Ecological Services of Constructed Two-Stage Agricultural Ditches

Abstract

Data collection occurred on geomorphic evolution, turbidity and denitrification potential at 8 constructed two-stage ditch sites in Ohio, Indiana, and Michigan. Systems ranged in age from one to nine years. Objectives were to: 1) demonstrate that two-stage channels maintain a stable geometry over time without excessive aggrading or degrading; 2) quantify the capacity of agricultural ditches using a two-stage management approach to reduce stream water turbidity and sediment export; 3) quantify if the two-stage ditch improves stream bottom habitat for macroinvertebrates, fishes, and mussels; 4) determine whether two-stage project sites of varying age differ in nitrogen removal capacity via microbial denitrification. All systems studied have been very stable, exhibited small adjustments on the constructed floodplains (benches), and have required little or no maintenance. Turbidity and sediment export decreased in the majority of two-stage ditches, compared to upstream control reaches. Results showed that older two-stage ditches (= 4 yrs) have proportionally more large substrates compared to conventional trapezoidal ditches, but younger two-stage sites (= 3 yrs old) do not. Therefore, the two-stage ditch may improve in-stream habitat by revealing larger substrate, but improvement in stream habitat may take several years to occur. Soil denitrification rates were highest on mature benches (>5 yrs). When both in-stream and floodplain bench rates were scaled to areal rates, N removal in a two-stage ditch was 2-14 times higher compared to conventional trapezoidal ditches. Two-stage designs enhanced the ability and capacity for ditches to permanently remove Nitrate-N from stream water via microbial denitrification.