Soil health and runoff water quality of broadbase terraces implemented with new tile inlet technologies
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[EMBARGOED UNTIL 12/01/2026] Soil erosion and waterlogging are persistent challenges in claypan landscapes of northern Missouri, where broad-based terraces are widely constructed to control runoff and sediment loss. However, the process of terrace construction involves extensive soil disturbance that can alter soil physical, chemical, and biological properties. The drainage inlets installed to improve drainage and reduce ponding may facilitate the transport of nutrients and herbicides out of the fields, potentially contributing to downstream water pollution. This research was conducted at the University of Missouri's Grace Greenley Conservation Showcase Research Farm near Leonard, Missouri, to evaluate the short-term impacts of terrace construction on soil health, and fertility, and to assess the performance of different tile inlet technologies, such as standard Hickenbottom riser (HBR), HBR with channel laterals (HBR+CL), water quality inlet (WQI), and blind inlet (BI) on nutrient, sediment, and herbicide transport from farm-scale terraced fields. Geo-referenced soil samples were collected before and after terrace construction from three topographic positions (shoulder, backslope, and footslope) and four depths (0-15, 15-30, 30-45, and 45-60 cm). Terrace construction resulted in substantial alterations in soil properties. Both sand and clay contents increased 32 g kg-1 and 29 g kg-1, while silt decreased 60 g kg-1, indicating a textural class change from silt clay loam to clay. Bulk density at 30-45 and 45-60 cm soil depth increased 7-13%, whereas at 0-15 and 15-30 cm decreased by 8-23% due to topsoil removal and redistribution. After terrace construction, soil temperature in the surface layer (0-15 cm) increased 1.7 °C, permanganate oxidizable carbon increased 12%, and 0.06 g kg-1 net increase in total nitrogen was observed compared to pre-construction values. In contrast, aggregate stability declined by 15-19%, and total carbon decreased by 0.54 g kg-1. Soil enzyme activity was reduced 26-50% including acid phosphatase, β-glucosidase, β-glucosaminidase, and arylsulfatase, confirming biological disturbance from soil mixing and compaction. Terrace construction also altered soil fertility and apparent electrical conductivity (ECa). Total exchange capacity (TEC) and soil test S, Mg, K, Na, and Fe increased, while soil pH, Ca, B, and Cu decreased, indicating both enrichment and dilution processes. An EM38-MK2 was used to record ECa values, which decreased by 19-36% across all four depths after the construction of terraces. Principal component analysis distinctly separated pre- and post-construction values, confirming terrace-induced changes in fertility, texture, and ECa distribution. To address ponding issues in terrace channels after terrace construction, tile inlet technologies were compared under no-till management using a randomized complete block design. Across the 2023-2025 monitoring period, HBR had the highest median daily discharge (2645 L ha-1), followed by HBR+CL (1901 L ha-1), WQI (1443 L ha-1), and BI (997 L ha-1). Additionally, HBR generated the greatest sediment and nutrient losses, while BI and WQI showed significant reductions of sediment and nutrient loads. In 2025, BI reduced TSS by 95% compared with HBR, largely due to adsorption and filtration within limestone gravel medium. Herbicide monitoring during 2024-2025 revealed that the highest chemical loads occurred immediately after the first rainfall following herbicide application, with discharge volume being the primary factor influencing the loads. The HBR and WQI had the highest herbicide losses, while BI consistently reduced loads across all chemicals due to tortuous flow through limestone gravel, resulting in large reductions observed in the 2025 corn season. Atrazine concentrations occasionally exceeded the USEPA maximum contaminant limits for discharge, highlighting potential water-quality risks associated with atrazine and the need for innovative research to reduce loss. Overall, this study demonstrates that terrace construction substantially alters soil physical, chemical, and biological properties across the soil profile, while improved inlet designs, such as BI, effectively reduce nutrient, sediment, and herbicide transport from terraced fields. The WQI effectively reduced sediments while increasing chemical loads compared to the BI. The adoption of these conservation drainage systems, combined with other conservation practices such as residue retention and cover cropping, can help restore soil health and protect downstream water quality in terrace-tile drained, intensively farmed agricultural fields in northern Missouri.
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M.S.