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Title: SHORT- AND MEDIUM-TERM CARBON AND NITROGEN DYNAMICS IN A GRASSLAND EXPOSED TO PAST AND FUTURE ATMOSPHERIC CO2

Author
item GILL, RICHARD - WASHINGTON STATE UNIV
item Polley, Herbert
item Johnson, Hyrum
item JACKSON, ROBERT - DUKE UNIVERSITY

Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 7/6/2003
Publication Date: 11/15/2003
Citation: Gill, R.A., Polley, H.W., Johnson, H.B., Jackson, R.B. 2003. Short- and medium-term carbon and nitrogen dynamics in a grassland exposed to past and future atmospheric CO2 [abstract]. Ecological Society of America Abstracts. p. 122.

Interpretive Summary:

Technical Abstract: Many important ecosystem processes have been influenced by historical increases in atmospheric CO2 which may lead to carbon sequestration in terrestrial ecosystems. Most plants exhibit increased C assimilation in response to experimentally doubled carbon dioxide, but it is unclear whether increased production can be maintained long-term because of decreases in mineral nutrient availability. It is also unclear whether increases in photosynthesis will result in ecosystem sequestration of C in soil pools with turnover times of decades to centuries. Our field experiment in an intact C3/C4 grassland in central Texas is unique in providing a continuous gradient of atmospheric CO2 from 200 to 550 ppm, allowing us to examine critical threshold and nonlinear responses to past, present, and future atmospheric CO2. Along this continuous gradient, increased CO2 promoted higher rates of decomposition in lab incubations for green, aboveground tissue from the dominant C3 forb but had no significant effect on decomposition rates of the dominant C4 grass. We found a positive, linear increase in the storage of C in fractions of soil organic matter (SOM) with decadal scale residence time in soil, with the SOM pools with the shortest residence time being most sensitive to changes in atmospheric CO2. However, the increases in soil C with decadal scale residence time were offset by losses in older soil C under elevated CO2, leading to no significant increases in total soil organic matter. We also suspect that the previously reported decrease in soil N availability with rising CO2 may be explained by higher rates of N immobilization during the decomposition of the dominant C3 forb.