In our latest post Jennifer Michel, ecologist at University of Lieja, presents her work ‘Vegetative stage and soil horizon respectively determine direction and magnitude of rhizosphere priming effects in contrasting tree line soils’. She highlights the treeline changes due to climate change, discusses positive and negative priming effects, and shares how artistic drawings help her to conquer her PhD.
About the paper
This paper is part of my PhD thesis, where we wondered how climate change induced tree line shifts in high latitudes and high altitudes will impact plant-soil-microbe interactions and especially the carbon cycle. The tree line ecotone is super sensitive to climate change, both in the mountains and in the subarctic. It’s quite fascinating because in these regions one can really observe the effects of climate change just by looking at the landscape: During the past decades the climate became milder higher up (uphill and up north), and certain tree lines followed this trend and “moved” upwards/northwards, that means shrubs and trees are starting to grow where previously there was only grasslands and tundra. This is often studied looking from above, like one can find satellite images showing literally a “greening of the arctic” and researchers looked quite a bit at changes in plant species composition. So here we decided to take another approach, that is looking at the belowground impact of tree line shifts, focussing on the soil and the rhizosphere. We were particularly interested in changes of SOM-mineralisation rates that occur when plants provide organic molecules to soil microbes. Depending on the productivity of the plant (also plant growth stage… and possibly the plant species as such and a trillion other factors), the quantity and composition of these organic inputs in the rhizosphere changes, and with it the energy and nutrients available to soil microbes. What one can then observe is either increases in SOM-mineralisation rates (positive rhizosphere priming effect) or decreases in SOM-mineralisation rates (negative rhizosphere priming effect). These changes in SOM-mineralisation rates translate back to CO2 released from the soil into the atmosphere and are hence a potential factor which could aggravate climate change and global warming.
About the research
At the time we did this experiment (2018), the scientific focus was still predominantly on positive priming effects, or they were at least vastly more studied/published than negative priming effects. This led to the opinion that priming per se is hence a risk for the carbon balance of major biomes as positive priming is often associated with enhanced soil CO2-emissions and decreasing soil organic carbon stocks. This is however only half of the truth, as negative priming effects are also common in natural ecosystems and we need experiments that evaluate the increased soil CO2 (if any) in relation to the increase above-ground biomass. In any case, negative priming makes a lot of sense from a plant nutrition point of view: positive priming, that is increased SOM-mineralisation, releases nutrients from the soil matrix which then become available for plant uptake. Negative priming on the other hand means that SOM-mineralisation rates decrease when microbes preferentially feed on labile organic substances freely available in the soil matrix, rather than on complex soil minerals, which can increase organic nutrient stocks in the soil via necromass accumulation. We had observed negative priming in a previous experiment using tree line soils and substrates, so we were keen to see what happens with SOM-mineralisation when we introduce living plants to these tree line soils and have a look at rhizosphere processes over a longer time period where plant and microbial nutrient supply and demand change.
The observation, in this experiment was that introducing fast-growing plant species to tree line soils first sped up carbon cycling (positive priming), but later this was reversed, so we observed reduced rates of SOM-mineralisation (negative priming) in almost all soils*. Overall, positive priming effects were perfectly balanced by the negative priming effects later – and we didn’t even account for the carbon fixed in plant and microbial biomass! So, while it may be soon prime time across the tree line, our paper does not necessarily indicate that that is a problem for the C balance*. Of course, more research is needed with much more sophisticated methods, especially for continuous isotopic labelling, and ideally in situ and at higher resolution over time, before we can really get an idea of what will happen at a larger scale. Meanwhile, studying rhizosphere priming effects in the context of plant nutrition and agronomy rather than purely soil carbon loss is really a nice way forward!
*The organic tundra soil from the Swedish subarctic is an exception here as priming was always positive and the net soil C balance was negative (net soil C loss).
About the author
I did a B.Sc. in Biology, then a M.Sc. in Plant Sciences and my Ph.D. is in Physical Geography, I guess that qualifies me as an ecologist… One of the best parts about ecology (THE best part possibly), is fieldwork. During my PhD, I was super lucky to have the opportunity to visit some of the most stunning places on planet Earth like the Andes and the subarctic. Seeing the sun rise at 3000m, above the clouds, is quite a sublime experience, and Northern Scandinavia is a hiking destination for a good reason (simply beautiful pristine nature, forests, tundra, treelines, wild rivers, caribou and gigantic mushrooms).
I think being in the outdoors, sometimes for days, often without WiFi, brings us back to the basics and ensures we don’t forget why we’re battling through all the other miseries of research (I mean praying for papers to be accepted, wishing nothing but loving kindness to reviewer 2 (or 1 or 3, sometimes all of them), running after funding, fixing stuff that doesn’t work all the time, trying not to collapse when R says “Error in match.fun(FUN): argument “FUN” is missing” hahaharrr).
Like the blog post? Read the research here.
