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Skip to 0 minutes and 12 seconds So can you tell me about what the nitrogen pollution that you’re adding on is doing to the soils here? Yeah. So one of things we’re most concerned about is actually the acidification of these soils, which is, obviously, a fairly serious environmental impact. The nitrogen coming in affects the soil chemistry by altering the amount of the base cations. And in particular in these soils, it’s calcium, magnesium, and potassium that act as a buffering system for the soils. When the nitrogen comes in, it can remove some of those base cations, which leads to the acidification of the soils. OK. So that can be quite a problem for the plant communities as well, can’t it? Indeed, yes.

Skip to 0 minutes and 58 seconds Those cations that get removed are actually nutrients for the plant. So not only is there an acidification effect, but there’s a nutrient depletion effect as well. The other thing that happens when the pH levels drop is that you begin to have more free aluminium in the soil. And aluminium is a potentially toxic element for microorganisms in the soil and for the plants themselves. And there’s many cases where there’s been a direct effect of aluminium toxicity that causes decreases in growth and potentially mortality in the plants, that kills the plants. OK. But we’re adding nitrogen, so that can’t be all of a bad thing, because that’s a nutrient for plants. That’s right.

Skip to 1 minute and 45 seconds So nitrogen appears a bit different than, say, in an agricultural field, where farmers add nitrogen to get greater crop yield. But the plants up here are adapted to low nitrogen levels. And part of that adaptation is by having low requirements for the nitrogen. So they have a very limited capacity to take up the nitrogen. That, in turn, allows there to be more free nitrogen in the soil, which, in turn, leads to more of that loss of base cations and the acidification of the soils. So you’ve got some samplers in here. Can you tell me what they’re for? Yeah. These samplers actually allow us to measure the chemical composition of the soil solution.

Skip to 2 minutes and 28 seconds What we’re mostly interested in, though, is the concentration of nitrate in the soils. And how is that changing with the addition of the nitrogen? So adding the nitrogen has resulted in an increase in the nitrate in the soil, which tells us that there isn’t complete uptake of that added nitrogen, that the plants aren’t taking up all of it, nor are the soil microbes. So that free nitrogen then allows there to be changes in the soil chemistry, the loss of the base cations and subsequent acidification. OK. So the other thing that I guess might be affected by the nitrogen is the amount of carbon in the soil. And we know how important that is around climate change. Right.

Skip to 3 minutes and 18 seconds How is that changing here? Interestingly, the amount of carbon coming out of the soil may have increased a little bit, but what’s fascinating from an ecological perspective is that the fraction of the soil carbon that wouldn’t usually be consumed by the microbial community is being consumed. So the nitrogen is actually resulting in some of the less easily degradable carbon being consumed by the soil microbes. And so how is the microbial community itself affected? So that shift in the consumption of the carbon is probably related to a shift from bacteria being the most important part of the microbial pool to having fungi be more important. And the fungi degrade the cellulose, the lignins that just don’t get consumed usually. OK.

Skip to 4 minutes and 15 seconds So that’s really interesting, because the nitrogen is changing the way the whole system functions. Exactly. Yeah. So we know that nitrogen is changing the whole system. What’s the future hold up here? We think that there’ll be an ongoing change, a rearrangement and changes in the abundance of the plant species up here. From the soil perspective, our concern is that there could be the continued loss of the base cations and continued acidification. And we’re actually seeing signs of acidification in the surface waters draining the ridge here. Oh, right. Yeah. And the bigger concern is the development of more aluminium toxicity up here.

Skip to 5 minutes and 2 seconds An interesting aspect of this is that a lot of those base cations are derived from dust that blows in here from the Colorado Plateau, which is hundreds of miles away. And yet, because of land use degradation, land use change and the degradation of the surface soils there, there’s been an increase in soils from that area being transported into here. That contributes a lot of the base cation buffering. And so that’s kind of a wild card that we don’t fully understand, how that may interact with the nitrogen deposition to alter the future of the soil chemistry and the ecosystems here. Wow. Well, thank you so much for allowing us to come and see your experiment in this amazing location. My pleasure.

More about soils and air pollution

Professor Bowman continues to talk about his research on nitrogen pollution in the Rocky Mountains.

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