How do various greenhouse gases differ?

In the context of climate change, we usually talk about CO₂, as this is the main gas emitted in terms of volume. Other gases are emitted in smaller quantities, but have a greater warming effect on the climate. This is why the term "global warming potential", or GWP for short, is often used. The GWP calculation is not without controversy and does not take into account the volumes of the gases emitted. However, GWP values are a way of putting the climate impact of different greenhouse gases into at least a rough context.

Figure 1: Global Warming Potential of CO₂, CH₄ and N₂O. The current estimates are based on the fifth assessment report of the IPCC (Intergovernmental Panel on Climate Change)

The study by Skinner et al. is mainly concerned with the outgassing of nitrous oxide (N₂O) and methane (CH₄) from agricultural soils managed in different ways. There is a lot to read about nitrous oxide in the results, but hardly anything about methane. Why is that?

The results of the methane measurements are less clear and more difficult to interpret. The slight uptake of methane measured for the BIODYN, BIOORG and CONMIN soils is not unusual and can occur depending on weather conditions and soil type. The comparatively large methane emission of the CONFYM soil can in turn be explained by the application of farmyard manure, which causes the methane-releasing organisms to multiply. This is particularly true when the soil water content is high, as it was at the beginning of the silage maize period.

Figure 2: Results of the methane measurements. The different agricultural systems and the overall results are outlined in red.

What is the “organic yield gap” and why does it occur in this study?

The "organic yield gap" in organic farming refers to the observation that organically and biodynamically farmed soils generally have a significantly lower yield than non-organically farmed soils. The average yield gap between the two organic systems and the two non-organic systems in the DOK plots is 19%. In this study, however, a yield gap of 27% was measured. The study authors explain this with the rather dry summer after the initial wetness, which affected the two organic systems more than the non-organic ones. In addition, the silage maize variety grown was developed for conventional systems with mineral fertiliser and not for organic systems. However, the considerable yield difference of 27% is offset by the fact that the biodynamic system BIODYN received 52% less fertiliser than the minerally fertilised system CONFYM. BIODYN is therefore more efficient and also more climate-friendly due to the lower fertiliser application.

What factors influence N₂O emissions?

Nitrogen fertiliser input – especially of mineral origin – is the main factor influencing nitrous oxide emissions. However, the soil pH value, the microbial biomass in the soil and the proportion of organic matter in the soil also play a role. According to the study authors, out of these secondary factors, the pH value explains nitrous oxide emissions best. The unfertilised NOFERT control plots have a comparatively low pH value. This can impair the conversion of N₂O to N₂ and thus lead to higher outgassing. The biodynamic plots have the highest pH value of all the soils analysed. BIODYN soils are therefore not as acidic, which in turn can be an influencing factor for lower nitrous oxide emissions. According to the available data, the increased pH can be attributed to management practices. For example, composted farmyard manure, as used in the BIODYN system, can lead to an increase in pH.

Is composted farmyard manure more emission-friendly than rotted farmyard manure?

According to the study results, systems that receive rotted farmyard manure have higher nitrous oxide emissions than BIODYN, which receives composted farmyard manure. However, the study did not investigate whether the composted farmyard manure emits additional nitrous oxide during the composting process, which does not end up on the trial plots and is therefore not included in the statistics.

Even if not everything about biodynamics is yet clear and understood, it is obvious that it has a lower climate impact than conventional agriculture due to the closing of nutrient cycles as well as the avoidance of chemical additives and long transport routes.

Sources and further links to this article

• Original study: Skinner, C., Gattinger, A., Krauss, M., Krause, H.-M., Mayer, J., van der Hayden, M.G.A., Mäder, P. The impact of long-term organic farming on soil-derived greenhouse gas emissions. Scientific Reports 9, Article 1702 (2019) https://doi.org/10.1038/s41598-018-38207-w
• General information on methane and nitrous oxide
• Image credits for GWP values