news & views BIOGEOCHEMISTRY
A faulty fertilizer
Elevated levels of CO2 can stimulate photosynthesis in plants and increase their uptake of atmospheric carbon. A five-year study in Minnesota grasslands shows that increased plant uptake of CO2 is restricted by the availability of vital nutrients and water.
Whendee L. Silver
CEDAR CREEK ECOSYSTEM SCIENCE RESERVE
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ncreasing atmospheric CO2 concentrations from fossil-fuel combustion and tropical deforestation are driving rapid changes in the Earth’s climate. Plants could help mitigate rising atmospheric CO2 levels: plants take CO2 from the atmosphere during photosynthesis, and convert it to biomass carbon and soil organic matter. Theoretically, elevated atmospheric CO2 concentrations could act as a fertilizer, increasing plant growth that in turn draws more CO2 from the atmosphere1,2. This CO2 fertilization of plants could potentially serve to offset some of the CO2 emissions from human activities and help slow climate change. Writing in Nature Geoscience, Reich and colleagues3 provide evidence to the contrary — that elevated levels of CO2 alone are unlikely to stimulate increased plant uptake of carbon because other resources in addition to CO2 simultaneously limit plant growth. Whether elevated atmospheric CO2 concentrations can act as a plant fertilizer depends on the availability of a suite of other resources that plants need for survival and growth. When these resources are in short supply, termed resource limitation, plant growth decreases or stops. When only one resource is limiting, an increase in the supply of that resource generally leads to a corresponding increase in growth. When two or more resources are equally lacking, a phenomenon known as multiple resource limitation, plants generally cannot increase growth unless there is an increase in all of the limiting resources. For example, grasslands are frequently simultaneously limited by both water and nitrogen4,5; under these conditions, an increase in water or nitrogen alone will not stimulate growth. This complexity of multiple limiting resources has made it difficult for scientists to predict the potential for CO2 fertilization effects. Most field studies on the impacts of elevated CO2 concentrations have been conducted over short time periods and have been limited to investigations of CO2 alone, or of elevated CO2 and changes in just one other resource. The results of these earlier
Figure 1 | The BioCON experiment in Minnesota, USA. It was originally designed to test the response of plant communities to three environmental changes occurring simultaneously: elevated atmospheric CO2, nitrogen deposition, and declining biodiversity.
experiments were mixed; plant growth increased in some cases, but not in others6–8. A better understanding of how several plant resources may act together to limit plant growth is needed to predict the potential for CO2 fertilization effects. Reich and colleagues3 addressed this gap in understanding with a five-year study of the effects of elevated levels of atmospheric CO2, nitrogen and rainfall on perennial grasslands planted as part of the larger Biodiversity, CO2, and Nitrogen (BioCON) project in Minnesota, USA9. The authors used 48 2 × 2 m plots that had been planted with nine grass species in 1997 (Fig. 1). Starting in 2007, they monitored the impact of different combinations of ambient and elevated CO2 and nitrogen, and ambient and reduced levels of rainfall on plant biomass production. Higher levels of CO2, nitrogen, and rainfall led to more biomass production compared with lower resource levels. However, this occurred only when at least
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one of the other two resources was supplied at a higher rate. The CO2 fertilization effect disappeared under ambient nitrogen and lower rainfall conditions. This means that the grassland plants were not able to take advantage of elevated CO2 concentrations unless another resource (water or nitrogen) was at its higher supply rate. Reich et al. present evidence that the treatment effects were direct, and not a result of indirect factors influencing nutrient availability, water use efficiency, or soil moisture. Understanding the limits of natural ecosystems to absorb extra CO2 must be considered when devising climate change mitigation strategies. However, rising atmospheric CO2 levels are not occurring in isolation. Fossil-fuel combustion and widespread fertilizer use have approximately doubled the amount of nitrogen cycling in the environment 10. Climate change is leading to warmer temperatures globally, more rainfall in some regions, and less in others. 1
news & views How these global and regional changes will influence the CO2 fertilization effect in the future is unclear. These results highlight some promise for increased CO2 uptake in managed lands that often receive nutrient or water amendments. Caution should be used when extrapolating the results from the Minnesotan grasslands to other regions. The grassland biome covers almost 40% of the global land surface and spans a widely diverse range of soil, plant, and climatic characteristics. An analysis of grassland response to elevated CO2 levels in California, USA, for example, showed little evidence for multiple resource limitation and no effect of elevated CO2 concentrations over five years6. In that study, only the addition of nitrogen consistently increased biomass production. The conflicting results across the two studies may have been due to differing plant communities (the Californian grassland was
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dominated by annual plants versus perennial grassland in Minnesota). It is also possible that other resources (possibly phosphorus) limited the ability of the Californian grassland to take advantage of the higher availability of CO2. These different responses to elevated CO2 highlight the need for more research and continued focus on the mechanisms driving CO2 fertilization. Reich and colleagues3 show that the availability of water and nitrogen in grasslands in Minnesota limit the amount of CO2 uptake by plants over a fiveyear period. The results imply that CO2 fertilization alone is unlikely to get us off the hook regarding climate change — we can’t depend upon natural ecosystems to solve the problem of rising atmospheric CO2 concentrations for us. Instead we need to take a more proactive approach. In addition to investigating the drivers of climate change, more effort should be
directed at identifying and testing other land-based solutions, particularly in our managed landscapes. Whendee L. Silver is in the Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720, USA. e-mail:
[email protected] References
1. Norby, R. et al. Proc. Natl Acad. Sci. USA 102, 18052–18056 (2005). 2. Delucia, E. et al. Science 284, 1177–1179 (1999). 3. Reich, P. B., Hobbie, S. E. & Lee, T. D. Nature Geosci. http://dx.doi. org/10.1038/ngeo2284 (2014). 4. Sala, O. E., Parton, W. J., Joyce, L. A. & Lauenroth, W. K. Ecology 69, 40–45 (1998). 5. Hooper D. U. & Johnson, L. Biogeochemistry 46, 247–293 (1999). 6. Dukes, J. S. et al. PLoS Biol. 3, 1829–1837 (2005). 7. Reich, P. B. et al. Nature 440, 922–925 (2006). 8. Morgan, J. A. et al. Nature 476, 202–205 (2011). 9. http://www.biocon.umn.edu 10. Fowler, D. et al. Phil. Trans. R. Soc. B 368, 20130164 (2013).
Published online: 2 November 2014
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