Nitrogen (N) inputs maintain terrestrial carbon sinks, which, in turn, affect the pace and magnitude of global climate change. Since the dawn of the industrial revolution, humans have increased the production of reactive N many fold through fossil fuel combustion and agricultural fertilizer applications. This N has been thought to stimulate carbon dioxide sequestration by fertilizing terrestrial vegetation. Alternatively, theory and observations point to increasing cases of N saturation of ecosystems in response to high N loadings, imparting a cascade of negative consequences including the production of the potent greenhouse gas nitrous oxide and the leaching of nitrates through water pathways, causing widespread human health and environmental risks. For more than two decades, global carbon models have highlighted the carbon uptake potential of anthropogenic nitrogen deposition. But recent studies have raised questions about the impact of weathered N from sedimentary and meta-sedimentary rocks on the global terrestrial carbon and nitrogen cycle. We hypothesize that N inputs via rock weathering fundamentally alters the fate of C x N interactions causing a disproportionate fraction of N deposition to bypass the terrestrial ecosystems. Using a global biogeochemistry model LPJ-GUESS, we investigate hitherto unrecognized C uptake and N saturation capacities of terrestrial ecosystems in response to N inputs via rock weathering, biological fixation and deposition from the mid-1800s to the end of the 21st century.
Results/Conclusions
We demonstrate that rock N inputs have contributed ~2 to 11 times more to modern terrestrial productivity and vegetation C sequestration than N deposition since pre-industrial times. Model simulation results based on the business as usual, RCP8.5 scenario ascribe 2 to 5 times more C uptake to rock N inputs and biological fixation, respectively, than anthropogenic N deposition though 2101. Since nitrogen from rocks partially satisfies baseline nitrogen demands, a disproportionate fraction of anthropogenic deposition inputs lead to nitrogen saturation and nitrogen losses from terrestrial ecosystems without contributing to plant productivity gains. Moreover, considering the undesirable nitrogen loss to the environment, we find rock weathered nitrogen to be more than 30 times more efficient at sequestering carbon than atmospheric deposition. Our findings reveal an alternative to the standard model: curbing anthropogenic nitrogen deposition will not necessarily reduce the terrestrial carbon uptake through 2101. Rather, measures aimed at reducing anthropogenic nitrogen deposition could substantially improve water and air quality and reduce N2O emissions, thereby revealing a win-win path for actions to reduce nitrogen pollution in the environment.