Glacier forelands reveal fundamental plant and microbial controls on short-term ecosystem nitrogen retention

Author Franciska de Vries gives her insight into newly published Journal of Ecology article: Glacier forelands reveal fundamental plant and microbial controls on short-term ecosystem nitrogen retention.

Franciska and her co-authors investigated how ecosystem nitrogen retention changed with succession across glacier forelands, in a project that was funded by a British Ecological Society research grant. Here you can find out more about the fieldwork that took place across three stunning glacial sites in Europe and how returning to analyse this previously collected data proved to be a surprisingly inspiring project during the global pandemic!

The amount of reactive nitrogen in the biosphere has been steadily increasing as a result of human activities, such as fossil fuel burning and intensive agriculture. The ability to take up and retain nitrogen is an important function of ecosystems, but despite extensive research in this area, the exact ecosystem properties that determine ecosystem nitrogen retention were not yet known. My PhD focussed on how soil microbial communities affect ecosystem nitrogen losses and retention, and then for my postdoc I explored how interactions between plants and microbes affect these processes. In several field studies I, and other researchers, found that ecosystems with slow-growing plants and microbial communities dominated by fungi are best at taking up and retaining nitrogen (De Vries et al., 2012, De Vries et al., 2006, Grigulis et al., 2013, Bardgett et al., 2003). But in a controlled experiment I found that fast-growing plants take up most of the added nitrogen, as well as soil microbes that need a lot of nitrogen for their biomass, which tend to be bacteria rather than fungi (De Vries & Bardgett, 2016). Other researchers found this too in controlled experiments (Myrold & Posavatz, 2007, Grassein et al., 2015). So why this difference?

At the end of my time as a postdoc, in 2012, I wanted to find out once and for all what the fundamental plant and microbial properties are that determine ecosystem nitrogen uptake and retention. When you want to find general patterns in ecology, it’s best to sample a wide variety of systems that differ strongly in the properties that you’re interested in. Glacier foreland chronosequences are not only very useful for this, they are also fantastically beautiful places. When glaciers melt, they expose barren rock, on which then ecosystem succession starts. First, a few plants start to grow, which trigger soil formation and the accumulation of organic matter, as a result of which soil microbial communities also develop and change. The further you get away from the melting glacier front, the older the ecosystems become, and they are also very different from the ones right in front of the glacier. The very young ecosystems are sparsely vegetated and dominated by a few pioneer species, while in the older ecosystems, the soil is fully vegetated by grasses and shrubs. Of course, the fact that these glaciers are melting is very depressing, but one useful thing is that people have been recording the position of the glacier front over time. This means that the ages of the sequence of ecosystems are known exactly – and this is what makes them such a great study system, because you can study how things change with ecosystem age.

The walk to the Damma glacier. Photo by Franciska de Vries.

So, I wrote a proposal for an early career research grant from the British Ecological Society to find out how ecosystem nitrogen retention changes with succession in three of these glacier forelands: the Ödenwinkelkees and Rotmoosferner glacier in Austria, and the Damma glacier in Switzerland. And it was successful – my first ever funded project!

After a lot of preparation and planning – contacting collaborators, sending materials to Innsbruck, booking mountain lodges, renting a car, making an itinerary, arranging permits – I was off to the Alps with my postdoc supervisor Richard Bardgett. Our first stop was at the Ödenwinkelkees glacier, where we met our collaborator Wolfgang Wanek. Essentially, what we did was walk down five kilometers from our hotel to the glacier front, collect about 80 kilograms of soil and intact soil cores from the various sites, and climb back up – with very heavy backpacks – to the hotel, where everything needed to be sorted and packed for shipping to Manchester. Repeat at the Rotmoosferner – but this walk was 10 kilometers there and back, and it was only the two of us – and repeat again at the Damma glacier, this time with collaborator Beat Frey. All while taking in the absolutely stunning scenery. This was my first time ever near, and even on top of, a glacier! While it was very emotional to see the speed at which these glaciers are melting, it was also impressive to see with my own eyes the process of succession, and soil formation, on such a compressed spatial scale.

(L) Me, at the location of the front of the Ödenwinkelkees glacier in 1997, illustrating how fast the glacier is melting. (R) The scenic walk to the Rotmoosferner glacier.

Alas, the fieldwork came to an end, and the samples needed to be sent to Manchester, so that I could submit them to a nitrogen addition experiment under controlled conditions. When all the boxes of soil samples had arrived in the lab, (supplemented with a hotel iron that Richard had accidentally packed!) the real hard work started. With the help of Caley Brown and my partner Victor van Velzen, and later in Aberdeen with Cécile Thion’s help, samples were scrutinised for plant and soil properties, and soil microbial biomass and community composition. I painstakingly added 15N labelled ammonium nitrate to the intact cores, gave them a rain shower after 48h, and dismantled the cores to trace where the nitrogen went.

Taking an intact soil core, in the field. Photo by Franciska de Vries.

We now fast forward to 2020. I was very busy in the meantime, starting an independent fellowship, setting up my own group, having babies, moving back to The Netherlands, and getting out the results from lots of other experiments that I did during my postdoc that were more urgent than this one. More urgent, because they were about the impacts of drought on soil food webs, plant-microbial interactions, and ecosystem functioning, which was (and is) a fast-moving area. So this glacier study ended up on the back burner for a while. Until – yes, I know this sounds counterintuitive – I needed something to focus on during the pandemic. Don’t ask me how, but I managed to reanalyse all the data, and write the paper, in a year with two lockdowns and with small children at home. It was the science-straw that I was holding on to.

Also, in the meantime, nitrogen had become very topical again in The Netherlands. I moved away from studying the behaviour of nitrogen in ecosystems because interest, and thus funding, was waning. But in 2019, the Dutch nitrogen crisis was back with a vengeance and it turned out that it had never really gone away – a legal trick made it possible to postpone measures to reduce nitrogen emissions. In 2019, the European court ruled that this was illegal, and The Netherlands had a nitrogen problem again.

(L) Me standing on the ice of the Rotmoosferner glacier (at this point I realised I hadn’t filled out a single health and safety form!) (R) Using a syringe to add labelled nitrogen to the soil cores.

So what did we find? Surprisingly, ecosystem nitrogen retention did not increase with succession, which was what I hypothesised, because with increasing age ecosystems become nitrogen-limited, soil organic matter builds up, and microbial communities become dominated by fungi. Instead, nitrogen retention was determined by plant and microbial community properties that did change during succession, but were highly variable. As I had found in my earlier pot experiment, plants and microbes with high nitrogen needs took up most of the added nitrogen. And as I had found in my field studies, nitrogen uptake by plants and microbes was higher when soil nitrogen availability was low. So finally, I can conclude that it’s not the fungi that are the cause of low ecosystem nitrogen losses. Rather, high abundance of fungi is a consequence of low soil nitrogen availability, which leads to higher nitrogen uptake by plants and microbes. And it’s the fast-growing plants and microbes that take up most nitrogen!

Now, why is this useful knowledge? Well, it will help us understand and predict the fate of nitrogen that enters ecosystems, for example via atmospheric nitrogen deposition. Are our ecosystems able to take up that nitrogen, or will it leach to ground and surface water? And what are the implications of chronic nitrogen enrichment, and other disturbances, for the ability of ecosystems to retain nitrogen? These results suggest that both nitrogen limitation and a high affinity of plants and microbes for nitrogen increase ecosystem nitrogen retention. Nitrogen deposition leads to high soil availability of nitrogen, but also selects for fast-growing, nitrogen-loving plants and microbes. I thought this study would be the end of my work on nitrogen retention, but I’d love to find out what the net effect of these opposing patterns is on nitrogen retention, and which systems become better or worse at taking up nitrogen… especially now that this is such an important topic in The Netherlands!

Franciska de Vries University of Amsterdam, The Netherlands & University of Manchester, UK

You can read the full article online: Glacier forelands reveal fundamental plant and microbial controls on short-term ecosystem nitrogen retention by de Vries et al.

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