Wim De Vries, Enzai Du, Klaus Butterbach Bahl, Lena Schulte Uebbing, and Frank Dentener
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
While the atmosphere consists of 78 percent di-nitrogen (N2), this form cannot be used by humans unless it is transferred in a so-called reactive form. This occurs through a microbial mediated process called biological nitrogen fixation (BNF), the main cause for the occurrence of nitrogen (N) in soils. Compared to the beginning of the 19th century, and especially since 1960, human activities have accelerated the global nitrogen cycle on land by approximately a factor of two. The current estimated global anthropogenic rate of N2 fixation largely exceeds the estimated natural rate of BNF in terrestrial ecosystems. Anthropogenic input of reactive N to the earth system occurs through industrial N fixation, intentional BNF in agricultural ecosystems, and unintentional NOx emissions from combustion processes. The most important source of intentional industrial fixation of atmospheric N2 is the Haber-Bosch process, mainly used in agriculture to create N fertilizer for crops to feed an ever-increasing global population. The intentional BNF from growing legumes is of the same order of magnitude as the unintentional N fixation by the emission of nitrogen oxides (NOx) from transport and industry.
Anthropogenically fixed N causes manifold impacts on ecosystem N and carbon (C) cycles and thus on the global warming potential (GWP), defined as the sum of the emissions of the greenhouse gases nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4), expressed in CO2-equivalents. First, elevated N use in agriculture leads to increased direct N2O emissions from agricultural systems. Furthermore, ammonia (NH3) emissions and nitrate (NO3) leaching from agricultural systems also cause indirect N2O emissions. Nitrogen use in agriculture, may, however, also cause changes in CO2 emission or uptake in agricultural soils due to N fertilization (direct effect) and in non-agricultural soils by NH3 deposition (indirect effect). Furthermore, NOx emissions due to combustion has elevated NOx deposition, further affecting CO2 exchange. Due to N limitation in most (semi-) natural ecosystems and marine systems, increased N deposition usually increases net primary production, thus stimulating C sequestration in those systems. Inversely, NOx emissions also induce ozone (O3) formation that leads to a reduction of net primary production, thus reducing C sequestration.
The quantitative impacts of human N fixation at world scale on CH4 exchange are insignificant compared to the impacts on N2O and CO2 exchange (emissions or uptake). The impact on the global warming potential thus mainly depends on the counteracting effect of CO2 uptake in terrestrial ecosystems as compared to (direct and indirect) N2O emissions from agricultural systems. Current knowledge suggests that CO2 uptake largely compensates N2O emissions, but on the long-term the effect on CO2 uptake may diminish due to growth limitations by other nutrients such as phosphorus. Furthermore, O3 exposure largely reduces the CO2 uptake and consequently, human N production causes overall an increase in global warming potential. These estimates are based on the contemporary state of the science and modelling approaches with respect to: (1) N inputs to various ecosystems, including NH3 and NOx emission estimates and related N (NH3 and NOx) deposition and O3 exposure; (2) N2O emissions in response to N inputs; and (3) carbon exchange in responses to N inputs (C-N response) and O3 exposure (C-O3 response), focusing on the global scale.