Posted on May 8, 2023
Every year, farmers fertilize their crops with nitrogen and phosphorus to ensure a good harvest. And every year some of that fertilizer inevitably makes its way into ponds and lakes, where it can trigger ecosystem-wrecking algal blooms. According to a new study published last month in Science of the Total Environment, though, there may be a relatively simple way to close the loop on this leaky system: using the nutrient-rich lake sediments as fertilizer.
“We’d be wise as a society to recycle these nutrients,” says University of Illinois Urbana-Champaign soil scientist Andrew Margenot, who was not involved in the research. “There’s no silver bullet, but there’s lots of tiny little solutions. And this is one of them.”
When excess nutrients from agricultural runoff build up in waterways, it can cause algal growth to explode in a process called eutrophication. After the algae die and decompose, carbon dioxide emissions soar and oxygen is sucked up, killing fish and other marine life. For decades, environmental managers have worked to restore lakes and improve water quality by dredging up the decaying organic matter in the sediments and often dumping it nearby or in landfills.
Priit Tammeorg and Olga Tammeorg, researchers at the University of Helsinki who specialize in fertilizer recycling and phosphorus dynamics, decided to try something different with a small shallow lake called Mustijärv in Estonia. In 2017, they recruited Mina Kiani, a graduate student at the university, to see whether she could use nutrient-rich sediments dredged from the lake to grow grass crops such as Kentucky bluegrass, ryegrass, and red fescue.
In a university lab, grasses planted in dredged lake sediments produced double the yield of crops grown in topsoil. Next, Kiani took the experiment outside. Over the next 4 years, she compared the growth of crops planted in dredged sediment plots with those in plots of regular soil, observing the plants’ nutrient uptake by baking and grinding the plants then identifying their elemental composition through spectroscopy. To assess the environmental footprint of the practice, she also monitored the different plots’ greenhouse gas emissions and how much nitrogen and phosphorus leached out into the surrounding water in the soil.
The team found that in the field, the grass crops grew just as well in the lake sediments as in regular soil, and took up more nutrients, absorbing 40% more nitrogen over the course of the trial. But as the sediment began to decompose in the field experiment, it emitted significantly more carbon dioxide. Also, because the recycled lake sediment was so packed with nutrients, the grass crops couldn’t absorb all of them. As a result, the unused nitrogen and phosphorus leached back into the dredged lake.
The high degree of leaching and emissions mean the nutrient-dense lake sediments aren’t a great primary soil for growing crops, says Priit Tammeorg, who adds that it’s like trying to grow crops in pure manure. But with lots of nutrients and low levels of heavy metals and other contaminants, the researchers say these lake sediments still hold some promise for crop production. Spread more thinly, it may work well as a supplemental fertilizer or soil additive, Kiani says, reducing farmers’ reliance on costly, carbon-emitting synthetic fertilizer. The researchers’ next field experiment will test that hypothesis, and initial greenhouse-based results look promising, they say.
Because unrestored eutrophic lakes produce significant methane emissions, restoring lakes and recycling the sediments for agriculture could mean a net decrease in greenhouse gases reaching the atmosphere on an ecosystem scale, Kiani says.
To Margenot, recycling dredged sediment as fertilizer makes sense, especially for farmers who already live near lakes. “If you’re the lucky farmer who lives by the lake, where the hauling costs are low, awesome,” he says.
Antonio Mallarino, a soil nutrient management specialist at Iowa State University, cautions that before farmers buy in, they’ll want to know the composition of the sediment—whether it’s silt, sand, or clay—as well as its nutrient content, whether it has any contaminants such as heavy metals, and whether it can be easily applied to their fields. Still, if shown to be cost-effective, “I believe some farmers would be receptive,” he says.
Kiani says even if sediment recycling isn’t economically feasible right now, world events could change that. The war in Ukraine saw fertilizer prices triple across Europe in a year, for example, and the COVID-19 pandemic saw similar price spikes in the United States. Some economists also expect fertilizer-grade phosphorus costs to skyrocket as the world’s supply of the nonrenewable resource dwindles. Using dredged lake sediment may become comparatively cheap if these traditional fertilizer sources get too expensive, Kiani notes.