The Use of Natural Abundance of 13 CO 2 to Determine Soil Respiration Components in an Agro-Ecosystem a School of Environmental Sciences, University of Guelph; b Agriculture and Agri-Food Canada, Québec City Introduction Methods Non-steady state chambers are used extensively in experiments to measure soil CO 2 flux. The isotope value of soil respiration (δ 13 C R ) can be used to estimate the contribution of soil organic matter, root respiration and crop residue to CO 2 flux. Tracking these components throughout the day and growing season will provide valuable information for soil C modelling and understanding of how different soil processes respond to environmental change. OBJECTIVES: To study the seasonal and diurnal variations of δ 13 C R for corn grown on a calcareous soil. To estimate the contribution of each respiring component, SOM, residue and corn roots, to the total soil respiration. Site History Results Location: Elora Research Station Soil type: London Loam Bedrock: Limestone pH: 7.8 Total C: 27.3 mg g -1 Organic C: 19.2 mg g -1 Inorganic C: 8.1 mg g -1 δ 13 C SOM (treated with HCl; i.e. without carbonates): -25.4‰ Fig. 1: The picture depicts chamber air being sampled in the bare soil plot at the Elora research station in δ 13 C soil (not treated with HCl; i.e. with carbonates): -20.8‰ Conclusion References Fig 6: δ 13 C R of the C-flux in bare soil and in the field with and without corn plants, with δ 13 C of substrates (dashed lines) Diurnal Seasonal Chambers were deployed for a total of 18 min per sampling period. During each sampling period 4 air samples were collected for CO 2 concentration analysis by gas chromatography. At the start and end of the sampling period air samples were taken to determine δ 13 C by mass spectrometry. A simple two member mass balance approach was used to calculate the δ 13 C R. These three plots were used to partition soil respiration into: SOM, residue and corn roots (Fig. 3). For seasonal C-flux and δ 13 C R trends, sampling occurred at 11:00h in June to July, and in October. Further seasonal measurements were conducted on P+R+SOM plots only throughout June to October at various times during daylight hours. Diurnal sampling occurred four times a day (2:00h, 8:00h, 14:00h and 20:00h) during late corn anthesis (August 4th – 13th). In 2008 there was a high frequency of precipitation events and below normal temperature during the growing season especially during corn anthesis (Fig. 4a and 4b). High soil water content (0.34 m 3 m -3 ) during August could have caused poor gas diffusion limiting respiration (Rochette, Flanagan and Gregorich. 1999, Nickerson and Risk 2009). Below are the trends observed during seasonal and diurnal sampling. Fig. 3: Layout of the plots used for diurnal and seasonal measurements of the C-flux and δ 13 C R. The legend is for all seasonal and diurnal graphs below. C-Flux: During early summer P+R+SOM had a substantially larger flux compared to R+SOM and SOM plots. SOM plots consistently had the lowest flux (Fig. 5). All fluxes were greatly reduced by fall due to corn maturity and cooler temperatures (Fig. 5). Corn roots, residue and soil organic matter contributed to the soil flux by approximately 48, 16 and 36% in the early summer, and 20, 30 and 50% in the fall, respectively. Throughout the growing season C-flux in the P+R+SOM plot was influenced by soil temperature and moisture. As corn reached anthesis a higher C-flux was expected, but cooler temperatures and increased precipitation are suspected to have decreased respiration (Fig. 4a and 5). Carbon Content: Description: δ 13 C signatures δ 13 C signatures: δ 13 C R : P+R+SOM plot had δ 13 C R values that were within -10 to -15‰ until mid September when the values shift to between -16 to -18‰ (Figure 4). This shift indicates that there was a reduction of respiration from the corn roots, and soil organic matter and residue were contributing relatively more to CO 2 flux (Fig. 5). The increase of δ 13 C R in October for all plots is possibly due to contamination from atmospheric CO 2 through convective transfer, after frost has taken place and soil respiration has decreased (Rochette et al. 1999). δ 13 C R : A diurnal trend was observed in R+SOM and P+R+SOM plots (Fig. 6). Largest enrichment occurred at 14:00h and most depleted at 2:00h. There was an average of ‰ difference between 2:00 to 14:00h for both the R+SOM and P+R+SOM plots. The δ 13 C R for P+R+SOM was consistently the most enriched. The δ 13 C R for SOM plot was inconsistent and did not show a diurnal trend, but was constantly the most depleted (Fig. 6). The δ 13 C R for the SOM plot (bare soil) was rarely close to the substrate δ 13 C SOM value of -25.4‰, suggesting an additional component was influencing soil respiration. A possible contributor was CaCO 3 derived from limestone within the soil profile. C-Flux: At 14:00h sampling on days 219 and 225, shading effects on the P+R+SOM plot created a different soil temperature than on the R+SOM plot, creating a larger C-flux in the R+SOM plot (Fig. 7). At 2:00h it was difficult to collect accurate samples due to the stagnant atmospheric air conditions. During the cool and wet climate conditions, this diurnal study observed that corn roots and soil organic matter were both the main contributors of the flux. At 8:00h, soil organic matter and corn roots contribution were 43%+4.7% and %. At 14:00h soil organic matter and corn root contributions were % and %. Seasonal trends of δ 13 C R and C-flux were observed through out this study. Diurnal trends for δ 13 C R and C-flux were more difficult to distinguish due to wet conditions. Both seasonal and diurnal trends were greatly influenced by soil temperature and soil moisture. Comparing the 3 plots: P+R+SOM, R+SOM and SOM, illustrates that corn roots and soil organic matter are significant contributors to the soil C-flux in a corn field. δ 13 C R values of the SOM plot were more enriched and varied throughout the season (-15 to -25‰), indicating a non-constant soil component such as carbonates could be contributing to the soil C-flux at this calcareous site. Further research is underway to characterize carbonates at various depth of the soil profile at this site. Texture: Silty Clay loam δ 13 C corn : -11.8‰ δ 13 C corn residue : -12.1‰ 4a 4b Nickerson N and Risk D Physical controls on the isotopic composition of soil- respired CO 2. J. Geophys. Res. 114:1-14. Rochette P. et al Measuring residue decomposition with CO 2 fluxes and 13 C natural abundance. Soil Sci. Soc. Am. J. 63: Rochette P, Flanagan L.B. Gregorich E.G Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13. Soil Sci. Soc. Am. J. 63: Corn Plant Roots Residue SOM Residue SOM SOM Plot: P+R+SOM Plot:R+SOM Plot: SOM Flow through non-steady state chambers (Fig. 1 and 2) were used, during June to October 2008 in a corn field to measure soil CO 2 flux and its δ 13 C R value. To partition respiration into its components, two additional plots were used: 1) a plot where corn plants were removed after emergence but corn residue was present (R+SOM), 2) a bare soil plot was created by removing turf in a nearby field, where corn has not been grown for over 30 years. M. Wilton 1,a, C. Wagner-Riddle 2,a, J. S. Warland 3,a, P. Rochette 3,b, R. P. Voroney a, S. E. Brown 1 1 MSc Candidate 2 Advisor 3 Advisory Committee Fig 4a and 4b: Air temperature and precipitation data from the Elora research station. 3a displays from May 1 st to October 31 st (seasonal study). 3b displays from August 4 th to 13 th (diurnal study). Fig 5: C-flux and δ 13 C R data from June to October at various daylight sampling times for plots: P+R+SOM, R+SOM and SOM. Fig 7: C-flux for plots: P+R+SOM, R+SOM and SOM during late anthesis of corn (August 4-10 th ), sampling occurred four times a day. 14h Fig 2: depicts the CT plot during 2h and 14h periods, right before C-flux and δ 13 C R sampling were initiated. 2h