Soils and air pollution

Even though it might not be the first thing that you think of, air pollution presents a real threat to soils. That air pollution can come in a wide variety of forms, including metals, nitrogen deposition, and sulphur deposition. The last two make up acid rain. I’m working in Colorado on the impacts of nitrogen deposition on natural communities. And there’s some really interesting work being done here on how nitrogen deposition is impacting soils in the alpine region. Professor Bill Bowman is an ecologist at the University of Colorado at Boulder. Bill, can you tell me about some of the work you’ve been doing?

Yes, for the past two decades, I’ve been doing work looking at the interactions between plants and soils, specifically looking at nutrient availability and how that influences plant diversity, plant function, but also how the plants themselves affect soil processes through their chemistry and the turnover of their organic matter. And you work a lot in the Rocky Mountains, in the alpine region. I do. Yeah. Most of my work has been in the alpine regions of the Rocky Mountains, that area above tree line dominated by short stature plants, mostly grasses and sedges, some forbs. They’re fairly slow growing. It makes a very good system for looking at questions related to plant-soil interactions.

So it’s a very remote region in the Rocky Mountains, but you’ve been working on air pollution. Is air pollution really a problem up there? Yes. You might not think so, but actually due to the way the wind blows from urban centres, both here in the front range, which is to the east of the mountains where we work, but also urban centres to the west, Salt Lake City, Los Angeles, Seattle, they contribute air pollution to those areas. And some of the pollutants coming in, in particular nitrogen deposition, is a nutrient for plants.

And yet, because the alpine plants are so well adapted to low nitrogen, they have a limited capacity to take up that nitrogen, and it has potentially a negative effect on the soils and their processing. Well, we’ll go and have a look at your experimental site in the alpine region.

Bill, can you tell me something about the work you’ve been doing here? Yes. For the past couple of decades, I’ve been working in the alpine here, looking at the relationships between plants and soils and factors that control the distribution of plants. In the past several years, we’ve become particularly concerned about the effects of air pollution on both soils and the plants that are dependent on them. And so we’ve been looking at impacts from mainly nitrogen pollution, because that’s the one that’s of main concern here. Here in the front range of the Colorado Rockies seems to be a hot spot of nitrogen deposition. And we know that nitrogen is really important in the context of both plant function and soil processes.

OK. Well, could we start by having a look at what the soil’s like here? Of course. Yeah. Right over here.

So can you tell me about these soils here? Yeah. Some real key characteristics to the soils here is that they’re derived from a granitic bedrock, which is made up of silicaceous minerals. As a result, they’re acidic to begin with. It’s also a very cold, dry climate. So soil development is quite slow as a result of that. Weathering processes are quite low. In addition, the decomposition of the organic matter in the soils is very low. So you have a build-up of an organic layer in here that’s quite pronounced that you can see here. The soils are quite gravelly. There is some sand mixed in there. Some areas have higher clay content, but right here, it’s mostly gravel and sand.

You’ve been doing research here on how air pollution is affecting the soils. Shall we go and have a look at your experiment? Good. Yeah. I’ve got some plots right up here.

So this is your experimental site? Yeah. So I’ve got an experiment going on here that I’ve been running for about 17 years now. It’s designed to look at the effect of nitrogen pollution, nitrogen deposition, on both the plants and the soils up here in the alpine. The alpine’s a great place for doing this because it’s a fairly sensitive system. If you compare this ecosystem and these soils with other ecosystems and soils, this is where we expect the first indications of a pollutant impact to be. So if we’re going to see a change anywhere, we’ll see it here. Exactly. We’ll see it here first. And so I’ve been running this experiment, as I say, for 17 years.

There are three different treatments of nitrogen input, which are designed to mimic what would happen, going on for the next couple of decades. You can think of it almost as a titration of a terrestrial ecosystem, looking it where do we see a threshold in the impacts. Where would we expect that to occur? And what will be the response? Will it be a linear response, a curvilinear, and so forth? And what’s the experimental design that you have here? So there are three different levels of nitrogen, which are designed to look at what happened over the next three to four decades. And so there are five replicate plots of each of the treatments, and then five blocks of those.

And the blocks actually incorporate the individual microsite variation. So there may be something unique about this area here that’s different over here. So we incorporate that little microsite variation by using those blocks. So I’ve got the three different treatments of nitrogen and a control to compare the nitrogen treatment impacts. Despite the fact it looks fairly homogeneous, we find that there’s pretty strong microsite variation that we need to take into account when we analyse the effects of the nitrogen treatments on the soils and the vegetation So that’s the very small-scale variability, where you get different plant species and different types of soil. Exactly. Yeah exactly.