Measuring Soil Respiration with a Gas Chromatograph

Abstract

Left: Soil sample. Source: Jason Johnson/USDA-NRCS. Above: Gas chromatograph. Source: Jason Warren. Farmers, ranchers, and home gardeners are interested in measuring and improving soil health. Indicators of soil health can include physical, chemical, or biological properties. The biological properties can include earthworm populations, the amount of organic matter in soils, or even measuring the microbial community. Depending upon what a researcher or farmer is interested in, they may assess the microbial community in terms of abundance, diversity, or activity. The methods to measure the microbial community vary; diversity is measured through molecular methods, whereas activity is measured as CO2 respiration. Measuring microbial activity is important for understanding nutrient cycling, which is why it is important data for agricultural landscapes. Currently, the methods for measuring soil respiration vary, making it difficult to compare results from one data set to the next. There is also a knowledge gap when it comes to what the activity of soil microbes means for management actions. Imagine if the results testing your cholesterol varied from lab to lab based on procedures. The outcome of this test could result in your physician putting you on a prescription to lower your cholesterol—and the dose of that drug would depend upon the results from the lab. When health interventions like this depend upon a lab test, the results of that test need to be consistent. Fortunately, these tests have been standardized across the medical field, and physicians can take action based on test results with certainty. Standardizing soil microbial respiration methods will make these data more meaningful and enable researchers to use measurements of microbial biomass to make management recommendations. Jason Warren, Associate Professor and Soil Management Extension Specialist at Oklahoma State University, is interested in using microbial respiration data to assess the microbial community. He and his colleagues developed a method to analyze soil respiration using a gas chromatograph (GC). They describe this method in a paper recently published in Agricultural & Environmental Letters (https://bit.ly/2N8KyhZ) and compare the GC method with the commonly used Solvita method using soil samples from nine diverse sites across Oklahoma. Both tests measure CO2 from dried, sieved soil samples. The Solvita method uses a gel, which absorbs CO2 as it is released from re-wet soil over 24 hours. The gel changes color but has a somewhat narrow range of detection. Therefore, when soils have high CO2 release, samples need to be diluted and re-tested. The GC method also requires a 24-hour waiting period between re-wetting and testing CO2 levels, but the GC reading is instantaneous, and the range of values that can be detected is broader. The authors also compared soil-drying temperatures. Soil samples need to be dried before processing for both tests, and depending upon the lab and equipment available, some may dry samples at room temperature while others would oven dry to speed up the process. The researchers were curious whether the drying temperature would influence the respiration results; therefore, they compared respiration observations for four drying temperatures (room temperature and 45, 65, and 105°C). The authors report that heat drying produced less variable results compared with air drying. In addition, for both the Solvita and GC method, soil CO2 emission increased as the drying temperature increased. However, samples dried at 105°C produced highly variable results, which the authors considered to be unreliable. When comparing the Solvita and GC values for respiration, the Solvita numbers were approximately six times greater than the GC values. However, the values were highly correlated and, if consistent, may be a data discrepancy that researchers can account for when comparing data sets. For those testing soils from agricultural fields, there is some pressure to get results to farmers as quickly as possible, and the potential for dilution and re-testing can make the Solvita method slow. While the GC method may be faster, a GC is an expensive piece of equipment, and not all labs would have access to one or be processing enough samples to justify purchasing one. Given the mix of methods in use, understanding how the results may differ and finding ways to compare the data in a meaningful way is important. “Now that we have a high-throughput method that provides reproducible results, we can start to generate the data needed to better understand the impacts of management on soil respiration as well as the temporal and spatial variation,” Warren says. He and his colleagues hope to continue their work to validate and calibrate methods for measuring CO2 respiration as an indicator of soil microbial activity and soil health. • Soil CO2 respiration is an indicator of microbial activity. • The gas chromatograph method for measuring respiration can eliminate the need for sample dilution. • Standardizing methods and validating results is necessary to enable researchers to use CO2 respiration to manage soil health. View the open access Agricultural & Environmental Letters article, “An Automated Laboratory Method for Measuring CO2 Emissions from Soils” at: https://bit.ly/2N8KyhZ.