What happens to nutrients and metals in soil long after biosolids—a byproduct from sewage treatment—have been applied to the land? That’s the question JoAnn Donald, a graduate student in the UF/IFAS Department of Soil, Water, and Ecosystem Sciences (SWES), set out to answer. The research study appears in the journal Geoderma Regional.

Working under the guidance of Dr. Yang Lin, SWES assistant professor of soil health, JoAnn studied a former Florida ranch that applied biosolids 14-years ago. The findings show how long these substances linger in the sandy soil and what that means for future environmental management. JoAnn further explains the research in this Q & A:

Can you tell us in basic terms what this study was about?

This research examined what happens to the nutrients and metals left behind in soil after sewage treatment leftovers—called biosolids—are spread on land. Specifically, we looked at a ranch in Seminole County, Florida, where biosolids were last applied 14 years ago. We wanted to know what remained in the soil and whether anything had moved deeper into the soil.

What exactly are biosolids, and why do some producers apply them to land?

Biosolids are what’s left after sewage is treated – cleaned and processed – which allows producers to use it on agricultural land. Biosolids are full of nutrients, like phosphorus (P), which plants need. So, people spread them on fields to improve soil and help crops grow. But because they may also contain things like heavy metals, there’s been concern about long-term effects.

What were the most surprising results from tracing biosolids in this study?

The biggest surprise was how much P was still in the soil after 14 years. It amounted to nearly three-quarters of what the ranch originally applied. That tells us P sticks around much longer than we expected, even in sandy soils where things usually wash away more easily. On the other hand, the heavy metals we tested for, like lead and nickel, were still at safe levels and had not built up in the soil.

Why is leftover phosphorus a concern?

When there’s too much P in the soil, it can wash into nearby rivers and lakes. That can cause algae blooms, which are harmful to fish, wildlife, and water quality. It’s called eutrophication, which can deplete oxygen in the water. It’s a significant environmental problem in Florida and many other places.

Did anything else stand out to you during the research?

Yes, we noticed that P wasn’t just sitting in the top layer of soil. It had moved downward along with calcium (Ca). In fact, Ca plays an important role in stabilizing P in soil, as we found a positive relationship between soil P and Ca levels. Our findings also mean P could eventually reach groundwater or affect deeper soil layers, which raises more questions about long-term impacts.

So, does this mean we shouldn’t use biosolids anymore?

Not necessarily. Biosolids are a great way to recycle nutrients and organic matter, but we need to manage how we use them and where. Our study shows that we all need to think more about the long-term effects, especially in places with sandy soils that don’t hold onto nutrients very well.

How did this tracing biosolids project come about?

I was a master’s student in SWES in Dr. Yang Lin’s lab. The idea was part of a bigger effort to understand how biosolids impact Florida’s unique sandy soils over time. I was lucky to be able to work with a great team and use a lot of interesting lab techniques.

What do you hope people take away from your research?

I hope people understand that what we put on the land doesn’t just disappear. Nutrients like P can remain in the soil for a long time and move through soils in ways we might not expect. It’s important to keep studying these effects so we can make better decisions about land and water management.

What’s next for you?

I’m planning to continue working on methods to remediate the phosphorus from biosolids. I really enjoy research that connects science to real-world problems, especially when it comes to protecting water and soil. I’m currently pursuing my PhD in SWES with Dr. Lin.

You can see the full journal article on tracing biosolids here: doi.org/10.1016/j.geodrs.2025.e00961

Featured image from Adobe Stock – Ludmila Smite

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