Organic Matter Transformations through Arroyos and Alluvial Fan Soils within a Native American Agroecosystem

Linked biochemical and fluvial processes in discontinuous ephemeral streams may support sustained productivity of soils farmed by southwestern Native Americans for ≥3000 yr. Ephemeral stream channels transport forest floor litter and soil materials from upland hillslopes to alluvial fans. Improved understanding of how ephemeral streams transport and process forest‐floor organic materials could improve conservation of ecologically important and productive headwater alluvial fans. We analyzed organic and mineral materials from source to mouth along two ephemeral streams and analyzed suspended sediments from four collection traps in each. Results suggest that decomposition processes differ by reach and frequent, low‐energy flows preferentially transport organic detritus as it decomposes. Processing of organic‐rich sediments in canyon reaches is dominated by microbial immobilization (low inorganic N and available P, high C/N ratio). Arroyo reaches receive organic materials chiefly from upstream so mineralization plays in increasing role as materials are transported and decomposed downstream without fresh inputs. The frequency of flow decreases in a downstream direction as water infiltrates sandy streambeds. In lower arroyo and fan reaches, inputs of organic‐rich sediments are infrequent. Relatively frequent wetting and drying stimulates mineralization of organic materials so concentrations of inorganic N and available P in detritus are relatively high. Results suggest that organic‐rich sediments processed through ephemeral streams and deposited on unincised alluvial fans are important in sustaining one of the most productive landscape positions in semiarid regions.