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Title: Spatial Patterns of Sediment and Phosphorus in a Riparian Buffer, Western Iowa

item Tomer, Mark
item Moorman, Thomas
item Kovar, John
item James, David

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2007
Publication Date: 10/15/2007
Citation: Tomer, M.D., Moorman, T.B., Kovar, J.L., James, D.E., Burkart, M. 2007. Spatial Patterns of Sediment and Phosphorus in a Riparian Buffer, Western Iowa. Journal of Soil and Water Conservation. 62(5):329-338.

Interpretive Summary: While riparian buffers are known to filter sediment and phosphorus (P) in runoff passing from agricultural fields towards streams, buffer performance is seldom looked at in terms of sediment and P accumulation in a buffer through time. We estimated accumulations of these contaminants in a buffer in western Iowa during a three year period, using sequential topographic surveys and soil sampling. We found significant variation in sediment accretion in the buffer, and that it's pattern corresponded to anticipated pathways of accumulated runoff from the adjoining hillslope. Total sediment accumulation was estimated at two tons per acre of contributing area per year. Because of the variation in sediment accumulation, P accumulation varied by nearly an order of magnitude (7 to 55 g m-2) among five plots. We also found dissolved P in soil water and ground water to be greater in areas where sediment accumulated, and that P in soil water was greatest beneath the switchgrass zone of the buffer, adjacent to the crop. These findings have implications for the design of conservation practices. First, although the buffer appeared to trap sediment effectively, it was located beneath a field managed in no-tillage, and the study occurred during a droughty period. Very little erosion should have occurred under these circumstances, yet sediment accumulation in the buffer was substantial. Results therefore emphasize the importance of using multiple conservation practices within and below fields to effectively control erosion in susceptible areas as are common in this region. Second, design of riparian buffers should consider the spatial pattern of runoff and season runoff is likely to occur. Warm season grasses like switchgrass may be best suited to interior zones of a buffer where runoff is most likely during early spring, or leaching of P may be increased. Results are of interest to conservationists and policy makers interested in improving the design of riparian buffers and understanding their effectiveness in combination with in-field erosion control measures.

Technical Abstract: Riparian buffers prevent sediment and phosphorus (P) from reaching streams, however their accumulation in buffers is seldom measured. This study’s objectives were to determine accumulations of sediment and P in a multi-species riparian buffer, and spatial-temporal patterns of P in soil water and ground water. The buffer was planted in 2000, below a steep-sloping field in row-crop production under no-tillage management in Iowa’s Loess Hills. Topographic surveys were conducted in 2002, after the buffer was fully established, and again in 2005. Mapped differences in elevation showed sediment accretion was associated with concentrated flow pathways and lateral flow along the buffer-crop margin. About 32% of the buffer’s outer switchgrass (Panicum virgatum L.) zone had sediment accumulations exceeding 4 cm, which totaled 14.5 Mg ha-1 contributing area (2 t/ac/yr). Among five soil plots, total P in accreted sediment varied from 7 to 55 g m-2, totaling 9.6 kg P ha-1 contributing area. Phosphorus concentrations in soil water were greatest beneath the switchgrass, compared to the crop, and inner vegetation zones of the buffer (p<0.05). Concentrations in soil water and ground water were also greater where sediment accumulated, presumably due to increased infiltration of runoff. Sediment and P trapping occurred despite no-tillage management in the buffer’s contributing area, and relatively dry conditions during the study. This emphasizes the importance of installing multiple, complementary conservation practices in sensitive environments. Considering seasonal risks of runoff when selecting species, and anticipated runoff patterns when planning buffer areas, could help minimize subsurface P loss through buffers.