From wetlands to fields: a two-year study on the impact of the surface-flow treatment wetland sediment on soil fertility and crop yield

Efficient agricultural production requires favourable weather conditions, healthy soil and an adequate supply of soil nutrients. In farming, common alternatives to artificial fertilizers include manure and digestates. Another potential soil amendment method is the addition of dredged bottom sediments from different water bodies. The use of lake sediments for agricultural purposes is well-documented; however, the potential of sediments from surface-flow treatment wetlands (TWs) designed to mitigate agricultural diffuse pollution remains largely unexplored. However, these sediments are rich in organic matter, nutrients such as phosphorus and nitrogen, and micro-nutrients like magnesium and calcium.

The primary objective of this study was to assess the impact of TW sediment on soil fertility and crop yield during a large-scale field experiment in Estonia across two vegetation periods. In 2023, the sediment effect was compared with a control (no amendment), digestate (a liquid by-product of biogas plant), and N:P:K mineral fertilizer. The longevity of the sediment effect was studied in 2024 using the same setup but with no additional amendments. Based on the initial soil composition and recommended fertilization rates, sediment, digestate, and mineral fertilizer were applied to three 48m² plots at the start of the 2023 season, just before sowing spring wheat. The buckwheat growth and yield were evaluated in the second season (2024). The experimental area had an onsite weather station. Each plot had 6 sampling points to monitor soil composition, root development, crop growth, greenhouse gas emissions, and continuous soil moisture, temperature, and electrical conductivity measurements.

One of the key findings of this study was that during the extremely dry spring of 2023, the sediment amendment significantly increased soil moisture content (median 38 % compared to 14 % on other plots), leading to earlier sprouting and grain ripening. During spring 2024, the soil moisture content was much lower (median 15 % with sediment and 11 % with others). On average, soil fertility and composition were more favourable when adding sediment. After the sediment application, the soil had an average total carbon content of 5.2 %, compared to only 1.7 % in the other plots. The carbon content was generally much lower in May 2024 but still the highest on sediment plot (median 2.4 %). The sediment amendment also resulted in much higher average plant-available Ca and Mg levels during 2023 and higher content during 2024 than on the other plots. The sediment amendment produced 2023 a significantly higher yield, median of 8.5 t/ha, while the other plots had lower yields (mineral fertilizer 7.2 t/ha; control 6.3 t/ha; and only 3.3 t/ha with digestate). With buckwheat, during 2024, we did not see any more such a clear effect of amendments applied at the beginning of the previous season.

Applying TW sediment as an agricultural soil amendment showed promising results for increasing crop yield in the application year. Further research is needed to determine if similar results can be achieved with other crops and combinations of different soil amendment practices.