Hillslope Soils and Organic Matter Dynamics within a Native American Agroecosystem on the Colorado Plateau

Abstract

Zuni farmers of western New Mexico demonstrate knowledge about soil and hydrological processes that link upland watersheds to alluvium‐derived soils that have crucial cultural, ecological, and hydrological functions. To define how hillslopes contribute to productivity of soils derived from alluvium, we studied soil‐vegetation‐landform parameters in three headwater drainages on the Zuni Indian Reservation. Analyses along eight summit to toeslope transects show that soil properties follow parabolic and linear trends with changes driven by elevation, lithology, and vegetation in the mesa‐canyon pinyon–juniper–Gambel oak ( Pinus edulis Engelm. –Juniperus spp. –Quercus gambelii Nutt.) woodlands. Total organic C, N, and P concentrations in surface horizons follow negative parabolic trends and are highest on wooded backslopes and lowest on summits and toeslopes. Inorganic N and available P concentrations and total organic to inorganic N and P ratios increase linearly from summit to toeslope. Taken together, soil, landform, and vegetation data suggest: (i) summit positions are relatively stable with immobilizing microbial environments; (ii) inorganic nutrients increase progressively down steep and erodible backslopes as inputs of forest litter are mixed with surface soil; (iii) influx of mixed sediment and organic materials from backslopes maintains concentrations of inorganic nutrients on footslopes and toeslopes. Entrenchment of drainage ways can circumvent these translocation processes. Without the influx of organic materials, footslopes and toeslopes may become nutrient‐depleted as immobilization becomes the dominant microbial process. The results underscore the importance of functional connectivity between upland hillslopes and alluvial soils.