The recent paper “Mitigation potential of soil carbon management overestimated by neglecting N2O emissions” sparked a lively debate among the scientific community working on climate change mitigation, which in turn brought the issue to the attention of different stakeholders. The study, which presents a process-based model application using about 8,000 soil sampling locations in the EU, may apparently dampen the enthusiasm for international initiatives on carbon sequestration in agricultural soils such as the ‘4p1000’. We interviewed Emanuele Lugato (EU Commission - JRC), lead author of the paper to understand a bit more about this study and the possible policy implications of these research findings.

Dr. Lugato, in your opinion, what is the main shortcoming of the 4p1000 initiative? Has it perhaps failed to take into account the N cycle?

“If you look at the scientific literature, there are a lot of inputs and comments about the 4p1000 initiative. I think it is a good initiative to raise awareness on soils and their potential for carbon sequestration. Indeed, there are many positive aspects. However, a criticism is that the initiative is focused on a global soil carbon pool and that regional characteristics and constraints are not sufficiently considered. For instance, I think that permafrost and high-latitude regions should be given particular attention. So, I believe that the initiative could be a bit more targeted to understand the local dimensions, in particular for agricultural soil that can be actively managed to sequester carbon. In fact, agricultural soils are, generally, far from carbon saturation compared to soils under other land use and not nitrogen limited, a constraint that was raised to criticize the global 4p1000 target.  On the other hand, to really mitigate climate change, we cannot look at the C cycle in isolation. In the paper, we explain that there may be a risk of offsetting the carbon sequestration capacity if C management is supplying additional N leading to possible higher N₂O emissions. 

For these reasons, the overall 4p1000 approach risks being too simplistic. I think we should regard this initiative as an important statement, whose value currently lies more in its broad message than in its scientific aspects”.

Reading your paper, the focus really seems to be on the nitrogen cycle: is there a need for further research in this field? Or where should future research look at?

“In the paper, we acknowledge that accumulating carbon in agricultural soils is, of course important, but at the same time, we stress that this does not automatically equate to climate change mitigation. While the application of nitrogen-fixing cover crops can yield important effects in terms of C accumulation in the short term, altering the nitrogen cycle actually leads to higher net emissions in the long term. So, when the 4p1000 talks about mitigation, it should not forget about the central role of nitrogen: as the carbon sink reaches equilibrium in the long-term, emissions of N₂O could continue permanently. It is in light of these important factors that we wanted to be somewhat provocative and explorative in our paper.

As far as scientific research is concerned, I would not say that there is a lack of research on the nitrogen cycle. The same could be said for the carbon cycle. What is actually needed is more integrated research that investigates the two cycles together, in a systematic way. Since there are feedbacks between the two, we should definitely look at them as combined issues.”

What would you respond if someone said that your model cannot be fully representative since it only includes two scenarios of mitigation practice options all over Europe? One may say that these two options are just a few from the whole puzzle.

“This is true, but the purpose of our paper was not to explore a full set of management practices. That would have not been possible in the scope of a single academic paper and perhaps is not even realistic to cover all the possible scenarios. What we wanted to do is to highlight the effects of large-scale feedback and to do that we chose two scenarios among the possible options. The purpose of our paper was purely to highlight the biophysical response of agroecosystems and to raise awareness about it. We did not aim to make recommendations. But I think that one of the strengths of our work is that we have the appropriate tools to help identify the best policy options in the European Union. So, if we could summarise the message of our paper in a few words, I think that it would be: “do not forget nitrogen if you want to mitigate climate change!”