Noémie A. Pichon, a PhD student in the Allan Lab, talks about her recent paper Decomposition disentangled: a test of the multiple mechanisms by which nitrogen enrichment alters litter decomposition, the background behind this paper and the next steps in this field.

What is the background behind your paper?

Biodiversity-ecosystem functioning experiments have been manipulating multiple environmental drivers separately (plant diversity, nitrogen enrichment), or considered them together in naturally assembled communities (land-use intensity direct and indirect effects). Due to the correlation between the different drivers, such as plant species diversity and functional composition; it is difficult to estimate the relative importance of their effect on ecosystem functioning. With the PaNDiv Experiment, we aimed at testing the effect on several ecosystem functions of these correlated drivers: nitrogen enrichment, plant species diversity and functional composition. The focus of this first paper, litter decomposition, enabled us to further disentangle these drivers effect through changes in litter quality (recalcitrance of plant litter material) or in soil quality (soil biotic and abiotic conditions). We could then answer the question: “What is the relative importance of nitrogen enrichment, species diversity and functional composition on decomposition through changes in soil or litter quality?”

Seraina Cappelli and Noémie Pichon on the PaNDiv experiment, May 2017. 
Credit: Hugo Vincent
Seraina Cappelli and Noémie Pichon on the PaNDiv experiment, May 2017.
Credit: Hugo Vincent

What are the key messages of your article?

In my opinion, the main message of this article, but also more broadly of the PaNDiv experiment, is that nitrogen direct and indirect effects are of comparable importance. Under nitrogen enrichment, plant species diversity decreases and the community shifts from slow growing to fast growing species composition. If we only consider nitrogen direct effects and not the effect of diversity loss or composition shift, we miss an important part of the information. In this paper, our results show that nitrogen enrichment increases decomposition, mainly by shifting species composition towards faster growing species producing high quality litter: high nitrogen and calcium concentrations, low fibres and leaf dry matter content. Another important point is that we were able to quantify the relative importance of soil and litter quality using a relatively simple litterbag setup and structural equation model. With this approach, further studies could estimate decomposition drivers’ relative importance in several different ecosystems and under different conditions.

What is the next step in this field going to be?

With the PaNDiv experiment, we only started to understand how nitrogen directly and indirectly affect functioning. We still lack a full understanding of what is happening belowground and the relative contribution of soil communities in amplifying or mitigating the effect of global changes on ecosystem functioning. But in order to do so, we would need to reduce the size of experimental setups. At least, it is what I would like to focus on next.

Did you have any problems setting up the experiment or gathering your data?

Litterbags decomposition experiment. Autumn 2017. Credit: Hugo Vincent
Litterbags decomposition experiment. Autumn 2017. Credit: Hugo Vincent

Our experiment consists of 336 plots of 2x2m separated by a 1m path. We were 2 PhDs working on the project, Seraina Cappelli and myself, and we had to set it up from the start. I still cannot fully explain how we managed to run such a large experiment. Of course, a lot of things did not work as expected, but exceptional colleagues make every hard work easy and enjoyable. I learned the most during field work, in particular the weeding sessions, when we had to manage teams of helpers weeding by hand the plant communities to maintain the different diversity levels and species compositions. I reached unexpected self-control levels while leading 15 people for two weeks under sometimes difficult weather conditions.

What’s your current position?

I completed my PhD at the end of January 2020 on the topic “Direct and indirect effects of nitrogen on ecosystem functioning”. This paper is the first chapter of my thesis to get published, my colleague Seraina Cappelli will also publish her first paper soon. I am extremely happy to see the results of our work being released. We are now both focusing on these publications before starting a postdoc position.

What is the best thing about being an ecologist?

The PaNDiv experiment during the weeding. Credit: Hugo VincentThe PaNDiv experiment during the weeding. Credit: Hugo Vincent
The PaNDiv experiment during the weeding. Credit: Hugo Vincent.

The diversity. To carry out the experiment described in this paper, I learned how to use a sewing machine, practiced and improved diplomatic skills with the farmers living around the experimental field, developed management skills by working in large teams, learned plant determination, structural equation modelling, and the local Swiss languages.

Read the paper in full here or the plain language summary here.