By Deirdre Griffin Lahue, Washington State University
Last month’s Research Short Story focused on some of the concepts and initiatives behind soil health, and this time we will go more in depth on a couple of measurements of active carbon (C) that have become common in soil health testing: permanganate-oxidizable C (POXC) and mineralizable C. These measurements are categorized by the Cornell Assessment of Soil Health (CASH) and the Soil Health Institute as biological indicators as they are chemical measurements of biological activity, or the potential for biological activity.
When soil microbes break down organic matter, they use it as a source of electrons (or energy), oxidizing the carbon molecules in the process. The POXC method uses a chemical oxidizer (potassium permanganate) to mimic the activity of microbes, taking electrons from whatever carbon is available within a set period of time (Weil et al., 2003). Carbon compounds that are easily accessed by the chemical oxidizer are defined as having potential to be “active carbon” while compounds that are protected inside soil aggregates, for example, are not. Soil organisms also release CO2 when they break down organic matter (mineralizing it as they turn it from an organic form to an inorganic form), and this CO2 is another chemical indicator of potential biological activity that we can measure. Though both are indicators of biologically active C, POXC has been associated with greater potential for organic matter stabilization and storage, while mineralizable C is associated with nutrient release (Figure 1; Hurisso et al., 2016).
POXC and mineralizable C have become more common in soil health work as our understanding of the importance of labile, or easily decomposable, carbon has evolved. As was mentioned in the last post, soil organisms drive many of the functions we look for in healthy soils, and these organisms need to be fed. Therefore, we now understand that it is not just the total stock of soil organic matter (which is ~50% C) that is important, but how much of that is C flowing through soil biota (Lehman and Kleber, 2015; See Collins and McGuire (2019) for more on this).
These active C pools tend to be more responsive to management changes in a shorter period of time (Culman et al., 2012), while total SOM can take years to show measurable differences. They also show seasonal dynamics as the amount of active C from plant root exudates or microbial biomass ebbs and flows (Figure 2 from Culman et al., 2013).
We wanted to see how biosolids applications have affected these active C pools, among other soil health metrics. In April 2019, we collected soil samples from a long-term (26-year) biosolids application trial in Douglas County that is in a dryland grain and oilseed cropping system. Biosolids have been applied every 4 years since the site was established (see Andy Bary’s Research Short Story for more on this experiment). We sampled from 0-2”, 2-6”, and 6-12” prior to biosolids application to investigate effects of historical biosolids applications on biological and chemical soil health parameters. We hypothesized that, given the surface applications in this no-till system, treatment differences would be most apparent in the surface samples (0-2”) but that with the longevity of biosolids applications at this site, treatments would also show effects deeper in the soil.
Figure 3a (left): Soil permanganate-oxidizable C (mg/kg dry soil) from 0-2”, 2-6”, and 6-12” in April 2019. 3b (right): Mineralizable C (mg CO2-C/kg soil/day) in an aerobic incubation, from 0-2”, 2-6”, and 6-12” in April 2019.
We found similar trends in both POXC and mineralizable C throughout the soil profile, with significantly higher levels at the 0-2” depth compared to 2-6” and 6-12” (Figures 3a and 3b). POXC showed treatment differences at the soil surface (0-2”) but not at the 2-6” or 6-12” depths (Figure 3b). Plots with historical biosolids applications of 4.5 dry ton/acre had the highest active C in surface soils and were significantly higher than both the synthetic fertilizer and unfertilized plots. Plots with 2 and 3 dry ton/acre historical applications had numerically higher POXC than non-biosolids plots but were statistically different only from the unfertilized treatment (Tukey’s HSD test, a = 0.05).
Soil mineralizable C showed treatment effects at both the 0-2” and 2-6” depths, with the highest values in the 3 and 4.5 dry ton/ac treatments from 0-2”, which at this depth were significantly different from the unfertilized control (Figure 3). The 2 dry ton/ac treatment was numerically higher than both the fertilized and unfertilized plots, but was not significantly different given the variability of the measurements. At the 2-6” depth, however, the non-biosolids treatments were significantly lower than all biosolids treatments.
It is clear that biosolids applications have increased the microbially available, active C in these soils, particularly at the soil surface. Our next steps are to look at the proportion of total soil C that is in these active pools, as well as to measure functional effects of increased microbial activity, such as soil water holding capacity, infiltration, and aggregate stability in order to continue to quantify the benefits that biosolids may have in these soils.
- Culman, S.W., S.S. Snapp, M. A. Freeman, M.E. Schipanski, J. Beniston, et al. 2012. Permanganate Oxidizable Carbon Reflects a Processed Soil Fraction that is Sensitive to Management. Soil Sci. Soc. Am. J. 76(2): 494–504. doi: 10.2136/sssaj2011.0286.
- Culman, S.W., S.S. Snapp, J.M. Green, and L.E. Gentry. 2013. Short- and long-term labile soil carbon and nitrogen dynamics reflect management and predict corn agronomic performance. Agron. J. 105(2): 493–502. doi: 10.2134/agronj2012.0382.
- Hurisso, T.T., S.W. Culman, W.R. Horwath, J. Wade, D. Cass, et al. 2016. Comparison of Permanganate-Oxidizable Carbon and Mineralizable Carbon for Assessment of Organic Matter Stabilization and Mineralization. Soil Sci. Soc. Am. J. doi: 10.2136/sssaj2016.04.0106.
- Lehmann, J., and M. Kleber. 2015. The contentious nature of soil organic matter. Nature 528: 60–68. doi: 10.1038/nature16069.
- Weil, R.R., K.R. Islam, M. a Stine, J.B. Gruver, and S.E. Samson-Liebig. 2003. Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use. Am. J. Altern. Agric. 18(1): 3–17. doi: 10.1079/ajaa2003003.