A unique ‘designer biochar’ made from waste can remove up to 41% of the fertilizer pollution flowing off of agricultural fields, a new study reveals. What’s more, it shows that this phosphorus-enriched material can then be sprinkled back onto farm fields, returning nutrients to the soil and enhancing the potential for circular fertilizer to reform our food systems. 

This striking win-win was created with two key ingredients: sawdust and lime sludge, byproducts of milling and wastewater treatment. By pyrolyzing these at high temperatures under low-oxygen conditions, researchers were able to turn the organic materials into a charcoal-like biochar, ready for the experiment. 

Biochar has already attracted accolades for locking away carbon, improving irrigation, enriching soils, and even filtering out microplastics from the earth. Meanwhile, previous lab studies have shown that the ‘designer’ sawdust-and-lime biochar that the researchers review in the new paper can also extract excess phosphorus nutrients from soil, before it trickles off downstream. The new Water Research study is the first to examine that potential on the field.

To run their experiment, the researchers relied on a piece of infrastructure that is commonplace on many farms: tile drainage systems, which are networks of pipes buried beneath crops, designed to capture water as it flows through the soil and channel it to the field edge, to stop fields becoming water-logged. 

Working on a 9.71 hectare farm field, the researchers kitted it out with a drainage system of their own, retrofitted with small chambers that they packed full of the biochar. These were situated close to the pipe outlet points, to allow rainwater to move through the biochar chamber before exiting the field. Their experiment ran in two phases of several months each, firstly in 2021, and secondly between 2021 and 2022, to capture the effects of seasonal differences in rainfall. 

They also tested out two types of biochar: one bunch of pellets larger than two centimeters, and another bunch that was smaller than one centimeter. 

Over the course of the study, two things became clear: that biochar pellets could slurp up a substantial amount of excess phosphorus, but that in order to do this, size really mattered. Across the two experimental periods, the researchers found that the larger biochar pellets were able to extract between three and 5.2 milligrams of nutrients depending on water flows, a small amount that gave them a phosphorus removal efficiency of 1.3 to 12%. The smaller pellets however, drew out between 27.2 mg and 30.4 mg of phosphorus, giving them a far greater removal efficiency of 38 to 41%. 

This chimed with the findings from the earlier lab experiments, which showed that the smaller the biochar pellets are, the more rapidly and efficiently they absorb phosphorus from water. 

And, that’s not the end of the road for the pellets—which started out as waste, helped remove waste, and could then be added to farm soils to incorporate the lost phosphorus back into crops, the researchers demonstrated in their field experiment.

Their final step was to do a lifecycle analysis on the designer biochar, including these final stages of its story. This revealed reductions of 12 to 200 kilograms of carbon dioxide equivalent were possible for each kilo of phosphorus removed from farmland run-off—partly because the recycled phosphorus returned to the soil would offset the addition of carbon-intensive virgin fertilizers. 

That lifecycle analysis also showed that their designer biochar would cost $413 per ton to make, which is less than half the cost currently to make regular biochar from other sources. So, for farmers looking to manage their environmental impact—but do so cost-effectively—they hope their new invention appeals.

Building that appeal is essential to move biochar beyond proof-of-concept and directly into the field, which is what the researchers hope to see more, on the back of this research. “There’s something very attractive about being able to reuse the pellets on the fields,” they add.

Zheng et. al. “Exploring the engineering-scale potential of designer biochar pellets for phosphorus loss reduction from tile-drained agroecosystems.” Water Research. 2024.

Image: ©Anthropocene Magazine