- Author: Jordon Wade
The concept of soil health brings soil chemical, physical, and biological traits together under a unifying framework. For more background information on the soil health paradigm, see our Focus Topic here (http://ucanr.edu/sites/Nutrient_Management_Solutions/stateofscience/Soil_Health_894/). To adapt to this new paradigm, new metrics must be developed and refined to better communicate changes in soil health. While we have many robust measurements for soil physical and chemical properties (such as bulk density, pH, and others), we need ways to reliably measure and communicate soil biological processes. However, measuring soil biology can be costly and difficult to translate into management recommendations. Mineralizable carbon (or respiration upon rewetting) has gained popularity as a soil health metric among researchers, extension, and soil conservationists largely because it addresses both of these issues simultaneously.
Much like humans, soil microbes emit carbon dioxide (CO2) as they go about their daily metabolic activities. This CO2 comes from the metabolic breakdown of more complex forms of carbon—a process known as carbon mineralization—to access the energy harnessed in these complex molecules. As microbes do more metabolic “work”, they are mineralizing more carbon, increasing respiration rates and ultimately producing more CO2. When a soil is dry, most of this metabolic activity subsides. Rewetting redistributes carbon-rich material throughout the soil profile and kicks the microbial community into overdrive for 24-72 hours. This burst of mineralization can then be measured in a soil test lab to measure soil microbial activity. While this process is very straightforward in theory, many of the specifics surrounding its implementation are still unclear. These remaining uncertainties must be resolved in order to provide reliable information to growers wanting to integrate soil health into their management decisions.
A recent study involving UC Davis researchers Will Horwath and Jordon Wade sought to assess the reliability of the metric using soils from across the United States, with California soils being well represented. They found that different labs came up with very different numbers for mineralizable carbon for the same soils, with average differences of ±20% between labs. Traditional soil metrics, on the other hand, vary by as little as 1% between different labs. With so much variability in how it is measured, this metric is not yet ready to stand alone as a reliable component of a soil health toolbox. However, extensive local calibration and replication can help to overcome this obstacle until a larger-scale solution is found.
This study was primarily interested in reliability within and between soil tests labs, although several methodological considerations were also examined. While the takeaways of this study are relatively straightforward, the intricacies of the methodology and inter-lab comparisons are a bit more involved.
Between lab variation: After sending a set of 7 California soils to three commercial labs and running them here at UC Davis, a very low level of agreement was found. Although there was agreement in the general trend—the high values were generally higher from all labs and the lower values were low from all labs—the absolute values varied considerably. On the surface, this is a seemingly contradictory finding. However, the graph below helps to illustrate the point using data from two of the labs. The results have an acceptable R-squared value (R2=0.61), but the individual points differ from one another (greater similarity between the two labs would be indicated by points being closer to the 1:1 dotted line). For example, the point circled in red would get a value of 40 from one lab and a value of 140 from another; an increase of approximately 250%. When a separate set of 20 soils from across the US were retested in commercial labs, the average variation between the labs was ±20% for a given soil. This value was much higher than traditional lab metrics, which had variations as low as 1%.
Within lab variation: While variation between labs was considerable, variation within a lab can also occur. To better understand how much variation can be expected within a lab, yet another set of 73 soils was each run in triplicate in the same lab to see how much mineralizable carbon values can differ. The variations ranged from 0.5 to 84.4%, with an average of 18% variation between the three replicates. When this finding was investigated more, they found that the variation in mineralizable carbon was largely soil-specific. In practice, this means that for a “true” mineralizable carbon value of 100, you would expect an average variation in mineralizable carbon values ranging from 82 to 118. However, the range could also be 99-101 or 16-184, depending on the soil. The results from the study are unclear in explaining what could be driving this soil-to-soil variability in mineralizable carbon.
The large amount of uncertainty associated with measuring mineralizable carbon makes it challenging to use this metric to inform management decisions. Differences between soils could be obscured or artificially created by this uncertainty. Therefore, two recommendations should be considered when utilizing this soil health metric.
- Exercise caution when interpreting mineralizable carbon values. This could involve a relaxation of the traditional statistical thresholds of pphttps://dl.sciencesocieties.org/publications/sssaj/articles/80/5/1352).
- Only analyze the mean of several measurements from the same sample. i.e. use analytical replication. While replication will increase overall cost of analysis, it will also allow for greater confidence in the values obtained. The greater the number of replicates, the greater the certainty in the mineralizable carbon values returned. For additional information and to read the full study, please click here (https://dl.sciencesocieties.org/publications/sssaj/articles/82/1/243).