Volume 9, Number 41: 11 October 2006
In the five journal reviews of the 19 Apr 2006 issue of CO2Science, we highlighted a number of studies that tend to repudiate the progressive nitrogen limitation hypothesis, which posits that insufficient quantities of soil nitrogen will ultimately curtail the ability of the aerial fertilization effect of rising atmospheric CO2 concentrations to indefinitely maintain increased plant growth rates and carbon sequestration rates in plants and soils. In the last of those five journal reviews, and in light of what was reported in them, we concluded that "as environmental and biological conditions change over time in ways that allow for greater rates of primary production and enhanced carbon inputs to soils, ecosystems experiencing such changes somehow find ways to bolster soil nitrogen inputs to accommodate the extra carbon." Since then, a new paper has appeared that provides important new evidence that supports that conclusion.
Working in an annual grassland at the Jasper Ridge Biological Preserve in the interior foothills of the central coast range south of San Francisco, California, USA, Zavaleta and Kettley (2006) examined patterns of production, standing biomass, carbon and nitrogen storage, community composition, and soil moisture along a 25-year chronosequence of sites that were in various stages of invasion by the woody shrub Baccharis pilularis. In doing so, they sought evidence for the hypothesis that "Baccharis-invaded sites would experience increasing nitrogen limitation as nitrogen was immobilized in biomass and litter," and that this phenomenon would preclude further increases in ecosystem biomass. At the conclusion of their study, however, they learned something quite different.
In progressing from initial grassland conditions to conditions that prevailed 25 years after shrub invasion began, the two researchers found that "net increases in biomass and tissue and soil C:N [carbon to nitrogen ratio] contributed to increases in total ecosystem carbon storage of over 125%." Even more stunning was their discovery that the increases in ecosystem biomass "drove increases in ecosystem nitrogen sequestration of ~700%." In terms of the soil alone, nitrogen content also "increased rapidly with shrub age," since the increase in soil nitrogen "was much larger than the increase in nitrogen immobilization in biomass and litter over time."
What was the source of the extra nitrogen? From whence did it come? The researchers mentioned several possibilities, but could not be sure of them. Nevertheless, their real-world observations clearly repudiate the myopic view, long held by many, that a soil of low initial nitrogen status must constrain long-term positive ecosystem responses to biomass-enhancing phenomena, such as woody plant invasions and atmospheric CO2 enrichment. Apparently, nature is smarter (and wiser!) than all of the nay-sayers.
In further discussing this aspect of the issue, Zavaleta and Kettley say that "while many climate models now incorporate the effects of short-term energy and resource exchanges between the atmosphere and the biosphere, most do not consider feedbacks associated with long-term vegetation changes." In this regard, they say their findings "illustrate the potential for important vegetation-mediated ecosystem responses and feedbacks to atmospheric CO2 and climate change," while additionally noting that "many of the changes [they] observed were progressive and did not saturate with time."
Truly, we can expect much more from the aerial fertilization effect of atmospheric CO2 enrichment than most climate alarmists and biological pessimists have been willing to acknowledge ... and for a much longer period of time.
Sherwood, Keith and Craig Idso
Reference
Zavaleta, E.S. and Kettley, L.S. 2006. Ecosystem change along a woody invasion chronosequence in a California grassland. Journal of Arid Environments 66: 290-306.