A new study published online this week in Nature Geoscience reports now that the climatic benefits from carbon sequestration are largely offset by increased nitrous oxide emissions, a further side-effect of human nitrogen additions to terrestrial ecosystems.
Human activities have more than doubled nitrogen inputs to the terrestrial biosphere since the 1860s. The two main causes for this are increased atmospheric nitrogen deposition from, for instance, fossil fuel burning, and the application of fertilizers in agriculture.
Nitrogen is an essential nutrient for plant and microbial growth, and one of the key limiting nutrients in many natural ecosystems. The anthropogenic perturbations of the nitrogen cycle are known to affect the terrestrial sources and sinks of greenhouse gases such as carbon dioxide (CO2) and nitrous oxide (N2O). These changes are potentially very important as they may significantly affect the climate system, but their magnitude is still unknown.
“When added to nitrogen-limited ecosystems, it [nitrogen] can stimulate plant growth and/or suppress soil respiration, thereby leading to increased ecosystem carbon storage” explains Sönke Zaehle. However, there are also potentially negative consequences for adding nitrogen to ecosystems, as increasing nitrogen availability may enhance nitrogen losses from ecosystems, and eventually even have damaging effects on plant health. Particularly relevant for climate are elevated emissions of ni-trous oxide, a long-lived greenhouse gas that is emitted from fertilised fields, as well as nitrogen-rich forest and grassland ecosystems.
Drawing on reconstructions of past and present anthropogenic nitrogen deposition and fertiliser applications, Sönke Zaehle and colleagues used a global computer model of the coupled terrestrial carbon and nitrogen cycles to better understand the consequences of this anthropogenic nitrogen perturbation for the climate system. Their results confirm that the anthropogenic nitrogen perturbation has profoundly affected terrestrial carbon dioxide and nitrous oxide fluxes. Human nitrogen additions are the principle cause for the increase in terrestrial nitrous oxide emission since 1960, and contribute to about one fifth of the current global net carbon uptake (1996-2005).Sönke Zaehle and colleagues then determined the effect of anthropogenic nitrogen on the at-mospheric concentrations of the greenhouse gases CO2 and N2O, and assessed the resulting consequences for present-day climate. The key finding is that the climatic effects of the anthropogenic nitrogen perturbation from both gases are very substantial but of opposite signs. The cooling effect due to enhanced carbon uptake of the terrestrial biosphere is more than compensated for by the warming effects from enhanced terrestrial N2O emissions.
Susanne Hermsmeier | Max-Planck-Institut
Further reports about: > CO2 > Geoscience > Illuminating Biomedical Research > N2O > Nature Geoscience > Nature Immunology > anthropogenic nitrogen > carbon dioxide > climate system > grassland ecosystem > greenhouse gas > natural ecosystem > nitrogen cycle > nitrogen deposition > nitrous oxide > terrestrial carbon
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