In a new publication, Postdoc Jared Onyango Nyang’au from the Department of Agroecology at Aarhus University and his collaborators have focused on a pioneering plasma treatment technique developed by a Norwegian company.  This technique enhances the fertilizer value of digestates from biogas plants, replacing more synthetic fertilizers as well as reducing environmental problems associated with emissions during storage and field applications of manure. 

Enhancing fertilizer value and pH stability 

The study delves into the transformative effects of plasma treatment on organic materials, particularly in digestates from biogas production.  The shift towards using high dry matter recalcitrant biomass in biogas plants coupled with sub-optimal operation conditions results in a low fertilizer value of digestates.  Moreover, it causes adverse environmental impacts during storage and field application of resultant digestates.  Using electricity, the plasma unit ionizes air with a strong electric field, making nitrogen and oxygen to form reactive nitrogen gas.  The reactive nitrogen is then absorbed into the digestates, enriching it with nitrogen in the nitrate and nitrite forms.  Notably, during plasma treatment, a significant reduction in pH occurs, offering a viable alternative to the costly and corrosive sulfuric acid-based acidification method commonly used by farmers. 

With a legal requirement in Denmark to store manure during autumn and winter, typically for 6 to 9 months prior to field application in spring, farmers are expected to employ mitigation measures to reduce greenhouse gas emissions, such as covering of storage facilities and slurry acidification.  Acid-based acidification is the widely adopted approach by farmers; however, it is associated with risks such as sulfur over-fertilization, use of large doses especially in digestates, making it costly and risks of acid corrosiveness.  Moreover, due to high digestate buffering capacity, the pH tends to return to its initial pH, reducing the effectiveness of the technique.  This may necessitate slurry re-acidification prior to field application to reduce ammonia losses. 

The study revealed that plasma treatment could achieve long-term pH stability and a one-time extended effect by acidifying the slurry to pH ≈ 5.0 or lower.  The low digestate pH could reduce emissions throughout the storage period as microbial activities are inhibited, and an equilibrium in favour of non-volatile ammonium-N is maintained, reducing ammonia losses. The potential of achieving and keeping a constant pH throughout the slurry/digestate storage period by one-time acidification using the plasma treatment technique could save farmers’ resources and protect the environment. 

Jared Onyango Nyang’au elucidates, "Our motivation was to address the challenge of low fertilizer value in digestates, a tradeoff of the biogas industry.  We sought to explore a technique that adds value to the digestates by potentially reducing emissions during storage and field applications and improving its fertilizer properties." 

Unveiling nitrification inhibition 

The study revealed a prolonged inhibition of nitrification in soil following the application of plasma-treated digestates.  By keeping the ammonium nitrogen intact and preventing its transformation into nitrate, this method shows potential for reducing nitrate leaching and lowering nitrous oxide emissions linked to the nitrification-denitrification process.  However, this benefit would be counter-balanced with potential risks of nitrogen losses from fixed nitrite and nitrate by the plasma. The technique via the plasma fixes reactive N in nitrate forms to the slurry, which potentially poses high risks of N2O emissions when applied to the field. This needs further testing.  

"We observed unexpected prolonged inhibition of ammonium-N nitrification," Jared Onyango Nyang’au reflects.  "This effect underscores the potential of plasma treatment to reshape agricultural practices towards greater sustainability." 

Practical implications and future directions 

The implications of this research extend beyond laboratory experiments, offering tangible benefits for farmers.  Plasma-treated digestates not only exhibit enhanced fertilizer value but also contribute to reduced methane emissions during long-term storage and lower ammonia losses during field application. 

Jared Onyango Nyang’au emphasizes the need for further investigations into the long-term effects and broader applicability of the technique.  He notes that the technique relies on an intense electricity-driven plasma, which may consume more energy than conventional methods of producing nitrogen in commercial fertilizers.  However, the operational costs could vary depending on the utilization of surplus electricity, particularly during periods of high electricity supply from renewable sources like wind and solar, leading to lower electricity prices. 

"We envision a pilot or farm-scale studies to assess the effects of plasma treatment on greenhouse gas emissions comprehensively.  Additionally, a life cycle analysis will provide insights into the economic and environmental viability of plasma treatment." 

A global solution for sustainable agriculture 

The plasma treatment technique holds promise for adoption worldwide.  Its potential to mitigate environmental risks, enhance fertilizer value, and promote sustainable farming practices positions it as a transformative tool in the quest for a greener future. 

"Investing in this technology could yield dual benefits: reducing emissions in the manure management chain and increasing crop yields," Jared concludes.  "It can be a step towards a more sustainable and resilient agricultural sector."