The Impact of Soil on Water Quality in Riparian Buffers

Most people agree that soil is a highly valuable natural resource, but how do we define soil? There are many definitions, and they often depend on the person writing the definition and their background. For instance, structural engineers will define soil as the surface layer of earth which can be compacted to support the foundation of a building. Agronomists may define soil as the medium to support plant growth. Geologists may even define soil as "overburden" because they are interested in the rocks, mineral deposits, and other subterranean resources that lie underneath. Still a more general definition of soil can be found through the lens of a soil scientist. Soil is a porous medium made up of solids (mineral and organic), liquids (water with dissolved ions and salts), and gas (oxygen, carbon dioxide, methane, nitrous oxide, and other gases of lower concentration) that is capable of supporting life. It is often thought that an optimal mineral soil will be composed of 50% solids, 25% liquids, and 25% gases by volume.

 Another fact about soil is that it is dynamic, which means it is always changing. Soils take hundreds if not thousands of years to form from weathered substrate (often rocks) called parent material. As soils form, living things start to grow within it. This includes plants, animals, and microbes. All living things that grow in soil eventually add organic matter rich in carbon and other nutrients to the soil that accumulate over time. It is generally thought that the greater amount of organic matter in a soil, the greater the soil's fertility. Soil fertility comes from organic matter being broken down by microbes over time in a process called decomposition, which allows the nutrients that were once in the organic matter to be broken down (mineralized) into plant-usable forms. There is no denying that soils formed from natural process will produce a tremendous natural resource that is capable of supporting life and that these soils are sustainable and stable under perennial vegetation.

This pie chart shows the general composition of a soil. The arrows represent that soil is dynamic in that what we do for land management practices will impact the relative soil composition. Source: Agriculture Victoria

However, once this perennial vegetation is cleared for development of farm fields, houses, larger urban developments, or other human-dominated landscapes, a shift can occur within these soils. If soils are ever left bare without vegetation for an extended period, they can be susceptible to erosion by both wind and water. This erosion can degrade and move valuable topsoil that is rich in organic matter to other parts of the landscape or directly to adjacent water bodies. Further, any sort of land management that involves heavy machinery (construction, farm equipment, etc.) can compact the soil, thus causing the soil to have a larger volume of solids than pore space taken up by liquids and gases. Compacted soils often have lower water infiltration rates, thus increasing surface runoff and erosion of even more topsoil. These severe management implications as well as others is what prompted the United Nation's Food and Agricultural Organization (UN FAO) to officially establish December 5^th as the annual World Soils Day which started in 2014. Each year there is a unique theme that revolves around soil management, but in general, December 5^th serves as a day to remember that what we do on our land affects this critical resource we call soil.

The best management strategies to improve our degraded soils come from following the example nature provides for us. Using perennial vegetation in critical portions of our watersheds is one such strategy. One example of this is the use of riparian buffers. Riparian buffers are perennial vegetation, often consisting of woody species, directly adjacent to surface water bodies like creeks and lakes. Planting these deep rooted, water loving woody species in former turf, farmland, or other human dominated landscapes can help develop larger pores, thus reducing compaction and increasing infiltration, and increase the amount of organic matter and carbon within a soil profile. These soil changes help enhance the soil's ability to act like a sponge to soak up water entering the riparian buffer from upslope, which in turn improves water quality before entering the adjacent water body.

Part of the soil core that was extracted from the new research and education buffer in Dauphin County, PA. These soil cores were taken 5 feet at a time to a depth of 10 feet total. Credit: Jennifer Fetter, Penn State

Quantifying this change is one of the goals of the new research and education buffer that Penn State planted in Dauphin County at the side of Spring Creek. To celebrate the true meaning of World Soils Day, soil cores were collected from this newly planted buffer to serve as a time zero sample of soil properties like carbon content and pore space. These measurements will help assess the soil's ability to act like a sponge. In addition, these soils will be processed for texture to determine if they are coarser, higher in sand, or finer, higher in clay. These texture profiles will aid in determining water movement within the buffer with the knowledge that water moves quicker in sandier soil than it does in soils with higher clay content. Another benefit of taking soil cores is that we were able to install shallow groundwater wells in place of the extracted soil. These wells will serve as sampling ports for our water quality analysis. Moving forward, soil cores will be collected once every 3 to 5 years to measure changes in our riparian buffer sponge as vegetation matures. Soil cores and water quality data from the shallow groundwater wells will serve as the essentials of our long-term data collection.  

Links

World Soils Day

Riparian Buffers (Penn State Extension)

New research and education buffer (Penn State Extension)

At the side of spring creek (Penn State Extension)

Sources

Agriculture Victoria: Agriculture Victoria, Victorian Resources Online. 2020. The Soil System and Soil Health Monitoring.  Accessed: 5 December 2021.