A New Model Search for the "Missing Sink" of Anthropogenic CO2
Esser, G., Kattge, J. and Sakalli, A. 2011. Feedback of carbon and nitrogen cycles enhances carbon sequestration in the terrestrial bioshere. Global Change Biology 17: 819-842.
In an attempt to hone in a little better on where the missing carbon might be, the three scientists note that efforts to explain the missing sink in recent years have included the role of nitrogen as an important constraint for biospheric carbon fluxes. And, hence, they used the Nitrogen Carbon Interaction Model (NCIM) of Esser (2007) "to investigate the carbon and nitrogen storage in the terrestrial biosphere, as influenced by the rising atmospheric CO2 concentration together with different combinations of the minor nitrogen exchange fluxes," concentrating on two different time intervals: the historical period of 1860-2002 and the projected future period of 2002 to 2100.
In pursuing this course of action, Esser et al. found that nitrogen fertilization of the biosphere in the absence of an increase in the air's CO2 concentration "would result in only minor additional carbon accumulation in plant biomass," while rising CO2 alone, without consideration of the nitrogen cycle, would bind roughly half of the carbon in the postulated carbon sink. And in the most realistic situation of all, they determined that "a complete ensemble of rising atmospheric CO2 and N2 fixation, denitrification, and leaching is necessary to achieve the 160 Pg C bound in the terrestrial biosphere between 1860 and 2002 as required by the missing sink concept."
A challenge to these results of their modeling efforts, however, is raised by the progressive nitrogen limitation hypothesis, which posits that low concentrations of soil nitrogen will curtail the ability of the productivity-enhancing effect of atmospheric CO2 enrichment to maintain increased plant growth and ecosystem carbon sequestration rates over the long term, which phenomenon is not observed in the NCIM. Esser et al.'s response is that the naturally slow rise of the air's CO2 content in the model simulations is responsible for the model's success, because in contrast to the sudden huge increase in the atmosphere's CO2 content that occurs in free-air CO2 enrichment and open-top chamber experiments (and that requires large amounts of nitrogen in order to fully capitalize upon the aerial fertilization effect of that instantaneous CO2 increase), "the naturally slow rise of CO2 used in the model experiments" does not have this immediate need for huge amounts of nitrogen. Thus, there is a very good chance that the aerial fertilization effect of atmospheric CO2 enrichment may well be doing a whole lot more for the planet in terms of sequestering carbon than what most people have long assumed.