Elevation-based soil sampling to assess temporal changes in soil constituents

Agricultural land currently is used not only for producing food, fiber, and other bioproducts, but also for disposing of and recycling industrial, municipal, and agricultural wastes. Because such uses may potentially have negative effects on the environment, the fate of applied elements or of those originally present in the soil are under intensive scrutiny. Temporal change in the mass of a given soil element or constituent per unit area typically is calculated as the difference between constituent masses in a fixed soil depth at two sampling times. This method, however, is adequate for only rare cases when soil volume remains unchanged (i.e., when there are no changes m soil bulk density or thickness). Other methods based on "fixed soil mass," "equivalent soil depth," or "cumulative mass coordinate" have been developed to account for bulk density changes but they still do not account for changes in soil mass, such as those associated with waste inputs or soil redistribution through erosion and deposition, or imports and exports. We propose an alternative method based on elevation-based soil sampling to account for the effects of changes in both soil bulk density and soil mass. Unlike methods that assume soil mass remains unchanged, the proposed method would also be applicable to sites with appreciable additions or removals of soil mass or volume. We discuss the merits of elevation-based soil sampling to assess temporal changes in soil constituents, and present an example of its application to a site receiving heavy applications of livestock manure for 30 yr.