Soils and invasive species

Hello. My name’s Arthur Broadbent. I’m a PhD student at Lancaster Environment Centre. And I’m studying invasive plant species and their impacts on the soil. So if we start by defining what an invasive species actually is, it’s a species that arrives and then spreads in a new habitat, a new region to which it isn’t native. And there are varying definitions. But the key thing is that it spreads by forming self-sustaining populations without any human assistance. So some invasive species can arrive naturally. But the big problem is that the rate of introduction has gone up dramatically because of human activities like a lot of other drivers of global change. Some invasive species were introduced intentionally.

For example, scotch broom which is a shrub that I study in New Zealand was brought over to try and stem hill erosion, slope erosion. And so they planted it. And now it’s become quite an invasive pest. And then other examples are pasture grasses, which people brought over, for example, also to New Zealand, Australia, and anywhere where European settlers went, to feed livestock. And when they brought the seeds of the grasses, within those mixes there are also seeds of other weeds and pests. So these unintentional introductions are probably the biggest way species arrive. And they’ve gone up dramatically since about the 1950s. And this is largely due to the increase in transportation and trade.

Planes and ships are excellent vectors for new species to arrive. So there are numerous reasons why we need to be concerned about invasive species arriving. One of them is their impacts on native species’ diversity which they can reduce, also ecosystem processes and services which they can alter. So this means that they ultimately cause large costs to society. In the US, for example, they expect invasive plants, just plants, to cause around $35 billion per year of damage. And largely this is due to reduced crop yields or reduced freshwater supplies. So one group of organisms that can cause a lot of damage are plants. And an interesting thing about plant invasions is that they also impact the soil.

So that’s something that I’m focusing on in my PhD, particularly how they impact soil processes such as nutrient cycling. So nutrient cycling is how a nutrient’s transferred from plants to animals and to the soil and then back through to the plants again and cycling through all these different sectors of the ecosystem. Nitrogen fixing plants, like scotch broom, which is what I’m studying in New Zealand, have the ability to fix atmospheric nitrogen via microbes that live within nodules in the plant. It’s a symbiotic relationship. And they have particularly large impacts when they invade low fertility systems where there’s a lack of nitrogen in the soil.

So the result of nitrogen fixing plants invading these low fertility systems is that you have this large influx of nitrogen into the soil, for example, when the plant dies and the roots and the matter decompose. And this can have knock on effects on soil carbon content, soil nitrogen content. The microbial community can switch from more fungi dominated communities to bacterially dominated communities. And in the end, the big picture is that nutrient cycling because of these things can speed up and decomposition can get faster as well. So it’s usually economically infeasible to eradicate an invasive species once it’s arrived and spread. But there may be enough resources to keep at a particular density or to control its spread.

So the question is, at what density do these plants start to have big impacts on the soil in the ecosystem? So the problem is that most of our understanding revolves around studies where we’ve had an uninvaded site, which has been compared with a heavily invaded site. So we don’t really know what happens in between these two endpoints, which is what my study is trying to find out in the context of scotch broom in New Zealand. So researchers at Landcare Research set up a field experiment where they’re looking at this increase of broom density and how it impacts on soil. And I was lucky enough to work with them on that, specifically looking at decomposition on nutrient cycling.

So our results have been quite interesting and unexpected. We found that as broom increases, the rate at which organic nitrogen in the soil is converted to inorganic nitrogen, which is then available for plants, increases quite dramatically. In fact, it’s a logarithmic relationship, which means that a very small increase in broom results in a much larger increase in this nitrogen mineralisation. This means that, that part of the nitrogen cycle is being speeded up substantially by broom invading. So we thought that this would increase litter decomposition as well, but actually we found it decreases litter decomposition. So this is something we’re investigating now, potentially through changes in the microbial community in the soil.

So this means that broom can have substantial impacts on nutrient cycling rates, on the soil. And we should keep it as low abundance as possible and remove it as soon as possible if we want to preserve the native ecosystem. So the next steps in my research will be to look at, do these impacts persist following the removal of broom? And also, why is it that when you do remove broom other invasive grasses start to inhabit these areas and native species don’t recover?