The paper “Greenhouse gas emissions in agricultural cultivated soils using animal waste-based digestates for crop fertilization“, published in The Journal of Agricultural Science, has been chosen as the latest Editorial Highlight.

There are many different types of agricultural waste that are generated on a daily basis from agricultural systems. Agricultural waste includes crop residue, livestock waste, feedstock and various substrates, with animal manure as the most common waste. Recently, the use of bio-substrates for biogas production has increased. During production of biogas in a reactor, organic matter is decomposed by microbes to produce one of the end products from the biogas system named digestate. The quality of the bio-substrate and its chemical composition are highly dependent on the raw material that is used for biogas production. During the process, most of the carbon is decomposed and mineralized nutrients that can be taken up by plants are made available in the digestate. The digestate is normally tested for quality and safety prior to field application to guard against the presence of heavy metals or other harmful elements or compounds.

Agricultural sources are responsible for the emission of three major greenhouse gases (GHG): carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). It is well known that CH₄ and N₂O emissions from all sources are significantly lower than CO₂, but the negative impact on the environment is many times higher. According to scientists, CH₄ molecules absorb up to 25 times more of the heat that is reflected from the earth than CO₂, and N₂O absorbs 298 times more than CO₂. Therefore, several studies have been conducted to assess the emissions of all three gases from agricultural sources such as organic farming in agricultural fields. Hence, our two-year study on the influence of animal waste-based digestate application on GHG emissions from an agricultural soil. The animal waste digestate, mainly from pig, cow and chicken manure, was applied to the soil and the emissions were compared to soil fertilized with synthetic nitrogen fertilizer. For each digestate application, the composition of the digestate was calculated according to its content of total nitrogen. The digestate was split-applied at 90 and 80 kg N/ha.   

During the two year-study, it was observed that CO₂ emissions from the soil during the cultivation of spring wheat increased significantly in 2018 after the first digestate fertilization application when compared to 2019, while the emissions decreased for both years in the digestate and synthetic N within the initial two to five days after the second fertilization application. One to two weeks after the second digestate application, a sudden increase in CO₂ was observed in the digestate and synthetic N treatments, but then it decreased rapidly and in some measurements, digestate emissions were even lower than those observed for synthetic N fertilizer.  It is probable that these emission levels were largely caused by the activity of soil microorganisms during that period, so this should be considered as a positive attribute of the use of the digestate. The N₂O and CH₄ emissions were relatively low during the two years of the study, with initial peaks observed after the digestate application. However, emissions decreased and flattened out over the course of cultivation for the two years.

These results, therefore, can be considered positive, because the increase in CO₂ emissions are likely due to the activation of microorganisms, which indicates the quality of the soil. Another important factor that plays a role in most cases of GHG emissions is soil moisture. We found that CO₂ and CH₄ emissions increased from the soil with an increase in soil moisture, but when the soil moisture was less than 15%, CH₄ emissions started to decrease. Further research to improve the knowledge of the impact of meteorological conditions on GHG emissions would be encouraged, while studying the long-term effect of digestate application on agricultural soils.

The outcome from these first two years of research provides hope that the use of organic amendments only momentarily and insignificantly increases GHG emissions. Therefore, taking into account its other positive properties, we can consider the use of digestate as a good alternative  to inorganic fertilizers for mitigating GHG emissions.

The Journal of Agricultural Science Editorial Highlights are selected by the Editor-in-Chief and are freely available for one month. View the recent selections here.