Grad Student Research Spotlight

“Phosphorus Release from Biochar of Various Feedstocks: Implications for Land Management”
Andressa Freitas (Soil & Water Sciences PhD 2019)


Phosphorus: Excessive fertilizer use can lead to nutrient loss from the soil resulting in algae growth in receiving waters. Reserves of the phosphate mineral are being depleted and could be eliminated in less than a century under current consumption levels.

Biochar: Environmental benefits include mitigation of greenhouse gas emissions, soil carbon sequestration, waste management, recycling nutrients, heavy metal remediation. Agronomic benefits include a decline in bulk density and increases in soil pH, water holding capacity, cation exchange capacity, microbial activity. It also stabilizes elements such as phosphorus.

Biochar Feedstocks (raw material): Animal-based feedstocks include poultry litter, biosolids, swine manure, and bones. Plant-based feedstocks include bagasse (sugarcane/sorghum), wood chips, sawdust, and tree bark/leaves/branches.

Research Questions

What is the proper feedstock to use – plant-based (hardwoods, maple, and pine) or animal-based (biosolids and poultry litter)?

What effect does biochar additions have on different soils?

What is the effect of biochar application on crop yield?

“I became interested in biochar, because of its discovery in my home country, Brazil, hundreds of years ago,” Andressa Freitas said. “My research started looking at characteristics of biochar and which feedstocks were suitable sources of phosphorus.”

The results showed animal-based biochar is more suitable as a source of phosphorus, while plant-based biochars are a potential source of other nutrients, such as potassium. Biosolids converted to biochar significantly increased total phosphorus and decreased water-soluble phosphorus without compromising phosphorus available to the plant. Chemical and physical characteristics of poultry litter biochar suggested that it could act as a slow P-release fertilizer. Freitas needed to validate these results in the field, which showed positive effects on crop yield.

Andressa Freitas, Soil and Water Sciences doctoral student, stands by corn grown with poultry-litter biochar.
Andressa Freitas stands by corn grown on a test field with biochar-amended soil.
Photo from Vimala Nair

“Pyrolysis (the process of making biochar) could be used to decrease the environmental risk of phosphorus loss from biosolids,” Freitas said. “This might still leave phosphorus available for the crops to use depending on the biosolids source.”

But what is the effect of biochar in Florida’s sandy soils? Freitas needed to determine phosphorus desorption from sandy soil that had been amended with biochar. Specifically, she wanted to know whether animal-based or plant-based biochar would release more phosphorus when applied at the same rate. Experiments were conducted on two different soils: Apopka (more phosphorus retentive) Ultisols and Candler (less phosphorus retentive) Entisols.

“We found plant-based biochar leached the same amount of phosphorus, regardless of the soil type,” Freitas said. “For the poultry-litter biochar, soils played a role. We had less phosphorus release in a more phosphorus retentive soil.”

The last part of the study was to test poultry-litter biochar in the field. Freitas compared it with an inorganic phosphorus fertilizer treatment and a control plot of no phosphorus. This was done in two locations with different soil types (Entisols and Spodosols) over two years (six cropping cycles). The results showed poultry-litter biochar can be a safe substitute for inorganic fertilizer as a phosphorus source.

“In addition, poultry-litter biochar generated higher cumulative biomass than an inorganic phosphorus treatment when applied to the Entisol site compared to the Spodosol one,” Freitas said.

“I’m surprised by the different biomass amounts generated when the same amount of poultry-litter biochar was applied in each location,” Freitas said. “I’m still trying to figure out what would be the cause of that, and I hope to better understand that with additional tests we will be doing.”

Freitas did the research under the guidance of the chair of her advisory committee, Dr. Vimala Nair, a research professor of environmental soil chemistry in the Soils and Water Sciences Department. Dr. Nair believes the research has considerable implications.

“With all of Andressa’s results, now we know which biochars might work and which ones might not work as a plant nutrient under site-specific conditions,” Nair explained.

“In the developing world where there’s a lack of fertilizer or fertilizer is too expensive and difficult to obtain, biochar could be an alternate nutrient source,” Nair continued. “Those populations have farm wastes that they need to dispose of so they’re not paying for the raw material, and the conversion to biochar can be done in their backyard or in community kilns.”

“The main thing I want people to take away from this work is that not all biochar is the same,” Freitas said. “What works well as a nutrient source in one soil for one crop, may not be beneficial for another in different soil.”

Making an Impact
Dr. Andressa Freitas (SWS ’19)

During her five years at UF, Andressa Freitas has made a noticeable impact in service and scholarship. Her resume includes working as a research assistant in the Soil and Water Sciences Department since she arrived in 2014. She also served as a teaching assistant for two courses; having a major role for one of those during a semester.

“I love teaching,” Freitas said. “I have a degree in agricultural education, and I was a teacher in Brazil before coming to UF.”

The Association for Academic Women at UF awarded her an Emerging Scholar Award for her dissertation. The Madelyn Lockhart Dissertation Awards Committee at UF selected Freitas for her “ability to articulate the importance and transformative potential” of her dissertation work.

As a member of the Soil Science Society of America (SSSA), Freitas was selected for the 2019 Congressional Visits Day. She was one of approximately 70 representatives of the SSSA, American Society of Agronomy, and Crop Science Society of America to lobby lawmakers in Washington, D.C., for more research funding.

“We spent about ten to 20 minutes just discussing some issues, mostly with congressional staff members,” Freitas explained. “For one of them, their district had Marianna, so I said ‘Oh, we have an experiment going on in Marianna,’ and we discussed what is needed.”

She calls it an eye-opening experience to see how the government works in America, compared to her native Brazil. The societies provided training for the participants before heading to the Capitol, another learning experience Freitas appreciated.

“We, as scientists, need to communicate with policymakers, to translate what the needs are and try to convince them what we need is necessary,” she said.

Freitas will be spending another year with the SWSD as a postdoc working with Dr. Nair, but her goal is to work with the United Nation’s Food and Agriculture Organization (FAO). The agency is leading the international effort to achieve food security for all by making sure everyone has regular access to high-quality food.

“My dream job is definitely FAO,” Freitas said. “There’s a long path until I get there, but that’s my main goal.”


Posted: May 24, 2019

Category: Crops, UF/IFAS Research
Tags: Andressa Freitas, Biochar, Poultry Litter, Soil And Water Sciences, Soil And Water Sciences Department, Soil Water And Ecosystem Sciences

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