Physical and Chemical Properties of Bench Sediments in Self-Formed Agricultural Drainage Channels

Abstract:

Two-stage ditches are a novel approach to managing agricultural drainage and are designed with floodplain benches set within the banks of a standard, trapezoidal channel. The floodplain bench serves to attenuate pollutant loads in surface waters through (1) capture of sediments, (2) nutrient assimilation by vegetation, and (3) transformation of C and residual N and P by indigenous microorganisms. Two-stage channels have been constructed in the tri-state region of Ohio-Michigan-Indiana over the last decade with initial results indicating C and P sequestration and enhanced N removal via denitrification. However, the sustainability and the net ecosystem services provided by these designs are relatively unknown beyond this timeframe. To better characterize the properties of two-stage ditches aged more than a decade, we examined the physical and chemical properties of sediments in unplanned, self-formed floodplain benches across 5 distinct Midwest ecoregions. Established benches were selected from 3 locations within each ecoregion and sampled along depth and bench-positional gradients from geo-referenced sites. The sediment-bound C, N, and P concentrations were quantified along with soil texture and channel geomorphology. Nutrient concentrations did not differ across bench position (upstream, downstream, near bank, or near channel); however, significant differences were observed between ecoregions. Steeper slopes and higher benches were associated with higher sand content than surrounding soils and promoted greater storage of C and N. Gradual slopes, on the other hand, were associated with higher clay and silt content. Across these specific ecoregions, P storage declined with increasing depth. However, this was unexplained by the particle size distribution at these depths. Further research is therefore needed to investigate whether P is released from waterlogged sediments or there is biological redistribution of this nutrient across the column depth.