
term, however, effects on ecosystem CO2 sequestration are likely to diminish due to growth limitations by other nutrients such as phosphorus. Furthermore, N induced O3 exposure reduces CO2 uptake, causing a net CO2 release which is estimated at 0.14 (0.07 - 0.21) Pg CO2-C eq. yr-1. Consequently, human N fixation causes an overall increase in net greenhouse gas emissions, which is estimated at 0.41 (-0.01 – 0.80) Pg CO2-C eq. yr-1. Even when considering all uncertainties, it is likely that net-impact of N-inputs on greenhouse gas emissions is positive (i.e. more inputs leads to more emissions, implying that a reduction in N-inputs is positive in view of climate change.
These estimates are based on most recent science and modelling approaches with respect to: (i) N inputs to various ecosystems, including NH3 and NOx emission estimates and related N (NH3 and NOx) deposition and O3 exposure, (ii) N2O emissions in response to N inputs, and (iii) carbon exchange in responses to N inputs (C–N response) and O3 exposure (C–O3 response), focusing on the global scale. Apart from presenting the current knowledge, the paper also gives an overview of changes in the estimates of those fluxes and C–N response factors over time, including debates on C–N responses in literature, the uncertainties in the various estimates and the potential for improving them.
nitrogen, ammonia, nitrogen oxides, nitrous oxides, fixation, emission, deposition, ozone, carbon sequestration, greenhouse gases
DE VRIES Wim;
DU Enzai;
BUTTERBACH BAHL K.;
SCHULTE-UEBING Lena;
DENTENER Franciscus;
2017-05-05
Oxford University Press
JRC101282
http://environmentalscience.oxfordre.com/view/10.1093/acrefore/9780199389414.001.0001/acrefore-9780199389414-e-13,
https://publications.jrc.ec.europa.eu/repository/handle/JRC101282,
10.1093/acrefore/9780199389414.013.13,
