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Biogas production reduces nitrogen leaching from manure

Scientists from Aarhus University have developed a new model that can estimate long-term nitrogen leaching from digested slurry compared to untreated slurry.

[Translate to English:] Afgasning af gylle øger kvælstoffets tilgængelighed for afgrøderne, hvilket alt andet lige kan føre til øget udbytte. Foto: Jesper Rais

Nitrogen from anaerobically digested slurry is more readily available to crops than nitrogen from untreated slurry. This is shown to be true even when considering long-term developments in the field. Calculations made at the Department of Agroecology, Aarhus University, have demonstrated this using a newly developed model, which to a greater extent than previously takes into consideration developments in the field over a significant time period. The results have been published in a report from DCA – Danish Centre for Food and Agriculture.


All things being equal, the plants’ increased access to nitrogen from digested slurry will result in increased crop yields and reduce leaching of nitrogen to the aquatic environment compared to using the same amount of untreated slurry. 


Plants benefit from mineralized nitrogen

Treatment of slurry in the biogas plant alters the nitrogen in the slurry. When the slurry is digested, the organic matter in the slurry degrades. This means that some of the organically bound nitrogen becomes mineralized.


This is good news for the crops as the slurry will then have a reduced content of organic nitrogen, which is not readily available to the plants, but a higher content of inorganic nitrogen, which is readily available to the plants. It is also good news for the environment because the more nitrogen that the crops can absorb and utilize, the less nitrogen is leached to the aquatic environment.


Mineralized nitrogen is immediately available to the plants whereas organic nitrogen has a higher risk of leaching. This happens because nitrogen is released over the course of the year, including periods during which the plants are unable to utilize it, and over a longer period; some nitrogen will even remain in the soil for more than 50 years.


The increased fertilizer value diminished over time

Scientists estimate that the fertilizing effect of untreated as well as digested manure is dependent on the manure’s proportion of inorganic nitrogen in the year in which the manure is applied to the soil and that the potential fertilizing effect will increase by 10-15 kg N per 100 kg manure N after biogas digestion in the year of application.


On the other hand, the subsequent effect of digested slurry is lower in the years following the application, and the long-term effect of the application is expected to be only half the effect in the year of application. This corresponds to an increased fertilizer effect of 5-8 kg N per 100 kg manure N seen in a long-term perspective.


Only limited amounts of energy crops should be applied

The effect of slurry digestion on nitrogen leaching depends on the amount of energy crops in the biomass. The higher the content of bioenergy crops, such as maize ensilage, in the biomass, the lower the reduction of nitrogen leaching.


This means there will be a reduced effect or even an increase in nitrogen leaching if too large amounts of energy crops are applied to the biomass. If more than 10 percent of the total nitrogen applied in the biogas plant comes from energy crops the result will be increased nitrogen leaching when compared to untreated slurry.


This project was initiated and financed by the Biogas Task Force of The Danish Energy Agency in cooperation with The Danish Enviromental Protection Agency and the Danish AgriFish Agency.


The report  ”Kvælstofudvaskning og gødningsvirkning ved anvendelse af afgasset biomasse” , DCA report no. 65, September 2015 (in Danish with an English summary) is available here.


For further information please contact: Senior researcher Peter Sørensen, Department of Agroecology, e-mail: ps@agro.au.dk, tel.: 8715 7738