Long-term use of biogas digestate stores carbon and enhances soil fertility

Every year, intensive agriculture is directly responsible of 10 to 14 percent of emissions of greenhouse gases globally, therefore developing sustainable farming practices is an important step to meet the Paris climate targets. The issue of GHG emissions from agriculture has lots to do with good or bad agricultural soil management. Bad soil management practices can lead to large emissions of carbon dioxide, methane and nitrogen oxide. On the other hand, soil can also become a powerful carbon sink (the EU estimates the soil sequestration potential in Europe can be as high as 1.5-1.7% of EU’s emission every year) and an improved soil management system can contribute to a substantial reduction of agricultural GHGs emissions by sequestering some carbon into the soil’s organic matter. In addition, good soil management can increase soil organic matter, maintains soil nitrogen (N), enhances fertility and productivity, biodiversity, and reduce soil erosion, leaching and water pollution.

In this scenario, agricultural biogas can play an important role in increasing the sustainability and the competitiveness of farms through the combined production of renewable energy and stable organic fertilizers in the form of digestate, while contributing to closing the carbon cycle in the farms by storing organic carbon into the soil. This approach is pursued by the BiogasDoneRight® model promoted by the Italian Biogas Consortium (CIB), an example of multifunctional and sustainable agriculture.

Sustainable agricultural intensification

The Italian biogas sector has grown quickly and today Italy is the second European producer of biogas after Germany. Most of the biogas is produced from agricultural anaerobic digestion plants, around 1,200 plants and 850MWe of installed capacity and some of those apply the BiogasDoneRight® model. Essentially BiogasDoneRight® is an agricultural biogas management system based on the efficient use of by-products (such as digestate used as fertilizer), reduced soil tillage, the application of double-cropping systems (a type of sustainable agricultural intensification where cover crops can be harvested for biomass without reducing food and feed) and the recovery of degraded lands.

A study conducted in a CIB member biogas farm located in the province of Turin in Northern Italy assessed the long-term effects of digestate application on soil fertility and carbon sequestration by applying the BiogasDoneRight® model on field. The study was carried out between 2009 and 2015, and eight different field plots (51 hectares totally) were fertilized each year using only digestate form the biogas plant. Soil sample were then analyzed in order to estimate the effects of digestate on important parameters affecting of soil fertility, organic matter content and the amount of stored carbon. In accordance with crop rotation plan each field was cultivated every year, with a single crop system (maize) or a double cropping system (Triticale/Lolium and Maize/Sorghum). Soil analysis were carried out on randomized soil samples to measure different soil parameters.

More carbon stored into the soil and fertility increase

The observations showed that the continuous application of digestate had a significant effect on the content of soil organic matter content (OM), which increased by 0.5%, from 2.2% to 2.7% in seven years. This confirms the positive effect of continuous input of organic matter on the increase of soil carbon stocks. In addition, also soil organic carbon (OC) increased significantly, from 1.3% to 1.6% during the period of observation. This demonstrates the potential role of soil in dynamic carbon sequestration. By contributing to storing carbon from the atmosphere into the soil, this model could represent an example of efficient and sustainable “carbon negative” agriculture.

Good pratice in soil management and the efficient use of digestate had significant results also on soil fertility. The increase of soil organic carbon significantly increased the Carbon-Nitrogen ratio (from 8,5 to 9 during the period) which indicates an increased stability equilibrium of available and organic nitrogen in the soil. A significant improvement of the soil’s capacity to keep and exchange nutrients was also observed, by means of increased soil cation exchange capacity (CEC). The enhanced stability of soil fertility parameters had a positive effect also on the concentration of available plant macro-nutrients, keeping a stable level of nitrogen (indicating reduced leaching effects) and inducing significant increase in phosporus and potassium (+50% and + 70% respectively during the period of observation).

These results demonstrate that digestate from agricultural biogas plants can be a good fertilizer and an effective mean to close the carbon cycle in the soil for a more sustainable agriculture. The increase of soil organic matter can enhance soil fertility and stability and maintain soil nitrogen. It can also increase soil biodiversity, while reducing erosion, leaching and water pollution.

This post is based on the paper “BiogasDoneRight®: Soil Carbon Sequestration and Efficiency in Agriculture” by Guido Bezzi, Lorenzo Maggioni, Carlo Pieroni – Consorzio Italiano Biogas, presented at the 24th European Biomass Conference and Exhibition.

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