The impact of the sediment from agricultural diffuse pollution control surface-flow treatment wetland on soil fertility and grain yield

Climate change has brought more extreme and unexpected weather conditions that result in frequent storm events and longer drought periods that are making agricultural production much harder. Efficient production needs in addition to favorable weather conditions also sufficient amount of nutrients in the soil. In ecological agriculture common alternatives to artificial fertilizers are manure and digestates. The main role of agricultural diffuse pollution control treatment wetlands (TWs) is to mitigate contamination that is coming from agricultural fields. Captured nutrients and organic matter are stored in the TWs mainly as sediments. It is quite common practice to use lake sediments as an agricultural soil amendment, however, there is less information about the use of TW sediments for improving soil health and nutrition. 

The main aim of the current study was to determine the impact of TWs’ sediment on the soil fertility and grain yield during a large-scale field experiment conducted during one vegetation period from May until September 2023, in Estonia. The amendment effect of the sediment was compared with control (no amendment), digestate, and N:P:K mineral fertilizer. According to the initial soil composition and recommended fertilization rate for nitrogen and phosphorus, the needed amount of sediment, digestate, and fertilizer was applied to four field plots (48 m2 each) before spring wheat sowing. The experimental area had an onsite weather station measuring humidity, precipitation, and air temperature. Each plot had a total of 6 sampling points for monitoring: soil composition, microbial and fungal communities and roots development, wheat growth, and GHG emissions (LI7810 and LI7820 analyzers, LICOR Biosciences); and constant measurement of soil moisture, temperature, and electrical conductivity (probes WET-150; Delta-T Devices).  

One of the most important outcomes of the study was that during the extremely dry spring of 2023, the sediment amendment had much higher soil moisture content, which resulted in much earlier sprouting and earlier grain ripening. The soil fertility and composition were on average more favorable with sediment addition. After amendment with sediment, soil had an average TOC content of 3.1% of C, compared to 1.7% on the other plots. Sediment amendment resulted in much higher average plant-available Ca and Mg content compared to other plots. The highest average NO3 contents was with digestate and lowest with sediment (443 vs 112 mg/kg). The considerably higher yield was gained with sediment (on average 9417 kg/ha) while all other plots had a similar yield to each other (on average 6510 kg/ha). Further analyses will show how sediment affected soil communities and root development. 

Overall, we can conclude that agricultural soil amendment with TW sediment gave promising results for ensuring higher crop yield. Further studies are needed to determine if similar results can be shown with other crops and with combinations of different soil amendment practices.