The Nebraska Sandhills contain massive dunes and upland prairies that make up approximately 90 percent of the landscape. Wide valleys between the dunes contain lakes and meadows/interdunes making up about 10 percent of the landscape. This undulating landscape with its slopes, seasonally wet low-lying areas, wetlands, and stream terraces creates variable regions of microclimate, hydrology, geology, and vegetation type, which in turn produce variability in the regional soils.
Dunes in the uplands may reach 400 feet in height and be 2 to 20 miles long. Topography affects how the forces of wind and water dislodge soil particles and transport and deposit them on the land surface, in turn influencing soil formation and development. Slopes in the Sandhills range from nearly level, or less than 2 percent slope, to steep, or more than 30 percent slope. Other features of the topography and landforms, such as elevation and aspect, can also have a significant impact on microclimates and vegetation and therefore on soils. For example, south-facing slopes receive higher solar radiation than north-facing slopes, which can affect dryness and wetness of the land. On the other hand, interdunal areas—called swales—tend to have high soil moisture and therefore can be more productive than the tops of the dune or the slopes.
Parent materials are the geologic materials from which soils are developed, and the nature of a soil is greatly affected by the characteristics of its parent materials. Most upland soils in the Sandhills are formed from parent materials of eolian sand (wind-blown deposits), and some are from stream-deposited materials in low-lying areas and stream terraces. Eolian sands undergo limited soil development due to the semiarid climate and the resistance of sand to breakdown or weathering. Weathering describes how parent materials are changed by physical, biological, and chemical processes into other soil components. Upland soils in the Sandhills are considered to be “young” or less mature in terms of soil development because they have not changed very much from their original condition at the time they were deposited.
The upland soils of the Sandhills are classified as Entisols, which means “recent.” An upland soil profile generally contains a thin, dark-color surface layer, called the A horizon, followed by unstructured subsoils that are mainly sand. Soil color tells part of the developmental story, as the different soil “shades” (i.e., red, yellow, gray, white, dark/brown) indicate the presence of certain minerals, biological activities, or the presence or absence of excess water. The dark color in the surface layer of the upland soil profile is characteristic of soil organic matter. Living soil organisms (flora and fauna) and other soil dwellers facilitate the decay of plant and animal residues and produce soil organic matter and release nutrients. One widespread Entisol soil type in the Sandhills is the Valentine series. Valentine soils have a surface layer of mineral material and organic matter that is one to five inches in depth, and these soils occur on slopes of up to 60 percent. They contain 78 to 98 percent sand, are well drained with low runoff, have high rates of water infiltration, and have low water holding capacity. Valentine series soils occur both in interdunes and on dunes and may be integrated with other soil types. These soils are neutral to slightly acidic, poor in nutrient holding capacity, and often contain less than 1 percent soil organic matter.
Loams are soils that are mixtures of sand, silt, and sometimes clay. The soils of meadows or of wide valleys with slopes of less than 3 percent range from loamy sand to sandy loam, with a sand content of 60–90 percent in the surface layer. Although sandy, these soils tend to be poorly drained and seasonally wet or frequently flooded because of their topographical position, the changing elevation of water table, or the presence of perennial creeks. Water tables in the wide valleys between the dunes can be within 2 and 4 feet of the soil surface throughout the growing season. These are referred to as sub-irrigated meadows, and they have greater overall plant productivity because water is available to support plant growth. Soils in the meadows are classified as either Mollisols (soft and dark from accumulation of calcium-rich prairie grasses) or Entisols. Unlike Entisols, Mollisols have a thicker surface horizon rich in soil organic matter, derived from the long-term addition and decay of grass roots and plant litter. Mollisols are rich in calcium and more fertile than Entisols and thus are extensively used for agricultural plant production. Excess moisture from water-table rise in these low-lying areas contributes to the accumulation of organic matter in the surface soil, which varies in concentration because of variability in the rate of decay and/or productivity and the degree of wetness. Organic matter can range from less than 1 percent to as high as 10 percent in frequently ponded meadow areas.
Soil is the largest storehouse of carbon, and worldwide, soils store double the quantity of carbon found in vegetation and the atmosphere. Grassland soils store significant soil carbon—commonly 50 percent more than the carbon stores in forests. Soil organic matter is important because Sandhills grasslands depend on nutrients from decaying organic matter to sustain productivity and ecosystem functions. Soil organic matter increases soil fertility, serving as the storehouse for the energy and nutrients used by plants and other organisms. It also exchanges chemical compounds much more readily than soil clay, which allows it to retain and release essential nutrients, such as calcium, magnesium, and potassium. Organic matter also helps to retain water and acts as a glue in binding soil particles. Increasing and maintaining soil organic matter is especially critical for sandy soils because their fertility and available water capacity are low.
Grazing animals’ excreta, such as dung and urine from grazing cattle, dead plant litter and trampled vegetation, decaying roots, and soil microorganisms serve as nutrient sources to grassland soils. The balance between these inputs and their decomposition determines soil carbon storage and release, as decomposition results in the biological transformation of the original organic materials and release of gaseous carbon dioxide and plant essential nutrients, such as nitrogen and phosphorus. Carbon inputs and their subsequent fate within Sandhills soils are controlled by a combination of factors, including weather (moisture and temperature), soil texture, grazing management practices, and topography. For instance, precipitation amount and duration can influence soil temperature and water content, which in turn affect microbial activity. Soil microorganisms (bacteria and fungi) are the main decomposers of soil organic matter, simultaneously impacting both soil carbon storage and the release of carbon to the atmosphere as gaseous carbon dioxide.
Improved grassland management can lead to soil carbon storage. Some Sandhill ranchers use high-intensity short-duration rotational grazing to increase soil organic matter by increasing trampling of standing vegetation. In the wide valleys, for example, grazing research has demonstrated annual litter accumulation of nearly two thousand pounds per acre—equivalent to about eight hundred pounds of plant carbon per acre. Synthesis of existing research also estimates that globally, grasslands have the potential to increase soil carbon sequestration by 0.25 ton (500 pounds) to more than 2.47 ton (4,940 pounds) per acre per year.
Yet in the Sandhills, overall knowledge of how grazing affects soil carbon stocks is quite limited. There is a need to evaluate Sandhills soils to better understand the fate and the patterns of nutrient return in pastures in relation to grazing strategies, climate, and topography. This research is essential in helping Sandhills ranchers manage their production and soil ecosystem functions using optimal grazing management strategies. Such strategies can help to mitigate the impacts of changes in precipitation and temperature and to stabilize landscapes by managing soil carbon for generations to come.