Brandt, K.

Curtin, D.

Cutforth, H.

McConkey, B.

Wang, H.

English

University of Saskatchewan Research Archive

Estimation of daily soil CO2 emission using instantaneous measurements H. Wang1, B. McConkey1, D. Curtin2, H. Cutforth1 and K. Brandt1  1Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Box 1030, Swift Current, SK S9H 3X2, Canada (e-mail: Hong.Wang@agr.gc.ca) 2New Zealand Institute for Plant & Food Research, Private Bag 4704, Christchurch, New Zealand.  Introduction   Soils play an important role in regulating atmospheric CO2 concentration, one of the primary greenhouse gases likely contributing to climate change. Research shows that if properly managed, agricultural lands could be an important sink for atmospheric CO2. However, there are also concerns that soils could become a significant source of CO2 to the atmosphere if global temperatures continue to rise.   Field measurement of CO2 fluxes is an effective tool for estimating and comparing impacts of different land management practices on the soil carbon balance. Because soil CO2 flux often varies markedly hour by hour, an automated continuous system would be the ideal way to measure the daily soil CO2 flux. However, it may be impractical to deploy continuous chambers over multiple treatments with several replications per treatment. Therefore, many researchers use a portable gas exchange system to measure instantaneous soil CO2 flux. Treatment means are used to calculate daily CO2 flux by directly extrapolating the measurement to 24 h. However, there are concerns as to whether measurements of instantaneous CO2 flux can adequately represent daily CO2 flux.  The objective of this study was to determine if the equation developed by Parkin and Kaspar (2003) can be used to estimate daily average CO2 flux in a semiarid agroecosystem.  Materials and Methods   Data collection: Instantaneous and diurnal CO2 fluxes were observed from various treatments in several field experiments on a gently sloping Swinton silt loam (Typic Haploboroll) near the Semiarid Prairie Agricultural Research Centre, Swift Current, Saskatchewan, Canada from 1995 to 1996 and from 2003 to 2007. Hourly Ta was recorded at a nearby weather station. Excluding rainy day measurements, 55 diurnal CO2 and corresponding soil temperatures (Ts) were taken under different treatments (measurements made at hourly intervals) in 2003, 2004, 2006 and 2007.   Model: Parkin and Kaspar (2003) used the following equation to estimate daily average CO2 emission with a single CO2 flux measurement:  Daily average CO2 Flux = R x Q(DAT-T)/10                where R is the measured CO2 flux at a specific hour, T is the temperature at the time of flux measurement, DAT is the daily average temperature, and Q is the Q10 factor (the ratio of respiration rates at temperatures differing by 10oC). They found that Eq. (1) using air temperature (Ta) successfully estimated pre-seeding soil daily CO2 emission in two fields under no-till management.  Parameterization and Validation: We randomly selected 30 diurnal measurements to calibrate Q10 factors. The remaining data (25 diurnals) was used for model validation. The calibration was performed using nonlinear regression analysis (PROC NLIN). To estimate the uncertainty associated with the model this analysis was done for 1000 bootstrap re-sampling datasets from the calibration data. Then, Eq. 1 was run with the calibrated mean Q10 factors to simulate daily average CO2 flux in the validation dataset and 1000 bootstrap datasets generated from the validation data.  Results and Discussion   Figure 1 shows mean diurnal changes of CO2 flux and soil and air temperatures over a single day. CO2 fluxes were lower than the daily mean in the morning and they were higher from 1100 to 2200 h. The CO2 flux generally followed the change of Ts and Ta. This indicates that, depending on the time during the working day (0800–1700 h) when the instantaneous CO2 flux is measured, the daily mean emission could either be over or under-estimated by linear extrapolation of the instantaneous value to 24 h. When precipitation did not occur, diurnal changes of soil moisture were small, therefore, had little effect on the change of CO2 flux (Fig. 1). The change of CO2 flux was closely associated with air and soil temperature.                             Day of year and hour of daySoil moisture (vol vol-1, %)1216202428Temperature (oC)0481216202428CO2 flux (µmol m-2 s-1)0.00.51.01.52.0Soil moisture Soil temperatureAir temperatureCO2 flux10     12     14     16     18     20     22     24     2     4     6     8      10178                                        179Fig. 1. Diurnal changes of soil CO2 flux, soil temperature, air temperature and volumetric water content. Calculated Q10 factors using the calibration data and bootstrap samples were 1.97 for Ts and 1.59 for Ta (Table 1). Upper and lower bootstrap confidence limits were very close to the means for both Ts and Ta. The model estimated daily CO2 emission with high precision (high Pearson correlation, r) and high accuracy (low root mean square error of prediction, RMSEP). Concordance correlation coefficient (CCC) is an indicator for both precision and accuracy. Values of CCC close to one indicate a good agreement between measurements and simulations. Table 3 shows that the CCC was high (> 0.82) for the model using either soil or air temperature. The model using Ts performed slightly better than that using Ta according to all the indicators (r, RMSEP and CCC). Validation results were very similar to calibration results, but the model using Ta performed slightly better than using Ts. Ta performed slightly better than using Ts.                          Different components of soil respiration (root respiration, respiration by heterotrophic organisms decomposing soil organic matter; respiration from surface residues) may respond differently to environmental factors. In a bare soil, where the heterotrophic respiration was the major source of CO2 production, diurnal changes of soil CO2 flux were highly correlated with soil surface temperature. Parkin and Kaspar (2003) indicated that Eq. (1) using Ts at 5 cm simulated daily average CO2 flux poorly, but simulation results were improved when Ta was used. This suggests that a substantial portion of the CO2 was emitted from surface residues. On cropped land, soil respiration was closely linked to current photosynthesis, therefore to irradance and Ta rather than to           1, 2Lower and upper bootstrap confidence limits at P = 0.05, respectively. 3All Pearson correlation coefficients were significant at P = 0.001. 4All concordance correlations were different from zero at P = 0.001. Table 1. Soil and air temperature-based Q10 factors calculated using calibration and bootstrap re-sampling data and assessments on these two Q10 factors for simulating daily average soil CO2 flux in the calibration and validation datasets and their bootstrap samples by Pearson correlation (r), root mean square error of prediction (RMSEP) and concordance correlation (CCC).  Soil temperature  Air temperature Model Mean Lower1 Upper2 Mean Lower1 Upper2 Calibration       Q10 factor 1.973 1.963 1.982 1.586 1.580 1.592 r3 0.899 0.898 0.900 0.887 0.886 0.888 RMSEP 0.052 0.051 0.052 0.059 0.059 0.060 CCC4 0.887 0.886 0.888 0.870 0.869 0.871 Validation       r3 0.835 0.834 0.835 0.856 0.856 0.857 RMSEP 0.049 0.048 0.049 0.044 0.044 0.045 CCC4 0.823 0.823 0.824 0.841 0.840 0.842 Ts. In our study, all simulations of daily average soil CO2 performed well, including both crop and fallow soils and under both no-till and conventional tillage using either Ts or Ta.  It is clear that a relationship between temperature and CO2 efflux described by Parkin and Kaspar (2003) can be used to estimate daily average CO2 emission in the semiarid prairies.  References  Parkin, T. B. and Kaspar, T. C. 2003. Temperature controls on diurnal carbon dioxide flux: Implications for estimating soil carbon loss. Soil Sci. Soc. Am. J. 67: 1763–1772. Curtin, D., Wang, H., Selles, F., McConkey, B. G. and Campbell, C. A. 2000. Tillage effects on carbon fluxes in continuous wheat and fallow-wheat. Soil Sci. Soc. Am. J. 64: 2080–2086.      

Estimation of daily soil CO2 emission using instantaneous measurements

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Estimation of daily soil CO2 emission using instantaneous measurements

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