Basis for Comparisons of Soil CO ₂ Respiration Test Procedures

McGowen et al. (2018) recently reported a gas chromatograph (GC) method for soil CO2 respiration, claiming advantages in time, labor, and precision compared with Solvita (Woods End Laboratories), a commercial soil respiration method. Herein we address core ideas that we find insufficiently supported and outline gaps in their description of methods. When two methods are compared, circular reasoning is invoked, and therefore it is ideal to have additional method comparisons. We applaud the fact that the authors agreed and provided our team the same set of refrigerated soils from which their conclusions were drawn. We performed several respiration tests, including Solvita and three differing infrared gas analyzers (IRGA). Two methods may be considered equivalent when the r2 is high, slope approaches 1.0, and the intercept is close to zero. The authors demonstrated significant correlation between GC and Solvita (r2 = 0.73). However, the slope was very low (0.226); thus, their GC method measured CO2 significantly lower than Solvita. We found that their GC method also gave much lower results compared with IRGA methods. Moreover, when we compared a LiCor 7000 CO2 IRGA to Solvita, we obtained a slope of 1.05 at a zero intercept (r2 = 0.887)—essentially, a perfect correspondence. We corroborated Solvita's accuracy by the new IRTH method, embedding an infrared sensor inside a Solvita jar during the test, resulting in r2 = 0.92 and a slope of 0.945. Incidentally, this confirms earlier work by Chitraker et al. (2006) showing a linear range of detection for Solvita compared with calibration gases from 0 to 3% CO2 (r2 = 0.99). In Table 1, we show several of the comparisons we made with the respective r2 values and slopes (intercepts adjusted to zero). We question the time advantage claimed for GC over Solvita. Both methods require similar setup and incubation. The authors report GC requires 8 min per test (McGowen et al., 2018). The Solvita digital reader enables four samples per minute, including labor. Recognizing GC autosamplers as generally labor saving, it is still difficult to comprehend how the GC method will ever allow the high-throughput per day for which Solvita is designed. We ran all our trials in similar-sized 475-mL jars, which is very common for IRGA soil methods. We obtained the best data with the 475-mL jars as compared to the 265-mL jars commonly used for Solvita. Increasing jar size would address the concerns raised in the original paper about oversaturation of Solvita at higher values (presumably >3% CO2). Larger jars dilute the CO2 concentration to be within an acceptable range. This is likely to eliminate or minimize the need for dilution and sample rerun with the Solvita method, which may have been part of the authors' calculation regarding time disadvantages. We had difficulty understanding the authors' reported free air space in GC vials, an important number since CO2 mass is calculated via the Ideal Gas Law. Normally, airspace is jar volume minus soil volume corrected for particle density. The authors refer to headspace as 0.015 L (vials were 0.020 L), suggesting that their 5-g soil sample equated to 5 mL, which would skew calculations. The authors indicated difficulty rewetting soils for respiration, which may explain differences comparing Solvita results. Presently, 50% water-filled pore space is favored over a blanket addition of water. Brinton and Burger (2015) reported the need for this new wetting protocol to avoid respiration suppression caused by the Haney method (Haney and Haney, 2010), which was discontinued in 2016 (Solvita, 2016). A factor that may influence the amount of CO2 determined after dried soil is rewetted is the ratio of headspace volume to soil in jars or tubes used. We are skeptical that micro-methods can be made reliable for soil respiration. We speculate that the miniaturized GC method is problematic, possibly because the small quantity of soil is difficult to moisten properly or because aerobic respiratory suppression occurs. Using our own data, we calculated that with the authors' jar configuration, CO2 concentrations as high as 7.75% of headspace may have resulted, whereas the GC was calibrated only to 40,270 μL L−1 CO2. A semiautomatic respiration method has been reported using a IR CO2 alarm detector limited to 0 to 2% CO2 (Haney et al., 2018), which presumably must be dealt with similarly by increasing jar size, reducing soil quantity, or recalibrating beyond the design specifications for the instrument with potential loss of resolution. Therefore, issues of accuracy related to calibration and design limits of instrumentation are a common feature for all methods of measuring soil respiration. We commend Oklahoma State University for the choice of soils in this study representing a wide range of respiratory activity, ideal for method comparisons, and for offering insight into soil health. The authors' team selected nine regions with paired comparisons of tilled and no-till farms. Our ANOVA analysis showed that all methods differentiated no-till versus conventional at a high degree of statistical significance. This implies that all these soil respiration tests do illuminate real differences in soil health. The path to method improvement will undoubtedly be never-ending; however, we should not lose sight of the enduring importance of distinguishing management impacts on soil health.