Author: Teal Potter
This article is part of a series if you’d like to read the others here they are: Part I Tillage, Soil Health, and Weeds: WSU Organic Transitions Project and Part II Watching the Waltz: Weed Seeds and Tillage.
In addition to examining the effects of tillage on physical soil properties, soil nutrients, and weed populations, we are also measuring soil biology. We chose to focus on one type of measurement, microbial biomass. Here we explain why we are including soil biology in this study, how we are measuring microbial biomass, and what we expect to learn.
Background on Soil Microbes
Soil biology is becoming a well-known contributor to soil health. Bacteria and fungi are some of the most studied groups of soil organisms and certainly the most studied types of micro-organisms. An individual micro-organism is typically a single cell. The term ‘micro-organism’, or simply ‘microbe’ for short, is an organism that cannot be seen by the naked eye, and thus a microscope is needed to examine these organisms. If you could line up some smaller types of bacteria end-to-end you could easily need 1000 cells to span the length of a millimeter! Thankfully we have options to quantify microbes in the soil besides microscopy because soils are densely populated with microbes. A single gram of soil may contain billions of microbial cells and a million genetically distinct types (Wilmes, 2006). You can see why it’s challenging to study microbial functions in soil with that much diverse life in the amount of soil you can fit on a US nickel!
Why we’re measuring microbial biomass
In this study we are comparing quantity of microbes—not diversity—among different tillage scenarios. The number of microbes in a soil can provide information about how much microbial activity is happening in a soil when a sample is collected. There are many different functions that soil microbes perform depending on the type and the soil conditions. A couple that are important in agriculture, for example, are microbial secretions of sticky substances that physically hold soil particles together. This helps stabilize soil and helps prevent erosion. Microbes also produce enzymes that decompose plant tissues. This is how nutrients are recycled in the soil for the plants that are actively growing nearby.
Also, more microbes means more dead microbes in the near future because microbes have short lifespans, often days to months. And dead microbes are now known as perhaps the most stable form of carbon in soil. More specifically, some parts of microbial bodies that contain a lot of carbon are slow to decompose (relative to plant tissues) when chemically bonded to soil minerals. However, a slight increase in stable carbon in the soil does not directly benefit crops because plants don’t need this carbon. Remember, plants get nearly all the carbon they need from CO2 in the air. Increasing stable carbon is considered good because we want to keep as much carbon in the soil as possible to slow climate warming.
The examples above suggest that increasing the number of microbes is good for agricultural soils. This is a bit simplistic because the link between microbial biomass and farming outcomes we’d considered good, like increased yields, are challenging to study and have not been demonstrated. However, assuming more microbes are better helps us make some predictions about the effects of tillage on soils.
How tillage is thought to impact microbial biomass
Tillage is soil disturbance. Physical soil disturbance can affect the number of microbes in a couple ways. In the short-term, tillage can increase microbial populations because the physical breaking of soil particles can expose new surfaces containing easily digestible molecules for microbes to consume, grow, and reproduce.
But if a field is intensively tilled year after year so many of the aggregates get destroyed that microbes will have less habitat (pore spaces where water and air interface with the soil) and food (various types of organic carbon). Physical soil disturbance also changes the temperature and moisture of the soil following a tillage event which would likely inhibit growth of some types of microbes but be favorable for others to grow and reproduce. Also, while single-celled microbes are likely too small to be physically harmed or destroyed by tillage implements, however, larger bodies soil organisms could be killed like earthworms and fungal hyphae (long filaments of fungal cells that compose the bodies of many fungal species). While this is not well studied, it is possible that tillage causes short term changes in the entire soil food web that would influence microbial population sizes.
Previous research on the effects of tillage on microbial biomass has shown that microbial biomass is consistently higher in very low disturbance fields like continuous pasture and continuous no-till systems compared to high disturbance cropping systems where there is frequent (e.g. annual use of) moldboard ploughs. These effects of high tillage can be seen in various soil types too. However, most farmers manage their fields at an intermediate level of disturbance compared to these extremes. Meta studies show that reducing tillage passes a little, for example, from conventional (medium high disturbance) to conservation tillage (medium low tillage) may or may not have a detectable effect on the microbial biomass in soils (Chen 2020). In these cases, soil type and climate play a larger role in whether decreasing tillage boosts microbial biomass. That being said, the soil types we have here in the Skagit Valley are more likely than other soil types to experience loss of organic carbon with heavy tillage (Nunes 2020). Thus, we expect that reintroducing tillage in these soil types could have a strong negative effect on microbial biomass in this study.
How we measure microbial biomass
Finally, for those of you interested in learning more about how the biomass measurement we are using actually works, here is a brief explanation. The measurement we are using to estimate microbial biomass is referred to as PLFA. This is short for phospholipid fatty acids. You may have learned about fatty acids in a high school or college biology class because they are essentially the molecules that make up the skin of cells. By measuring all the phospholipid fatty acids in a soil sample, we are able to make a rough estimate of the mass of the microbes they come from.
Measuring PLFAs is a popular way to estimate microbial biomass in agricultural studies and on-farm tests because we can use the total biomass data or choose to look at the component groups that make up the total such as mycorrhizal fungi and gram-negative vs gram-positive bacteria which have different roles in the soil ecosystem. These groups can be numerically separated because of slight differences in the molecular structure of their phospholipid fatty acids (Kaur 2005). You likely already know that fatty acids differ slightly among plants species if you have decided on a particular type of cooking/vegetable oil for health or cooking temperature reasons. Saturated fat like in coconut oil has more carbon molecules on the fatty acid molecule chain than unsaturated fats like olive oil! By the way, this makes the cell structure more rigid at higher temperatures and the reason why coconut oil is solid at room temperature and more stable for cooking with at hot temperatures than unsaturated oils. The arrangement of C on fatty acids is the same biology that allows for distinguishing microbial groups using PLFAs!
Another reason the PLFA method is sometimes preferred over DNA methods is because fatty acids degrade quickly when cells die compared to the DNA in the cell. Thus, the PLFAs that exist in the soil represent microbial biomass at that point in time whereas DNA methods represent both living and dead cells. (Kaur 2005)
Now that you know a bit more about how microbes are measured, how microbial mass may be affected by tillage, and why it could matter, you should be better able to interpret other research you hear about and the future results from this study.
Chen, H., Dai, Z., Veach, A. M., Zheng, J., Xu, J., & Schadt, C. W. (2020). Global meta-analyses show that conservation tillage practices promote soil fungal and bacterial biomass. Agriculture, Ecosystems & Environment, 293, 106841.
Kaur, A., Chaudhary, A., Kaur, A., Choudhary, R., & Kaushik, R. (2005). Phospholipid fatty acid–a bioindicator of environment monitoring and assessment in soil ecosystem. Current Science, 1103-1112.
Nunes, M. R., Karlen, D. L., Veum, K. S., Moorman, T. B., & Cambardella, C. A. (2020). Biological soil health indicators respond to tillage intensity: A US meta-analysis. Geoderma, 369, 114335.
Wilmes, P., & Bond, P. L. (2006). Metaproteomics: studying functional gene expression in microbial ecosystems. Trends in microbiology, 14(2), 92-97.