With concerns over global climate change growing, scientists have commenced research on all possible effects on different ecosystems. Richard Phillips, assistant professors of biology at Indiana University, studies the effects of the rising atmospheric carbon concentration on woody forests, and how nitrogen in the soil limits plant growth.

On Friday, Sept. 18, during his seminar on the nitrogen cycle and CO₂ in forests, Phillips stated that, “most studies see a strong trend toward an increase in [plant] biomass under elevated CO₂.” As the concentration of carbon dioxide in the atmosphere increases, more CO₂ will be available to plants for the process of photosynthesis. With increased rates of photosynthesis, the vegetation is then able to fix more carbon into a sugar form, C₆H₁₂O₆, that the plant burns for energy to promote growth. With the increased availability of carbon due to atmospheric loading, Phillips asked what resources will be the limiting factors for the increase of vegetative biomass.

In plant care, the widely accepted routine consists of water, wind and sunlight. However, this fails to acknowledge a crucial factor in plant growth and a limiting factor in the increased rate of biomass growth under elevated levels of CO₂: the nitrogen cycle.

“The early predictions were that nitrogen would be a strong predictor of when and if the biomass growth rate would plateau in terms of CO₂ enrichment, and that the potential for negative feedbacks would actually lower that particular level as well. So understanding the relationship between nitrogen and the way that it mediated the CO₂ response is essential to understanding the total productivity of the ecosystem,” Phillips said.

Woody forests lack a gardener that might provide N-enriched fertilizer, so their nutrient availability is dependent on the microbial productivity in the soil. However, according to Phillips, since “there was very little change in mobilization and mineralization … there were other methods through which trees were accessing nitrogen.”

With sources of nitrogen being relatively constant within the soil, Phillips sought to determine whether plants with CO₂-accelerated growth had found a method to increase nutrient availability in the soil. He postulated that perhaps the plants were trading stored carbon for nitrogen with the rhizosphere — a network of nutrients and living organisms within the vicinity of the plant’s root system.

To test their hypothesis, Phillips and his team at Indiana University looked to the carbon that is stored underground in the roots of the plants. This carbon is released into the rhizosphere through a process known as exudation, which differs from the way that carbon is released during decomposition in the organic compounds it uses. To test the patterns of exudation and its effect on the microbial and enzyme activity in the soil, Phillips used free-air CO₂ enrichment technology and fertilization to model different scenarios.

The results of the study suggested that carbon exudation in forests exposed to elevated CO₂ accelerates the rate of nitrogen cycling in soil. In nutrient poor soils, carbon exudation rates were greater, which significantly increased microbial activity and nitrogen mineralization. There were no significant results in nutrient rich soil. Therefore, when nitrogen is abundant and not a limiting factor in vegetative growth, less exudation is necessary to stimulate nitrogen cycling to increase abundance.

By minimizing the limiting factor on plant growth, trees are more readily able to grow more rapidly through the uptake of additional CO₂. Ultimately, this innovative process of trading through exudation enables the vegetation to grow faster and decrease the concentration of CO₂ in the atmosphere. The researchers hope this could be a driving force for ecosystem health, as well as a small deterrent against global climate change.