An article in the April 13 issue of Nature, “Nitrogen limitations constrains sustainability of ecosystem response to CO2” (subscription required) reports on a six-year study of the role of nitrogen on biomass accumulation when CO2 concentrations increase.

Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO2. Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation, soil N supply is probably an important constraint on global terrestrial responses to elevated CO2.

This seems pretty important, since one of the arguments you hear from global warming optimists is that increased CO2 will result in increased biomass, which enables more carbon sequestration and may enhance agricultural output.

Although some models indicate a considerable capacity of land ecosystems to sequester large amounts of C in the coming century, this C accumulation is likely to be constrained over time by N availability … Estimating the role of terrestrial ecosystems as current and future carbon sinks hinges on the ability to understand the biogeochemical consequences of variability in soil N limitations and N deposition rates worldwide.

Those of you who understand the various overlapping sciences here, help me out. Is this another feedback loop we need to pay more attention to? As limits in natural nitrogen levels put the brakes on biomass accumulation, natural CO2 sequestration processes may also slow, and thus atmospheric CO2 buildup may accelerate.

Yikes! Seems like yet another way the climate may do an end run around us.