A new study in Plant and Soil found that the input of nitrogen from the atmosphere, via a process of rain funneling through rocks, created levels of nitrogen that are adequate to support plant growth in post-glacial alpine soil, challenging the common view that the element is the primary limiting factor in deglaciated areas.
In fact, the researchers found that phosphorous, due to the low-weathering rates and high nitrogen deposition of the region, is the element in soil which limits post-glacial plant life colonization. The team was lead by Hans Göransson of the University of Natural Resources and Life Sciences, Vienna.
The finding challenges the widely-held view that the only plants capable of colonizing post-glacial environments are species that are able to fix nitrogen from the atmosphere. Instead, as this work shows, natural processes enable other plants to become colonizers in the European Alps.
This research may have implications for future ecosystem plant colonization and biodiversity, a topic of interest to scientists as glaciers retreat and expose new soils in many regions of the world.
The team, focusing on Damma Glacier in Switzerland, found that the expected nitrogen-fixing plants were usually absent in the early stages of these post-glacial sites, contradicting what previous research has suggested. The study’s findings differ from the research done in recently deglaciated areas in Glacier Bay in Alaska and on the Franz Josef glacier in New Zealand, which show an abundance of nitrogen-fixing plants in post-glacial sites.
Most plants are unable to process atmospheric nitrogen directly and can only absorb it once it undergoes a transformation within the soil. (Nitrogen is essential for plant growth.) Some plants, however, undergo a process called nitrogen fixation which converts atmospheric nitrogen into a form useful for them. Bacteria in the plant’s roots help, as they are able to convert the nitrogen into a usable form.
Because of this capacity, nitrogen-fixing plants are generally thought of as the colonizing species in post-glacial sites, since these rocky areas are typically so low in soil nitrogen that plants that cannot fix nitrogen would not be able to grow. Once the nitrogen-fixing plants begin to die and the nutrients from them return to the soil, a more diverse second generation of plants can grow.
The team set out to explore how plants in the region were colonizing even when nitrogen-fixing plants were not present. They found that nitrogen from the atmosphere was deposited into the soil by newly exposed rocks, which acted as funnels when it rained. This process provided sufficient amounts of nitrogen for plant growth, and thus allowed non-nitrogen fixing plants to grow in these areas.
The researchers divided the Damma post-glacial area into a total of 21 sites across three time periods related to the age of the soil since the glacier retreated: pioneer (fewer than 16 years since deglaciation), intermediate (57-80 years), and late-stage (108-137 years). The Damma glacier has had a long and well-tracked retreat, making the separation of time periods easy.
The team used ion exchange resin bags at each site that measure the amount of nitrogen in the soil. They also took the above-soil measurements by collecting the biomass growing at the sites and analyzing the nitrogen levels.
They found that nitrogen levels were high in the pioneer stage, followed by low levels in the intermediate, and high levels again in the late stage.
As the nitrogen channels through rocks and into the soil, it creates an overabundance of nitrogen at first, since there is little or noplant life to use the element. This process eventually creates hotspots of plant growth, but as soil and organic matter increases, the rocks become covered. Once the rocks are covered, the atmospheric nitrogen can no longer be deposited into the soil. This, along with the increased presence of plants using the soil nitrogen, leads to a decrease in nitrogen availability within the soil in the intermediate stage.
High levels of nitrogen return in the late-stage sites once the vegetation has matured and therefore requires less of the element for growth. With more plant cover, nitrogen increases as plants die and the nutrients are returned to the soil through decomposition.
The team also looked at phosphorous, another important element for plant growth, and found little difference in its levels in the soil, regardless of the time since deglaciation. Since nitrogen levels changed with time, the ratio of phosphorus to nitrogen also varied. The researchers found that phosphorus stabilized at a low level. When the nitrogen levels were high, in the pioneer and late stages, phosphorus was the limiting element. This relationship flipped in the intermediate stage when nitrogen availability was low. Thus, as the nitrogen availability changes, so too does the element that is limiting plant growth.
The researchers concluded that colonizing plants found in the bedrock typical to the Alps are more likely to be limited by phosphorous due to the high levels of nitrogen deposition and the low weathering rates needed to release phosphorus from minerals. This gives an advantage to plants that can use the phosphorus from mineral sources, thus affecting the composition of the plant life in those areas throughout the different stages of deglaciation.
“In succession, the next set of species coming in is dependent on [those] already present. Thus a change in primary succession may lead to dramatic change in the plant community later on,” Göransson, the lead author, told GlacierHub in an email interview.