As the world grapples with the environmental impacts of modern agriculture, researchers are exploring innovative ways to enhance the sustainability of farming practices. One such effort, led by a team at the University of Wisconsin-Madison, has uncovered a promising solution: harnessing the power of manure to improve the efficiency of synthetic fertilizers.
At the heart of this study is the complex issue of nitrogen management. Nitrogen is a critical nutrient for crop growth, but its application as a synthetic fertilizer often leads to unintended consequences, such as water pollution and greenhouse gas emissions.
“The core issue we were trying to address is the fact that not all the nitrogen we apply as fertilizer ends up being used by the plants,” explains Josh Mirabella, the PhD student with the University of Wisconsin-Madison who spearheaded this research. “A lot of it can be lost through runoff, leaching, or gaseous emissions, which is not only a financial cost to farmers, but also has significant environmental consequences.”
The big-picture goal of this research was to find ways to increase how much nitrogen could be retained by soil, Mirabella adds. “A number of new technologies for processing liquid dairy manure have hit the market in the last decade, and a lot of them haven’t really been tested in a field setting,” he says. “So our goal was to see how the use of a couple of these processed manure solids interacted with chemical fertilizer applications. And our goal was to see if any of them are better than others for helping soil hold on to fertilizer.”
Searching for answers
To better understand this challenge, researchers designed a study that mimicked a corn silage system where combinations of different rates of chemical nitrogen fertilizer were applied in the form of ammonium sulphate in combination with several different manure products.
Mirabella explains that this study employed dairy manure, and he stresses the fact that results could be different had this research been conducted using other forms of manure, including swine or poultry manure.
The team’s experimental design focused on testing how dairy manure might impact the fate of synthetic fertilizer. “We looked at two different types of processed manure solids – one that was chemically separated and one that was physically separated from the liquid fraction,” he says. “We also included a typical liquid dairy manure treatment and a control treatment with no manure at all.”
Mirabella notes that the liquid dairy manure and solid manure products differed in many ways. “Two of the most important include dry matter and the form of nitrogen,” he says. “While the liquid manure was less than 3% dry matter and had the majority of its nitrogen in the form of simple inorganic ammonium, the manure solids were about 90% dry matter and had most of their nitrogen as less-available organic nitrogen.”
Each type of manure was combined with different rates of chemical fertilizer, and researchers devised an ingenious way to track the journey of the applied nitrogen. “We used a special technique where we applied the nitrogen fertilizer as an isotope,” say Mirabella. “This allowed us to physically track where the different molecules of that fertilizer nitrogen ended up – in the plants, in the soil, or leaving the system entirely as nitrous oxide or something else.”
By applying these different manure treatments in combination with varying rates of synthetic nitrogen fertilizer, the researchers could observe how the manure influenced the nitrogen’s behavior.
Surprising insights
The results of the study, published in the journal Biology and Fertility of Soils, revealed some unexpected insights. “The biggest difference we saw was that both the chemically- and physically separated manure solids actually increased the amount of fertilizer nitrogen that we were able to find in the soil one year after we applied the fertilizer,” say Mirabella.
In contrast, the liquid dairy manure treatment did not have the same effect. “When we came back a year later and measured the soil, we found significantly more of the applied fertilizer nitrogen in the topsoil where the manure solids had been used, compared to the liquid manure and the control with no manure,” he says.
This suggests that the processed manure solids may have a unique ability to help the soil “hold on” to the applied synthetic nitrogen, reducing the potential for losses.
However, more isn’t always better as Mirabella explains that there’s a limit to how much nitrogen plants can take up.
“I think the most surprising thing to me personally was that there wasn’t a big crop response to the different manure treatments,” he says. “For example, if you compare how much nitrogen the crops took up with the liquid manure compared to the different manure solids, it took up the same amount of nitrogen, essentially, regardless of what kind of manure we applied. And that was even true, to some extent, with the no-manure area, because we did apply chemical nitrogen fertilizer.”
That said, researchers found that the manure solids had a positive impact on nitrous oxide emissions. “We measured the nitrous oxide emissions throughout the growing season, and we found that the manure solids treatments had significantly lower emissions compared to the liquid manure plots,” says Mirabella.
Nitrous oxide is a potent greenhouse gas, and reducing its release from agricultural soils is a crucial step in mitigating the environmental impact of farming.
Implications for farmers
The findings of this study have important implications for farmers looking to optimize their nitrogen management practices. “The key takeaway is that the form of the manure you apply can make a big difference in how effectively the synthetic fertilizer nitrogen is retained by your soils and kept available for your crops,” says Mirabella.
Traditionally, many farmers have relied on applying liquid dairy manure directly to their fields, often in combination with synthetic fertilizers. However, this research indicates that investing in manure processing technologies to extract the solid fraction may be a more effective strategy.
“The solid manure fractions, whether they’re chemically or physically separated, seem to be better at helping the soil retain applied nitrogen fertilizer,” says Mirabella. “This could translate to reduced losses through runoff, leaching, or gaseous emissions, ultimately improving your nitrogen use efficiency and saving you money on fertilizer inputs.”
Of course, the specific benefits will depend on the individual farm’s circumstances, such as soil type, climate, and existing manure management practices.
“It’s important to remember that this study was focused on dairy manure, so the results may not directly translate to other livestock operations,” Mirabella says. “But the general principle of considering how manure is processed and applied is likely to be relevant across different farming systems.”
For farmers interested in exploring the potential of manure-processed solids, Mirabella recommends starting by evaluating your current manure management practices and considering whether investing in processing technologies could be a worthwhile investment.
“Talk to your local extension agent about the manure processing options available in your area,” says Mirabella. “They can help you assess the costs and benefits, and determine if it might be a good fit for your operation.”
Additionally, Mirabella encourages farmers to stay up to date on the latest research in this area. “As new technologies and management strategies emerge, it’s important to keep an open mind and be willing to try new approaches,” he says. “The more we can learn about how to effectively harness the power of manure to improve fertilizer efficiency, the better we’ll be able to enhance the sustainability and profitability of our farms.”
The challenge of managing nitrogen in agricultural systems is a complex one, but this research offers a promising solution. By exploring the influence of different manure products on the fate of synthetic fertilizers, researchers have uncovered a path forward for improving nitrogen use efficiency and reducing the environmental impact of our farming practices.
By embracing the power of manure, farmers can unlock new opportunities to enhance the productivity and resilience of their operations, all while protecting the natural resources we all depend on.
