Vertical distribution of total carbon, nitrogen and phosphorus in riparian soils of Walnut Creek, southern Iowa

Subsurface lithology plays an important role in many riparian zone processes, but few studies have examined how sediment nutrient concentrations vary with depth. In this study, we evaluated concentrations of nutrients (N, C and P) with depth in a riparian zone of the glaciated Midwest. A total of 146 sediment samples were collected from 24 cores that extended to a maximum depth of 3.6 m at eight sites in the riparian zone of Walnut Creek. Subsurface deposits were predominantly silt loam, becoming coarser and more variable with depth. Nitrogen and carbon content ranged from < 0.01 to 0.42% and < 0.01 to 7.08%, respectively, and exhibited a strong trend of decreasing nutrient content with depth. In contrast, P concentrations averaged 574 mg/kg and did not vary systematically. Systematic variations in texture and nutrient content with depth largely corresponded to stratigraphic differentiation among the Camp Creek, Roberts Creek and Gunder members of the regionally recognized Holocene-age DeForest Formation. Variations in subsurface nutrient content were not found to be significantly related to present land cover, but land cover may have influenced nutrient content at the time of original sediment accumulation. Subsurface lithology and stratigraphy should be considered an important component in riparian zone studies where nutrient losses to streams via streambank erosion or groundwater discharge are assessed.