Nitrogen loss in manure is an age-old problem, but manure additives and processes can help preserve nitrogen.
Melissa Wilson, University of Minnesota, explained the use of nitrification inhibitors. “Manure includes two forms of nitrogen from urine and feces,” she said. “From urine, there’s urea, or uric acid. This is considered ‘fast’ nitrogen. Even though it’s in organic form, it quickly converts to ammonium.”
Feces contain “slow” nitrogen, which is released over time. “Some is readily mineralized into ammonium, some takes longer,” said Wilson. “Readily mineralized slowly releases during the first year.” Residual components can take up to several years to release.
The way in which manure is excreted and stored dictates the ammonium, or inorganic concentration, of manure. Manure stored as a liquid or slurry typically has a higher concentration of ammonium.
The role of nitrogen cycling in soil is key to understanding nitrogen activity in soil. “We know plants take up ammonium and nitrate, but animals aren’t giving us ammonium and nitrate,” said Wilson. “They’re giving us ammonium and organic nitrogen. Cattle and poultry manure tend to have a higher proportion in the organic nitrogen form, where swine manure tends to have more ammonium nitrogen form. That’s important because it means it’s going to move more or less quickly through the nitrogen cycle in the soil.”
Organic nitrogen undergoes mineralization, which takes several years, and converts organic nitrogen to ammonium nitrogen. The concern with ammonium nitrogen is gaseous losses through volatilization – ammonium turns into ammonia gas and is lost to the atmosphere.
“The other way ammonium transforms is into nitrate nitrogen through nitrification,” said Wilson. “This is typically done by bacteria in the soil.”
After the conversion is complete, nitrate can be lost in several ways.
“It can be lost as a gas through denitrification process,” said Wilson. “This is typically happens in saturated soils. The other process through which nitrate can be lost is through leaching – water carries nitrates away, deeper into the soil profile where the roots can no longer get it.” Nitrification inhibitors disrupt soil microbes that cause the nitrification process.
Wilson described a research project aimed at determining whether manure application timing, with and without the additive nitrapyrin (Instinct®), affected corn grain yield and nitrogen availability in soil. Testing was done both before and after soil temperatures reached 50º.
Testing was conducted in early October and in November when temperatures were lower. In October, more of the nitrogen remained in the ammonium form with the use of nitrapyrin. In November, results were similar with more ammonium remaining in the soil.
Yields are an important consideration when using an additive. In October, without nitrapyrin, yields were significantly lower than with nitrapyrin. In November, there was no statistical difference in yields.
Wilson said this shows that using nitrapyrin or another nitrification inhibitor in October before soil temperatures are cooler is worthwhile. However, any increase in yield would have to offset the cost of applying the product. When a nitrification inhibitor is used in November, when soil temperatures are cooler, yield increases are lower and may not justify product cost.
Dr. Joe Sanford, University of Wisconsin, discussed role of biochar as an additive in manure management. Biochar is a product of pyrolysis, which involves placing organic biomass such as wood chips, corn stover, straw or manure in a reactor with low oxygen content at high temperatures (600º to 900º F or higher).
The products of pyrolysis are syngas and bio-oil. “Both of these products can be used for energy production or heating,” said Sanford. “We also end up biochar, which has interesting characteristics similar to activated carbon. It has high carbon content, high surface area, alkaline pH and a fairly good exchange capacity with nutrients and other elements.”
Biochar isn’t new and has been used primarily as a soil amendment in a variety of production systems. “It’s been shown to improve nutrient retention,” said Sanford. “Soils amended with biochar that received manure application had significant reduction in total nitrogen leaching and nitrate leaching. It’s also been found to increase water use efficiency in sandier soils, increase aeration and increase porosity in clay soils.”
Sanford’s research group is studying nutrient retention and how biochar might be used in manure management. “One area is compost,” he said. “This is probably one of the most studied areas when it comes to waste management. Biochar has been found to have benefits in composting, particularly around nitrogen retention in the form of nitrogen in the compost.”
The group is also examining how to best use manure solids. “If we’re separating solids, drying them down and converting to biochar, that’s another potential export from the farm,” said Sanford. “We’ve also been looking at using it as a cover on the top of manure storages and incorporating it into anaerobic digestion systems.”
Changes in the ag industry, particularly regarding water use, have resulted in manure storages without a natural crust, which increases ammonium emissions. Sanford said there’s potential for biochar to be used as a surface cover over liquid manure storage to reduce ammonia emissions and greenhouse gases.
Wood chips and corn stover used as manure storage covers reduced ammonia emissions, but when biochar was added to the mix, emissions were almost eliminated from manure storage.
“Cover stability is important,” said Sanford. “Biochar is fairly hydrophobic, and the cover staying on the surface of manure storage is essential. After about a month, the biomass started to sink but the biochar was just about as good as it was on day one.”
Sanford said one complaint from producers who use a permeable storage cover as a mechanism for ammonia management is that such covers sink and require frequent reapplication. “Biochar seems to be a potential method to reduce the applications of a permeable cover and help reduce ammonia emissions,” he said.
A major problem with anaerobic digestion, particularly with dairy manure, is that dairy diets are high in sulfur. Sulfur in anaerobic conditions turns into hydrogen sulfide through sulfur reduction. Hydrogen sulfide removal is a costly portion of the digestion budget – can biochar help reduce hydrogen sulfide?
“By adding biochar, particularly low-temperature biochars, we could significantly reduce hydrogen sulfide production,” said Sanford. “High temperature biochars are not great at reducing hydrogen sulfide. Not all biochar is the same – temperature and feedstock is important for the end goal.”
The negative aspect of biochar production is high cost with a high capital investment to install a biochar production system, particularly on-farm. “With carbon credits and the way anaerobic digestion is moving,” said Sanford, “there may be future incentives for lowering the cost of biochar.”
by Sally Colby