I sometimes think that degrees are prizes found in boxes of high fructose children's breakfast cereal. This paper makes me suspicious.

Plants apparently do much less than previously thought to counteract global warming, according to a paper to be published in next week's online edition of Proceedings of the National Academy of Sciences.

The authors, including Bruce Hungate of Northern Arizona University and lead author Kees-Jan van Groenigen of UC Davis, discovered that plants are limited in their impact on global warming because of their dependence on nitrogen and other trace elements. These elements are essential to photosynthesis, whereby plants remove carbon dioxide, a greenhouse gas, from the air and transfer carbon back into the soil.

"What our paper shows is that in order for soils to lock away more carbon as carbon dioxide rises, there has to be quite a bit of extra nitrogen available--far more than what is normally available in most ecosystems," said Hungate of NAU's Merriam-Powell Center for Environmental Research.

The paper notes that various plants can pump nitrogen from the air into soils, and some researchers expected rising carbon dioxide to speed up this natural nitrogen pump, providing the nitrogen needed to store soil carbon. However, the research team found that this process, called nitrogen fixation, cannot keep up with increasing carbon dioxide unless other essential nutrients, such as potassium, phosphorus and molybdenum, are added as fertilizers.

But, it's not that simple. Many previous studies have established that while there are limits that they change over time as ecologies adapt to increased CO2 levels.

One part of that is that as soil carbon increases less nutrients are needed for equivalent benefit beacuse soil chemistry is improved. More plants can grow with the exisitng nutrients.

Another factor is that soil organisms adapt as CO2 slowly increases to function in that new environment. To see this the experiments must be long term and mimic the slow rise. It's difficult since they don't have time to do it at a realistic pace, but they can get some idea of the process and do some guesswork.

Humans only account for about 5% of the carbon in the biosphere. It seems doubtful that this is enough to stress adaptive mechanisms. Still, there are always limits and probably not very many localities where CO2 is the limiting nutrient while all others are avaliable in abundance. There are some, but this isn't the general case.

This can be the case for arable land in production since nutrients are explicity added. Past practices were often insensitive to soil carbon - just tilling the soil causes the loss of huge amounts - so there is great potential for carbon sequestration in crop lands. Carbon lost in the past can be recaptured, and this will improve the soil too. It will grow more plants with less added nutrients.

Update:

More about the complexity of evaluating plant response to elevated CO2.

His research, which focused on the regrowth of the plant, reveals that alfalfa grows more with elevated concentrations of carbon dioxide (CO2), in particular when this condition coincides with high temperatures. The effects can be affected by other variables, such as the availability of water in the soil, which would reduce its growth and can modify its response to CO2. In addition, in the study it was confirmed that the process of photosynthesis can be stimulated or reduced by CO2, depending on the growth phase of the plant.

As this study highlighted, one of the most interesting aspects of this type of plant is the increase in nutrient storage in the roots, especially of proteins, when the plant is periodically cut back. These nutrient reserves contribute to rapid regrowth and to maintaining the perenniality of this crop. Similarly, it has been shown that a moderately dry climate maintained over time favors the accumulation of these reserve proteins, which can stimulate the growth of the plants during the following regrowth.

The results show the great variability of plant response to increases in CO2. Thus, a greater availability of CO2, which in principle should stimulate growth through increase photosynthesis, when it interacts with other variables such as the temperature or availability of water, can modify significantly the response of the alfalfa, depending on its stage of growth.