Digestate, a co-product of biogas production used as organic fertiliser, has the potential to transform Europe’s agricultural sector offering an alternative to commonly used chemical fertilisers. A recent article published by Ghent University in collaboration with the EBA shows that digestate has a high fertiliser value and has the potential to meaningfully contribute to carbon storing in agriculture.
Europe is currently focusing on ways to reduce its dependence on Russian gas and the impact of soaring energy prices on farmers’ energy bills. Besides fuel prices, fertiliser prices have also reached record levels. The EU is largely dependent on imports for most of chemical fertilisers and 30% of them come from Russia. Russian halt to fertiliser exports will threaten supplies for European farmers and put at risk world food supply. Additionally, today’s production and transportation of Russian fertilisers is energy intensive and contributes to GHG emissions. Alternative solutions are therefore needed to the use of less conventional fertilisers. Digestate is a valuable organic fertiliser than can substitute fossil-based alternatives whilst stepping up on the ambition for the climate targets.
Using digestate to nurture our soils
The paper “Assessment of the Carbon and Nitrogen Mineralisation of Digestates Elaborated from Distinct Feedstock Profiles” analyses the nitrogen and carbon mineralisation rates of five of the most representative feedstock streams in the European biogas sector (pig manure, poultry manure, energy crops, sewage sludge, food waste). Nitrogen mineralisation is a biological process and vital part of soil fertility by which organic nitrogen is converted to inorganic forms which can be absorbed by plants. In agriculture, nitrogen is applied to crops in the form of fertiliser, helping them grow stronger and better. The paper shows that digestates have a high fertiliser value due to their content of nitrogen. In agriculture, nitrogen is essential to make sure plants are healthy as they develop and nutritious to eat after they are harvested. In addition to their fertilising properties, digestates have a strong potential to store carbon in the soil. The close monitoring of the mineralisation rates allowed the authors to predict the carbon sequestration potential of digestate.
Making the most out of the AD process
Anaerobic digestion (AD) is best known for the biogas it produces, as this can be converted into energy (renewable electricity and heat or even advanced biofuel). However, AD also produces digestate which is an excellent fertiliser containing all nutrients and micronutrients necessary for farming. As with energy recovery from biogas, the issue of digestate recovery should be a systematic part of AD project to exploit all its potential benefits. Indeed, the digestate has a double agronomic interest: making mineral elements available to plants and contributing to carbon storage in our soils.
Closing nutrient loops
Among other things, nutrients, such as nitrogen, are fully conserved in the AD process. They even become more available, partially changing from an organic to a mineral form, preferred by plants. The results show that the nitrogen mineralisation of digestates ranges from 21 to 39%, underlining the fertilising property of the digestates included in the study. Additionally, the observed variability in nitrogen mineralisation indicates that some digestates have a higher fertiliser value than others. For example, digestate from pig manure (39%) reach almost double the value of digestate from sewage sludge (21%).
Enabling carbon storage
On top of its primary function as organic fertiliser, digestate has also the potential to store carbon in agricultural soils. This increases the amount of CO2 stored in the soil, reducing its presence in the atmosphere in line with Bioenergy Carbon Capture and Storage or Utilisation (BECCSU) practices.
The paper demonstrates that we can store in the soil 50 to 80% of carbon from digestates used as organic fertiliser. This represents between 205kg and 553kg of sequestrable carbon for 170kg of digestate used per hectare.
A supportive regulatory context
The cross-sectoral ‘Fit for 55’ package has provided the regulatory context to accelerate the EU’s commitment to achieve net emission reductions of at least 55% by 2030. All sectors will need to contribute to this 2030 emission reduction target, including land use, forestry and agriculture sector. Between 2021 and 2030, EU Member States have to ensure that accounted greenhouse gas emissions from land use, land use change or forestry are balanced by at least an equivalent accounted removal of CO2 from the atmosphere. On a broad scale, this vision will involve the upscaling and incentivisation of carbon farming initiatives intending to increase carbon removal and storage in the land sector. In December 2021, the European Commission acknowledged benefits of natural CO2 storage and utilisation, applied in biogas production. This communication was a positive step towards the full recognition of the environmental benefits of sustainable biogas and biomethane production.
The importance of the carbon removal imprint on current and future EU policies relating to agricultural systems makes it even more pertinent to examine digestate not only from a fertilising point of view but also as a possible contributor to carbon storage.