Carbon Dynamics and Estimates of Primary Production by Harvest, ^(14)C Dilution, and ^(14)C Turnover

Large plots of native shortgrass steppe were labeled with 1 4 C 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 1 4 C dilution, 1 4 C turnover, and traditional harvest methods. Stabilization of labile 1 4 C via translocation, incorporation into structural tissue, and respiration and exudation required one growing season. Exudation was 17% of plant 1 4 C after stabilization. Estimates of turnover time for leaves, crowns, and roots by 1 4 C 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 1 4 C 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. 1 4 C turnover appeared to provide reliable estimates of aboveground, crown, and root production. In contrast to reliable estimates by 1 4 C turnover, 1 4 C dilution estimates of root production were anomalous. The anomalous estimates were attributed to a nonuniform labeling of tissue age classes resulting in differential decomposition/herbivory of 1 4 C: 1 2 C through time, as well as movement and loss of labile 1 4 C 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 1 4 C dilution were unrealistic. 1 4 C turnover methodology provided reliable estimates of production in this community.