Volume 14, Number 19: 11 May 2011
In an important new paper, Phillips et al. (2011) write that "in forests with low nitrogen (N) availability, rising atmospheric CO2 can exacerbate N limitation," which phenomenon is often claimed by climate alarmists to gradually erode the initial growth-enhancing benefit of the increased CO2 concentration, as described in what has come to be called the progressive nitrogen limitation hypothesis; but the three researchers note that in such situations atmospheric CO2 enrichment often leads to increased belowground carbon allocation in the form of enhanced fine root production that may enable the trees to access additional N. However, they indicate that "the vast majority of N in forest soils is stored in organic forms requiring depolymerization by microbially derived extracellular enzymes prior to root uptake," citing the work of Schimel and Bennett (2004). And, therefore, they state that "increased root exploration alone is unlikely to sustain plant nitrogen requirements under rising CO2 unless accompanied by the concomitant stimulation of soil microbial activity and the release of nutrients from soil organic matter." But despite the presumed importance of root exudates in this scenario, they say that "no studies have quantified the effects of CO2 enrichment on exudation by mature trees," which is what they thus set out to do, in an effort to better understand why progressive nitrogen limitation has not been observed in some long-term studies of trees growing on nutrient poor soil (Langley et al., 2009; McCarthy et al., 2010).
Working at the Duke Forest FACE facility located within a loblolly pine (Pinus taeda) plantation in Orange County, North Carolina (USA) -- where an extra 11.2 g N/m2/year were applied to half of each nitrogen-poor ambient and CO2-enriched (to 200 ppm above ambient) plot from 2005-2009 -- the three researchers examined plant-microbe interactions in the rhizospheres and bulk soils of the various treatments, measuring differences in rhizosphere microbial activity and root exudation rates. So what did they find?
Phillips et al. report that, on an annual basis, "exudation increased by c. 50% for trees enriched with CO2 in non-fertilized plots," but they say that trees were unaffected in this manner by CO2 enrichment in fertilized plots, in a dramatic demonstration of the fact that "increased root carbon efflux from CO2-enriched trees stimulates rhizosphere N cycling in low fertility soils," providing additional evidence that "rhizosphere microbes such as actinomycetes, which produce NAGase enzymes and respond strongly to CO2 at this site (Billings and Ziegler, 2008), are using energy derived from exudates to synthesize enzymes that release nitrogen from soil organic matter (Cheng and Kuzyakov, 2005)." And they emphasize that "this dramatic contrast between the fertilized and unfertilized treatments provides evidence that enhanced exudation is a mechanism trees employ for increasing nitrogen availability."
In consequence of their experimental findings, Phillips et al. say their study demonstrates that "the enhanced carbon flux from roots to soil in low fertility forests exposed to elevated CO2 creates hotspots for microbial activity that are associated with faster rates of soil organic matter turnover and N cycling," which phenomenon provides the trees the extra nitrogen they need to take full advantage of the enhanced potential for growth that is provided by atmospheric CO2 enrichment, thereby overcoming the incorrect implications of the progressive nitrogen limitation hypothesis. And to make this point perfectly clear, they state that their results "provide field-based empirical support suggesting that sustained growth responses of forests to elevated CO2 in low fertility soils are maintained by enhanced rates of microbial activity and N cycling fueled by inputs of root-derived carbon."
Sherwood, Keith and Craig Idso
References
Billings, S.A. and Ziegler, S.E. 2008. Altered patterns of soil carbon substrate usage and heterotrophic respiration in a pine forest with elevated CO2 and N fertilization. Global Change Biology 14: 1025-1036.
Cheng, W. and Kuzyakov, Y. 2005. Root effects on soil organic matter decomposition. In: Zobel, R. and Wright, S. (Eds.), Roots and Soil Management: Interactions Between Roots and the Soil. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, Madison, Wisconsin, USA, pp. 119-143.
Langley, J.A., McKinley, D.C., Wolf, A.A., Hungate, B.A., Drake, B.G. and Megonigal, J.P. 2009. Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2. Soil Biology and Biochemistry 41: 54-60.
McCarthy, H,.R., Oren, R., Johnsen, K.H., Gallet-Budynek, A., Pritchard, S.G., Cook, C.W., LaDeau, S.L., Jackson, R.B. and Finzi, A.C. 2010. Re-assessment of plant carbon dynamics at the Duke free-air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development. New Phytologist 185: 514-528.
Phillips, R.P., Finzi, A.C. and Bernhardt, E.S. 2011. Enhanced root exudation induces microbial feedbacks to N cycling in a pine forest under long-term CO2 fumigation. Ecology Letters 14: 187-194.
Schimel, J.P. and Bennett, J. 2004. Nitrogen mineralization: challenges of a changing paradigm. Ecology 85: 591-602.