New Study Finds Nitrogen Availability Affects the Evolution of Future Carbon Sinks in Northern Eurasia
Writer(s): Logan Judy
A new study coauthored by an EAPS professor has yielded an interesting result – enhanced nitrogen availability from permafrost degradation, the abandonment of agricultural land, and atmospheric nitrogen deposition may account for 30-50% of the carbon sink projected for Northern Eurasia in the future. These factors also play a large role on how the distribution of carbon sources and sinks within the region evolve over time.
The study, coauthored by EAPS Professor and University Faculty Scholar Dr. Qianlai Zhuang, and published in Nature Communications, analyzed the potential impact of future nitrogen availability on carbon sequestration in Northern Eurasia. Nitrogen is an important nutrient for plant growth, and more vegetation, particularly trees, is one key to increasing carbon sequestration. Nitrogen’s importance is not limited to the atmosphere – in the Arctic region, for example, Nitrogen has been shown to be more limited in the soil than in the atmosphere.
This project is part of the NASA Land-Cover/Land-Use Change (LCLUC) Program, using several decades of satellite data to map the globe in detail, with different projects covering different regions. Dr. Zhuang is the primary investigator of the project investigating the Northern Eurasia region. The first phase of the project was spent analyzing the historical data over a period of four to five years. Equipped with that information, the team is now turning its sights to the future.
“We want to know, ‘Will this region be a carbon sink or a carbon source?’” Dr. Zhuang said. “Nitrogen can play a big role in that. So we looked at how nitrogen availability has changed, and how that change will affect future carbon activities.”
Nitrogen availability is impacted by a variety of factors, such as atmospheric nitrogen deposition from fossil fuel combustion, the application of nitrogen fertilizers, and decomposition of organic matter newly exposed by thawing permafrost. Predicting these outcomes can be complicated. As permafrost thaws, enhanced decomposition of soil organic matter increases the release of carbon dioxide, but the associated release of Nitrogen during decomposition may enhance later carbon sequestration in vegetation, particularly in trees. According to the model used in the study, thawing permafrost in the early part of the 21st century diminished carbon sequestration, but the enhanced Nitrogen helped forests recover from timber harvest more rapidly to enhance carbon sequestration later in the century.
Because organic matter is constantly being removed from agricultural ecosystems, nitrogen availability, including enhanced availability from the application of nitrogen fertilizers, has little effect on carbon sequestration in these ecosystems. With the abandonment of agricultural land, however, the continued decomposition of organic matter on this land provides a source of nitrogen not normally available to natural vegetation to enhance carbon sequestration. The study found that the application of fertilizers to croplands left a legacy that enhanced the loss of carbon from organic matter decomposition immediately after abandonment, but increased carbon sequestration in the regrowing trees later. The asynchronous timing of carbon gained by trees from the enhanced nitrogen availability versus, the associated carbon loss from soils, both from permafrost degradation and the abandonment of agricultural land, causes the size and distribution of important carbon sources and sinks within the region to evolve over time.
All things considered, the study finds that the region is predicted overall to continue to be a carbon sink in the absence of significant changes in the occurrence and severity of wildfires. The relative importance of permafrost degradation, agricultural land abandonment, and atmospheric nitrogen deposition on carbon sequestration in Northern Eurasia, however, depends on the climate and land-use policies being implemented.
In their model, the researchers considered both a “business-as-usual” scenario and a scenario in which the climate stabilized. Permafrost degradation had a larger effect on carbon sequestration in trees than atmospheric nitrogen deposition or agricultural land abandonment under a “business-as-usual” scenario. In contrast, abandonment of agricultural land had a much larger effect on carbon sequestration in trees than permafrost degradation or atmospheric nitrogen deposition under a climate stabilization scenario. In both cases, forests currently underlain by permafrost are projected to account for about twice their current share of the region’s annual carbon sink by the end of the 21st century.
This research is supported by NASA in collaboration with the Marine Biological Laboratory’s Ecosystems Center, MIT’s Joint Program on the Science and Policy of Global Change, and the University of California-Davis.