Previous studies have documented several ways that C3 and C4 grasses differ with respect to their impacts on ecosystem properties. To further this understanding, we designed a field experiment to address the broad objective of identifying and quantifying C3-C4 differences in belowground carbon-cycling processes, based on measurements of stable isotopes of C. The experiment contained four linear 15-m × 90-m blocks, half of which were planted to C3 grasses and the other half to C4s, in an otherwise uniform environment in central Iowa. Over three growing seasons we undertook regular sampling, measured soil CO2 fluxes, and applied within-plot manipulations to segregate different CO2 sources. Whereas root-free soil chambers, 75 cm deep, were installed to assess the δ13CO2 of soil organic matter decay, sampled grasses served to assess the δ13C of root-respired CO2. The formerly cropped land had soil emissions with δ13CO2 values that averaged -20.8 ‰, which were between those of the C3 and C4 grasses, at ‑27.5 and ‑12.1 ‰, respectively.
Results/Conclusions
Over three years of study we observed consistent seasonal patterns in the δ13C values of CO2 collected from the soil chambers installed in year 1 of the study. In each year, samples collected after soil thawing, in April and early May, had values that were greater than -20 ‰, which were greater than those observed later in the growing season, until October. Less discrimination at cooler soil temperatures has previously been observed for soil CO2 emissions. At our site, this was also true in root-free chambers that were installed in years 2 and 3, but only in C3 plots, not from C4-influenced soils. We also observed a significant relationship between δ13C and soil temperature in chambers with intact roots, but only in C3 and not in C4 plots. The δ13C of soil-derived CO2 is temperature dependent in C3 but not C4 grasslands, at our site. This finding is consistent with a lower temperature sensitivity for decomposition of more labile organic matter.