by Bob Henson
What do lightning and fertilized crops have in common? Both play a role in the global nitrogen cycle. See the sidebar for details. (Photos by Carlye Calvin.)
As Earth’s atmosphere warms and fossil-fuel emissions climb, some researchers point to the possibility of more rapid plant growth as a potential silver (or green) lining. With more carbon dioxide in the air and fewer areas prone to freezing, one might expect agriculture to thrive and many natural ecosystems to prosper.
The future may not unfold so simply, though. Plants need more than CO2 to thrive. Changes in temperature and precipitation—their average values and variability—influence the course of the biosphere as well. There’s also the matter of nutrients. Chief among those is nitrogen, the cornerstone of most fertilizers as well as a byproduct of burning fossil fuels.
Will there be enough nitrogen in usable forms to support a boom in plant growth, one that might help reduce the global rise in atmospheric CO2? A subset of climate modelers, faced with questions like this one for years, is gradually assembling the tools needed to provide answers. Recent progress in linking ocean, atmosphere, and land through interactive global models has paved the way for including nitrogen, carbon, and other aspects of air chemistry. The feedbacks are hugely complex and the work is painstaking, but the researchers are getting glimmers of insight.
On the rise—but where?
“Human inputs have clearly altered the biogeochemistry of N [nitrogen] on a global basis,” notes the U.S. Nitrogen Science Plan (see On the Web). It adds, “Among the cycles of elements important to life, the N cycle has been the most perturbed beyond its natural state by human activities.”
At first blush, it’s easy to see what’s happening with atmospheric nitrogen: there’s more of it. Four out of five molecules in the air are made up of inert nitrogen gas (N2). The far smaller amounts of reactive nitrogen are what make a difference, though. Most of this nitrogen enters the air from fertilizers and industrial emissions as nitrogen oxides (NOx) and ammonia (NH3) in roughly equal amounts. About a third comes from legume and rice crops, whose acreage is growing worldwide.
Together, these sources boosted reactive nitrogen to more than tenfold its 1950 level by 2000. Fertilizers are the main culprit, responsible for roughly half of the increase. Though they’re designed to enrich the soil, fertilizers end up losing most of their nitrogen to the atmosphere. “It’s a relatively inefficient process,” says ecologist Peter Thornton, one of NCAR’s specialists in nitrogen modeling.
Where does that extra nitrogen go? It’s a question Elisabeth Holland addressed that question in a recent paper in Ecological Applications. Holland, the head of NCAR’s biogeochemistry initiative, joined colleagues at the University of New Hampshire to study parts of the nitrogen budget in the United States and Europe.
Since much reactive nitrogen falls to Earth in rain or snow (see sidebar), the prevailing flow of the atmosphere may be carrying nitrogen from North America eastward. Holland and colleagues found that between 1978 and 1994, only about 40% of the reactive nitrogen produced by the United States ended up being deposited there. Western Europe received nearly five times the nitrogen deposition per unit area than the U.S. average.
“According to our study, the United States is exporting nitrogen,” says Holland, “and there’s some evidence that a portion may end up in Western Europe.”
A growing group of modelers
NCAR's specialists in nitrogen and carbon modeling include (left to right) Elisabeth Holland, Jean-François Lamarque, and Peter Thornton. (Photos by Carlye Calvin.)
More scientists are joining Holland on the N-deposition trail. NCAR’s Jean-François Lamarque is working with 10 U.S. and European labs on a six-model study of nitrogen cycling in support of the next Intergovernmental Panel on Climate Change (IPCC) assessment. The team formed at a 2003 workshop organized by Lamarque and NCAR colleague Jeffrey Kiehl. They dubbed the collaboration that emerged SANTA FE (Scientific Analysis of Nitrogen cycle Towards Atmospheric Forcing Estimation).
“We decided that looking at nitrogen deposition and how that would influence the carbon cycle was an interesting new question that hadn’t really been addressed,” says Lamarque. At the time of the first SANTA FE meeting, none of the vegetation models linked to major global climate models could handle nitrogen cycling. The SANTA FE researchers have met several more times, comparing notes on how their embryonic nitrogen models are doing. “By looking at a variety of models and a variety of climates, it gives a feel for how uncertain those numbers are,” says Lamarque.
In findings soon to be published, the team estimates that the average amount of nitrogen deposited over the planet’s land areas could more than double by from the present day to the year 2100 over present-day values. According to Lamarque, most of this appears to be due to the sheer rise in nitrogen emissions.
Also feeding into the next IPCC report will be a separate cross-model study led by Frank Dentener (European Commission Institute for Environment and Sustainability) that analyzes nitrogen deposition from nitrogen oxides and ammonia. Like the SANTA FE team, Dentener is finding that emissions are the main driver behind the rise in nitrogen deposition projected for 2030. However, he says that climate change can have some feedback on deposition, depending on how it affects rainfall and vegetation (see sidebar) and how it influences the rate at which reactive nitrogen is produced by lightning and by bacteria in the soil.
Even as they peer into nitrogen’s future, the leaders of both of these model-comparison projects are keenly aware that their picture of reactive nitrogen in the present isn’t complete, either. One of their biggest concerns is the scant data on regional and temporal differences in both emissions and deposition. It’s the first of many critical questions cited in the 2004 U.S. Nitrogen Plan. “Little is known about emission factors of ammonia outside Europe and North America,” says Dentener.
When C meets N
NCAR’s Thornton is digging still deeper into the nitrogen-climate connection. He’s getting his hands dirty—virtually if not literally—by examining the feedbacks that involve both carbon and nitrogen exchange among soils, microbes, flora, and the atmosphere.
An offshoot of the SANTA FE project, Thornton’s nitrogen model
is the first to be linked interactively to a global climate model.
For now, it’s a one-way linkage. Thornton is using the nitrogen
deposition data that emerges from Lamarque’s atmospheric chemistry
model, an experimental component of the NCAR-based Community Climate
System Model. In short, nitrogen goes from the CCSM into Thornton’s
soil/flora model, but it doesn’t return to the CCSM. Getting
the biosphere and chemistry components to interact fully is a very
expensive step, says Thornton.
Still, there’s much to be learned from the current arrangement. “Our
focus is on how the land responds to the atmosphere with respect
to nitrogen, rather than the other way around,” says Thornton.
Because there’s a strong and rapid cycling of nitrogen between
soil and plants, Thornton’s model can track that cycle as changes
in the atmosphere unfold.
New work at NCAR shows that the regional patterns of nitrogen demand and deposition maximize the potential impact of global nitrogen increases on climate. (Illustrations courtesy Peter Thornton and Jean-François Lamarque.)
(Click on the model images for larger versions of each.)
This illustration is the annual nitrogen availability (proportion of annual demand) projected by the new carbon-nitrogen coupling capability of the Community Climate System Model. Pale blue, for instance, denotes areas where only around 75% of the nitrogen that plants could use is available.
This illustration is nitrogen deposition (teragrams per square meter) projected by NCAR’s atmospheric chemistry model, coupled to the Community Atmosphere Model, for the year 2100, based on the IPCC’s A2 emissions scenario. Areas in orange and red show the largest increases in deposition. These largely coincide with those land areas shown at left where plant growth is most strongly limited by nitrogen, such as eastern North America, Europe, and southern Asia.
The implications of depicting nitrogen in global climate models could be substantial. One of the main brakes on CO2 accumulation in the atmosphere is uptake from plants. Not only do warmer temperatures speed up much plant growth, but they also hasten the decomposition of organic matter in the soil, which pumps CO2 back into the atmosphere. Thus, even as plants are pulling greater amounts of carbon dioxide from the air, microbes are returning even more, producing a net addition of CO2 to the air.
Even through nitrogen has ill effects as a pollutant, it can help nourish plants that remove CO2 from the air. Up to now, the global models examined by IPCC haven’t considered whether a lack of nitrogen could hinder plant growth. A 2003 paper in Science by Bruce Hungate (Northern Arizona University), with colleagues at the Carnegie Institution of Washington and the University of Oklahoma, estimated that even a rise in global nitrogen deposition to current U.S. levels wouldn’t be enough to support the increased plant growth produced in most global climate models.
Thornton’s own modeling work adds another twist to the story.
Soil microbes convert nitrogen into the mineral forms that plants
use. By including this process in his model, and watching it intensify
as temperatures rise, Thornton discovered that the additional nitrogen
could enhance plant growth enough to partially or completely offset
the net CO2 emissions from
the biosphere as climate warms. For at least one ecosystem—a
temperate deciduous forest—the addition of nitrogen to the
model appears to produce a net carbon sink instead of a source.
“That doesn’t mean the CO2 concentration in the atmosphere goes down by any means,” Thornton notes. “It means it’s not going up as fast.”
Whether this finding will hold true on a global basis remains to be seen. Thornton and colleagues are doing as much modeling as possible in time for the next IPCC report, after which they’ll consider the many other limits—both nutritional and climatological—that they’d like to weave into coupled climate-chemistry models.
“In order to get the best possible answer, you’ve got to have N and all the other limiting nutrients,” says Thornton. Phosphorus, he adds, is “probably the next thing to tackle. Then there’s sulfur and lots of others. But N gets you at least part of the way to the right answer.”