Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/26/2011
Publication Date: 2/12/2012
Publication URL: http://handle.nal.usda.gov/10113/57474
Citation: Fassbinder, J.J., Griffis, T.J., Baker, J.M. 2012. Interannual, seasonal, and diel variability in the carbon isotope composition of respiration in a C3/C4 agricultural ecosystem. Agricultural and Forest Meteorology. 153:144-153. Interpretive Summary: In carbon cycle research it is often desirable to partition ecosystem respiration into its autotrophic and heterotrophic components. Recently developed tunable diode laser spectroscopy systems (TDLAS) can help to do this by separately measuring the release of the two stable isotopes of CO2, 12- and 13-CO2. C4 crops like corn exhibit less discrimination against the heavier 13-CO2 than C3 crops like soybean and most other plants. This can be exploited when corn is planted in soils that primarily developed under C3 vegetation. We measured the exchange of both forms of CO2 in such a system using a TDLAS. We found that the contribution of root (autotrophic) respiration to total soil respiration increased linearly during the early growing season, reaching a peak of 55%. We also found diurnal variability in the total isotope composition of respiration, suggesting either different temperature sensitivities for autotrophic and heterotrophic respiration, or different source depths. This method should prove useful in further carbon balance studies, particularly in developing management strategies to minimize heterotrophic respiration.
Technical Abstract: The stable carbon isotope ratio 13CO2/12CO2 is a valuable tool for understanding the processes controlling the autotrophic (FRa) and heterotrophic (FRh) contributions to ecosystem respiration (FR) as well as influences of photosynthesis on respiration. There is increasing interest in the temporal variability of the carbon isotope composition of ecosystem respiration (dR) and its potential influence on ecosystem flux partitioning based on isotope methods. In this study, an automated chamber system was combined with stable carbon isotope techniques (chamber-TDL method) to examine the main factors controlling variability in FR and dR on the seasonal and diurnal time scales. Results indicated that root respiration linearly increased from early to peak growth of a C4 (corn) growing season, accounting for up to 55% of the total soil respiration (FRs). In addition, dR was consistently 1 to 4 permil more enriched than nightly averages of the isotope composition of soil respiration (dRs) during the same time period. The relatively enriched dR signal indicates the strong influence of above-ground C4 respiration on the total isotope composition of ecosystem respiration. The data and analyses also revealed a strong diurnal pattern in dRs. During peak corn growth, the diurnal pattern of dRs varied by as much as 2 permil and was strongly influenced by the isotope composition of root respiration. In addition, microbial consumption of root exudates may have also affected diurnal variability in dRs. Finally, this study examined the potential influence of turbulent mixing on chamber measurement of dRs.