Low-Fe lake sediments recycled to agriculture improved grass growth- and hardwood biochar reduced leaching of N and P

 

In AgriChar research group, we are trying to find ways to best close the phosphorus (P) cycles in the environment by recycling nutrients in sediments back to the agriculture and restoring eutrophic lakes.

The nutrients including Phosphorus (P) in upstream agricultural soils are washed out by erosion and reach the bottom of the lake sediment. Excessive load of (P) causes eutrophication problems in many lakes. One of the effective restoration methods especially for small shallow lakes is removal of sediments enriched with nutrients. We examined the factors behind P release from sediment to lake water after removal of sediments from a 1-ha shallow eutrophic lake during two-year follow up period. Removing 7 500 m3 of nutrient-rich sediment removed 6 400 kg of P, including the potential source of P for algae. 

 

However, high pool of releasable P was rebuilt soon after the removal of sediment due to high external P loading, resulting in high internal P loading which sustained eutrophication of the lake. So ways that help to control nutrient losses from fields upstream are of great importance! Our study documented clear strong linkage between sediment P and iron dynamics after the restoration. We also found that the Fe-P and labile P fractions were the most important sources of P release evidenced by their considerable seasonal and interannual changes after the sediment removal. 

Our story did not stop at this point. In the next step, during a nine-month lysimeter experiment, we examined the potential solutions for the disposal of large amounts of excavated lake materials after restoration procedures, and sustaining agricultural crops yields without increasing the nutrients leaching from soil. We also tried self-made biochar with Kon-Tiki kiln in reducing the leaching of nutrients- and were surprised to see that indeed EBC premium-grade biochar can be made fully DIY:

 

From this study we learned that reusing excavated nutrient-rich sediment from a eutrophic lake, with a low iron (Fe) to P ratio, improved properties of a nutrient-deficient sandy loam surrounding the lake by increasing the availability of P and other nutrients in soil. Although the soil chemical fertility enhancement of the depleted soils alone would suffice to make this application advisable, the substantial increase in plant growth and P uptake in treatments with sediment-amendments adds further interest. 

 Of all components of soil P, the Fe-P fraction was the most important contributor to plant P uptake. Considering the environmental impact, applying a relatively thick layer of sediment on a 25-cm topsoil layer did not increase the risk of phosphate and mineral nitrogen leaching. Also, a biochar layer was  a promising addition for reducing P and N leaching from sediment.

Please check the following recently published articles for more interesting information:

Kiani, M., Tammeorg, P., Niemistö, J., Simojoki, A. and Tammeorg, O., 2020. Internal phosphorus loading in a small shallow Lake: Response after sediment removal. Science of The Total Environment, p.138279.

Kiani, M., Raave, H., Simojoki, A., Tammeorg, O. and Tammeorg, P., 2021. Recycling lake sediment to agriculture: Effects on plant growth, nutrient availability, and leaching. Science of The Total Environment, p.141984.