# Soils run deep, but digging isn't always easy ## What is "deep soil" and why is it important? Soils are an environmental interface. They exist in the special place within the Earth system where rock, life, air, and water meet[^sphere]. As you move deeper into the [[Soils form in profiles and horizons| soil profile]], soils converge with the underlying rocks. The "deep soil" bridges the interface between soils near the surface and the bedrock below. As with most soil properties, there is a lot of variation in soil depth. Soils can be anywhere from tens of centimeters to tens of meters deep[^how]. In practical terms, soil scientists often consider the "deep soil" to be material beyond 1 m depth from the surface [^deep]. Studying the deep soil is crucial to understanding the history of any soil and landscape. We can only come to know the origins of a soil and its full range of functions within an ecosystem by digging deeper. Only looking at the surface soil leaves us with an incomplete view, because the deep soil allows us to interpret how, exactly, that soil has developed from the underlying rock into a dynamic ecosystem that supports life. We need this historical perspective to predict how soils (and the ecological functions they support) will change in response to new environmental conditions in the future. Most of the ecological processes that occur in soils interact, in some way, with the deep soil environment. Ecological processes are interactions between organisms and their environment. Water, a requisite for life, moves and transports material from the surface, through the deep soil, and into the groundwater. Plant roots grow within the surface soil and often expand deep into the soil profile. And then there's the carbon. It turns out that deep soil horizons hold on to a whole lot of carbon[^deep]. This carbon gets into the deep soil when it is deposited by roots and associated fungi (known as mycorrhizae) or when transported by the aid of water. The carbon remains in the deep soil because it is protected from microbial decomposition because microorganisms tend to be more concentrated towards the surface [^schmidt]. In general, the concentration of carbon (that is, the amount of carbon per unit soil) is higher in the surface soil[^exception]. While the concentration of carbon may be higher at the surface, the deep soil often occupies more space. So, when we total up the amount of carbon stored within an entire soil profile, deep soil horizons contribute a greater amount of carbon to the total[^75]. This matters because we need a reliable assessment of how much carbon is actually held in soils if we want to make accurate predictions about global climate change. Despite all of this, a lot of soil science studies do not collect samples in the deep soil[^tansley][^adam]. Even more concerning, the depth to which we study soils has been decreasing in recent decades[^30]. Now more than ever we need to expand our view of soils rather than narrowing it. [^deep]: Research by Moreland et al. (2021) in the Sierra Nevada Mountain Range revealed that the majority of soil carbon was stored below the surface (A horizon) and within the weathered bedrock https://iopscience.iop.org/article/10.1088/1748-9326/ac3bfe [^sphere]: We call these intersecting and interacting realms of the Earth "spheres" - the biosphere (life), lithosphere (rocks), atmosphere (air), hydrosphere (water), and pedosphere (soils) [^how]: Soils can be tens of meters deep: https://www.jstor.org/stable/1312764 [^schmidt]: Schmidt et al. (2011) provide a great review of the reasons why carbon sticks around in soils https://www.nature.com/articles/nature10386 [^exception]: A notable exception would be in buried soil horizons where the organic carbon content decreases with depth at first, and then increases again at the point where the soil was buried. [^75]: Up to 74% in this study from of Sierra Nevada soils! https://iopscience.iop.org/article/10.1088/1748-9326/ac3bfe [^30]: Yost & Hartemink (2020) found this trend after reviewing studies published in four soil science journals https://link.springer.com/article/10.1007/s11104-020-04550-z [^tansley]: Richter & Billings (2015) further discuss the limitations of focusing our study on shallow soils and argue in support of the critical zone concept https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.13338 [^adam]: As I was writing this, I saw Adam Laehn posted his take home messages from the recent North American Forest Soils Conference, one of which was that we need more data beyond 30 cm deep https://www.instagram.com/p/CvI6vGmvbEL/?img_index=1 ## Why don't all soil studies extend into the deep soil? From my perspective, there are two reasons why soil science research doesn't always dig very deep. First, there are logistical constraints. Whether we like to admit it or not, no research study is perfect, and all study designs have compromises because research is a human endeavor. In the case of deep soils, we may not have the capacity to dig very deep into the soil for every sampling campaign. Especially when we are trying to sample soils across a large spatial extent (like a landscape), there is a tradeoff between how deep we can collect a sample versus how much area we can cover. This is because we often can only process a certain number of soil samples at a time due to laboratory capacity or budgetary limitations. So, we have to make decisions about whether we want to collect soils across a larger area, or if we want to collect across a smaller area, but to greater depths. In this tradeoff, spatial breadth seems to win over depth. Another logistical constraint is the physical work of collecting a single soil sample. Soil sampling often doesn't come easily. We might have a hard time physically getting deep into the soil, especially in clayey or rocky soils, where it becomes difficult to dig with hand tools alone. Soil scientists have devised methods to dig deeper, including hydraulic probes mounted on the back of trucks, backhoes to dig large, deep holes, and mechanized corers to cut into permafrost. These options may not always be feasible for remote research locations like an alpine area in the Rocky Mountains. We also want to minimize the disturbance caused by our soil sampling process, so heavy equipment is not always appropriate for sensitive ecosystems. ![[Edited-3719_Original.jpg]] *Dr. Kholodov preparing to core into deep permafrost horizons during the 2021 Alaska Soils Field Course. Photo by Douglas Freese.* A second reason we may not sample the deep soil is because of an **ecological timescale bias**. There's a general sense that ecological processes including interactions between plants, soils, and microorganisms occur on relatively short timescales (years and decades), whereas the processes of rock weathering and soil formation occur on longer timescales (centuries and millennia). Ecological processes occur more significantly in the surface soil horizons in which plant roots exert a greater influence. Soil processes in the surface horizons tend to be more dynamic over shorter timescales than those deeper in the profile. As a result, researchers asking ecological questions about the soil environment tend to focus on surface soil horizons. This is not a bad thing - we need many people from many backgrounds studying soils. However, it does leave us with a narrower view of soil processes. Given how foundational soils are to just about everything that happens on land, there is a lot of interest in studying soils by biologists, ecologists, hydrologists, botanists, among many others. But it's hard for one person to study it all. The integrative nature of soils require that we collaborate in diverse teams to extend our view of soils in many directions. I should note here that despite recognizing the importance of deep soils and full profile investigations, I, myself, am not immune to either logistical constraints or the ecological timescale bias. My own research in soil ecology has primarily focused on surface soils (though my teaching extends much deeper). ## The concept of the critical zone can help us integrate the deep soil So, how do we solve the challenges of studying the deep soil? Here's where the concept of the critical zone comes in. The critical zone is the continuous layer of the Earth's surface extending from the tops of plants deep into the soil to the bedrock below[^tansley]. This is the zone that is critical to supporting life. In the last few decades, we've seen the emergence of Critical Zone Observatories and Networks in the US[^czo]. These research hubs and coordinated efforts allow scientists to build on each other's expertise by working across disciplines of aspects of the critical zone. By coordinating efforts at specific research sites, we can also overcome some of the logistical challenges of digging deeper. [^czo]: Read more about the history and future outlook of critical zone science in the United States here: https://eos.org/features/critical-zone-science-comes-of-age ## Take home message Studying the deep soil environment is critical to our understanding of how soils form and function across space and time. Even so, it's not always easy to do. Articulating the concept of the critical zone as a scientific community has helped us push back against our tendency to study aspects of the nature in isolation. Instead, critical zone science encourages intentional study across Earth's many environmental interfaces. We learn a lot when we open ourselves to deeper explorations with folks who think differently than we do. #### Postscript > This essay is part of [[One thing about soil - an educational series]] created by Dr. Yamina Pressler. The essay was originally shared on substack. *updated Aug 19, 2023*