New Mexico Geological Society Annual Spring Meeting — Abstracts

In the past, active floodplain soils were considered much too young to contain any significant amount of soil development, such that few soil studies have been conducted on floodplains to determine rates of soil formation in time scales of years or decades. This is particularly so in arid regions, where rates of soil development are generally slower than those in more humid regions. The Middle Rio Grande floodplain at the Rio Grande Nature Center in Albuquerque provides an opportunity to study a unique floodplain environment, where the surface has been artificially stabilized by the Cochiti dam construction in 1942. Soil stratigraphy in this region is characterized by a complex sequence of overbank flooding deposits and lateral accretionary deposits which have been subjected to pedogenesis for the past 57 years.

Eight soil pits were excavated along north-south and east-west transects ofthe Rio Grande River in order to characterize the soil spatial variability. Soil stratigraphy, vegetation species, and vegetation density are variable along both transects. Pedogenic features such as O and A development, root diameters and densities, and degree of sedimentary structure preservation vary noticeably with changing vegetation. Typically, the better-developed soils are observed in the forested areas. Laboratory analyses revealed textural and vegetation controls on both organic matter and carbonate content. Organic carbon weight percentages are higher and penetration of organic matter is deeper in riparian forest soils than in desert patchland soils. Although weight percent calcium carbonate is less than 0.5 % in both the desert patchland and riparian forest, carbonate penetration is deeper and concentrations are lower in the riparian forest soils. A positive relationship also exists between finer-grained sediments and organic content in both soils. Our findings suggest that: (1) short-term pedogenesis is suprisingly significant on the Middle Rio Grande floodplain when contrasted with soils of deposits of similar age elsewhere in the region; and (2) vegetation exerts some control over the rate of incipient stages of soil development. In other words, one must consider that the apparently accelerated soil formation is largely attributable to the construction of Cochiti dam.