CARBON DYNAMICS AND ESTIMATES OF PRIMARY PRODUCTION BY HARVEST, C-14 DILUTION, AND C-14 TURNOVER

Large plots of native shortgrass steppe were labeled with C-14 to assess short-term patterns of carbon allocation and the long-term process of herbivory, death, and decomposition, and to compare estimates of net aboveground, crown, and root primary production using C-14 dilution, C-14 turnover, and traditional harvest methods. Stabilization of labile C-14 via translocation, incorporation into structural tissue, and respiration and exudation required one growing season. Exudation was 17% of plant C-14 after stabilization. Estimates of turnover time for leaves, crowns, and roots by C-14 turnover were 3, 5, and 8 yr, respectively, yielding estimates of belowground production that were much lower than previously thought. Estimates of aboveground production by C-14 turnover were close to those obtained by harvest of peak-standing crop, but lower than reported values obtained by harvest maxima-minima. Estimates of root production by harvest maxima-minima were zero in 2 of 4 yr. C-14 turnover appeared to provide reliable estimates of aboveground, crown, and root production. In contrast to reliable estimates by C-14 turnover, C-14 dilution estimates of root production were anomalous. The anomalous estimates were attributed to a nonuniform labeling of tissue age classes resulting in differential decompostion/herbivory of C-14: C-12 through time, as well as movement and loss of labile C-14 through the first growing season. Isotope-dilution methodologies may be unreliable for any estimate of pool turnover when the labeling period is not as long as pool-turnover time. Problems and biases associated with traditional harvest maxima-minima methods of estimating aboveground primary production are well known, but are greatly exacerbated when the method is used to estimate root production. Estimates of root production by C-14 dilution were unrealistic. C-14 turnover methodology provided reliable estimates of production in this community.