Sixteen years of two-stage floodplain development: Long-term changes in soil chemistry

High concentrations of nitrogen (N) and phosphorus (P) are often found in subsurface tile drain effluent from agricultural fields resulting in eutrophication and subsequent dead zones in downstream bodies of water. Although two-stage ditches are widely implemented as floodplain restoration practices to improve channel stability and enhance nutrient retention, their long-term (>10 years) soil biogeochemical development and capacity to function as sustained nutrient sinks remain poorly quantified. To evaluate the long-term nutrient retention potential of two-stage ditches, we assessed soil biogeochemical properties across a chronosequence of restored floodplain sections in the Shatto Ditch Watershed, IN, USA, spanning 5–16 years since construction. Additionally, we compared floodplain soils with both the on-floodplain flowpaths of tile drain effluent and adjacent agricultural fields across depth profiles, providing context to evaluate how soil properties changed relative to the fields from which the floodplains were constructed. We found that soil % organic matter (OM) on 5- and 6-year floodplains was comparable to their respective adjacent fields. In contrast, % OM on the 16-year floodplain was significantly higher than both its adjacent field and the younger floodplains, consistent with progressive OM enrichment over time. We found that soil N and P also suggested progressive accumulation with floodplain age, with N significantly higher in the 16-year floodplain relative to both its adjacent field and the younger floodplains, and P trending higher in older floodplains. Our results indicate that two-stage ditches accumulate OM, N, and P over 16 years, highlighting their potential as long-term nutrient retention features in agricultural landscapes.