Carbon Indicators of Soil Health

Since plants use carbon dioxide [CO₂] during photosynthesis, it is obvious that they are sequestering carbon [C] from the atmosphere. However, when these plants die, they begin to decay and their sequestered C is released back into the air. Thus, farming that involves the growing of crops is considered a major source of C emissions. That is why there is increased interest in promoting farming practices that sequester C in the soil so that it is not released back into the atmosphere. Thus, agricultural practices that increase C sequestration in the soil are being promoted.

The USDA-NRCS defines soil health as “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”. They further state that “this definition speaks to the importance of managing soils so they are sustainable for future generations”. In a 2016 publication, the case was made that soil health emphasizes the biological component of soil because health refers to something that is living. Thus, it follows that a measure of soil health should necessarily involve measurements of biological indicators since they should reflect both direct and indirect impacts of living organisms that inhabit the soil.

Assessing a soil’s “healthiness” involves having a standard measurement or measurements that is/are accepted and used by those who are conducting research that will either directly or indirectly affect the health of a soil that is used to grow crops. Such a measurement or measurements should be capable of assessing a soil’s inherent properties that are used to determine a soil’s healthiness so that the soil in question is declared as “staying” healthy or “improving” in healthiness as a result of management practices used at the site.

Soil organic matter [SOM] with its high C content forms the basis of the food source for soil microbes, which in turn are linked to soil functions that are related to soil health. A publication titled “An evaluation of carbon indicators of soil health in long-term agricultural experiments” by Liptzin et al. [108 authors from Canada, Mexico, and the U.S.] reports results from a study that was conducted with the goal of evaluating six potential C indicators of soil health. The indicators used in this study were: 1) soil organic carbon [SOC–closely tied to soil health]; 2) 96-hr C mineralization [Cmin-96]; 3) 24-hr C mineralization [Cmin-24]; 4) permanganate oxidizable C [POX-C]; 5) water extractable organic C [WEOC]; and 6) ß -Glucosidase enzyme [BG–can degrade plant cell walls and act as a proxy for C cycling]. Details about the conduct of each test are provided in the above-linked article.

Soil samples were collected from 124 long-term experiments in North America that had documented management histories to compare the effects of tillage, cover cropping, crop rotation, crop count, nutrient amendments, irrigation, or livestock grazing intensity on soil health. Samples were collected between spring thaw and planting in northern locations and between crops at southern sites. The overall goal was to evaluate how to select an indicator of soil C dynamics because it is not practical or economically feasible to measure all 6 of the indicators. The analysis methods quantified the response of all indicators to adoption of decreased tillage, inclusion of cover crops, application of organic nutrients, increased crop count, crop rotation diversity, and residue retention.

Pertinent results from the study follow.

•    Any soil indicator or indicators that is/are selected should be 1) responsive to management, 2) easy and inexpensive to collect and measure, and 3) interpretable and understood by users.

•    Tested indicators generally responded to soil health practices other than increasing crop count and rotational diversity, and were significantly higher in response to 1) addition of organic nutrients, 2) decreased tillage, 3) residue retention, and 4) use of cover crops.

•     A major finding from the analysis is the importance of the consideration of site characteristics–e.g. soil texture, climate–when interpreting the absolute values of the indicators. However, the analyses reported in this article show that C mineralization assays are less dependent on site characteristics.

•    The analysis reported here suggests that BG and WEOC were more sensitive to soil texture than the other indicators.

•    The C indicators examined in this study were greater in colder and wetter sites with more clay content.

•    There was little evidence that increasing the number of annual crop species [i.e. crop count or the number of cash crops in a rotation] or including crops other than grains affected the C indicators examined in this study.

•    Generally, sites with grain-legume rotations had lower SOC than those with continuous grain rotations.

•    In general, soil C stocks were greater in soil near the surface.

•    In a majority of the sites, most indicators responded similarly to soil health practices. This was especially so for their response to cover cropping, applying organic nutrients, and residue retention.

•    A majority of the sites used in the study had a consistent positive response of SOC to decreased tillage, especially in the short-term.

•    All C indicators in this study were predictable with the same site-level variables, were strongly correlated with each other, and responded to soil health practices.

•    POX-C was most highly correlated with SOC, and Cmin-24 and BG were more variable within a treatment. Cmin-96 was the only indicator that did not respond significantly to decreased tillage and residue retention.

•    Both POX-C and Cmin-24 have field tests that are available; thus, samples do not have to be sent to a lab for these tests.

•    WEOC is cheaper to measure than the other indicators, which are similar in price.

•    The authors state that “as long as the same indicator is used over time or across management practices at the same time, all of these indicators can provide insight into C dynamics”. They advise against substituting one indicator for another.

•    And finally, the authors state that “balancing the cost, sensitivity, ease of interpretation, and availability at commercial labs, a 24-hr potential C mineralization assay [Cmin-24] could deliver the most benefit as a measurement in conjunction with SOC”.

Click here for a White Paper on this website that provides information about soil health and its importance to sustainability.

Composed by Larry G. Heatherly, Oct. 2023, larryh91746@gmail.com