Interference with global n and p cycles
I’m going to talk with you about human impacts on the global nitrogen and phosphorus cycles.
The global biogeochemical cycles link the living and the non-living parts of the Earth system. Chemical relationships control the general patterns of interaction between geological and biological processes. It’s the way that the nutrient elements flow through land, ocean and atmosphere that ultimately regulate all of life on the planet. And human activities are changing all of these basic cycles.
As we explore this topic, I’ll make reference to very simplified diagrams like this one, that show the present day cycles and the human changes. The arrows just show the overall annual flows of the elements between the Earth’s crust, atmosphere, oceans and the biosphere, that is all the living organisms on Earth.
It’s important to keep in mind that these flows are the result of many different Earth system processes, and at the same time it’s also important to remember that in the boxes, atmosphere, ocean, biosphere, and so on, we see many processes and feedbacks happening all the time. The Earth system is a very dynamic system, and the simple diagram is a very simple representation.
Because all of these boxes are connected to each other through these different processes, human activities in one will play out in all of the others. It’s these consequences that mean that we need a planetary boundary for nutrient elements.
Piecing together the global budgets, in other words putting the numbers on the up and down arrows in those diagrams, is one of the major achievements of the last 50 years. It takes international and interdisciplinary research, and it involves scientists working in all kinds of environments around the world.
People are most familiar with the global carbon cycle because changes in carbon are so tightly linked to the climate system. The figure shows some of the processes that are happening at the moment. And here is the first of the simple flow diagrams I talked about.
The numbers show the annual flows between the different stocks of carbon in the Earth system, and the red arrow shows the human disturbance. Every year the biggest single change that we make to the carbon cycle is that we’re moving carbon from underground fossil fuel stocks into the atmosphere.
We also contribute carbon dioxide into the atmosphere through deforestation and agriculture. But our main concern is that this carbon dioxide is accumulating in the atmosphere where it acts as a greenhouse gas. Climate is changing as the concentration increases.
But carbon isn’t the only element that we’re changing. People are much less familiar with the other elemental cycles, but they’re just as important, and actually they’re tightly linked to carbon and the climate system too.
In this picture you can see the main changes in the nitrogen cycle. Nitrogen is one of the essential elements that sustains all of life on Earth. We use it in our proteins that make up our muscles, for example. The flow diagram shows the natural changes in white and the red arrows show the major changes that humans are causing in the system. The biggest change is the transformation of atmospheric nitrogen into reactive forms, like nitrate and ammonia. We do this because we need that nitrogen as fertilizer for the food production.
Right now the red arrow of nitrogen fixation is bigger than the natural arrow of natural biogenic fixation of nitrogen. We are currently more than doubling the natural rate of drawing down of nitrogen from the atmosphere into the biosphere, but you can see that that isn’t all that we’re doing. This nitrogen is not all taken up by crops. A large amount is being released back into the atmosphere where it causes air pollution problems and acid rain, and is a climate greenhouse gas, and the rest is released into rivers and oceans where we have problems of nitrogen enrichment and eutrophication. I’ll explain what that is in a little while.
This is happening all around the world. When rivers, lakes, and coastal zones have very high concentrations of nitrogen, and other nutrient elements, the most responsive organisms draw up this nutrient most quickly. We end up with algal blooms, microorganisms are thriving in these water conditions.
The image shows areas where there are very large amounts of marine algae, reflecting the highest concentrations of nutrient elements.
In the nitrogen cycle we have another particular challenge, which is that a large amount of the nitrogen is transported all around the world, very large distances, in the atmosphere. We see biogeochemical and ecological tipping points in area far away from the most intense sources. The densely populated areas have the most intense nitrogen pollution problems, but you can see from the figure that in this century of industrial activity, nitrogen really has become a global problem.
The issue is not just a regional one. And nitrogen is not the only element that’s causing us concern at the moment. The phosphorus cycle is also being changed very substantially by human activity, and phosphorus is another one of these essential nutrient elements.
The figure again shows the processes that are happening in the Earth system, and the simple diagram shows the natural flows in white and the major human perturbation in red. Unlike nitrogen and carbon, phosphorus isn’t really affecting the atmosphere. The biggest change is that human activities are mining it from the ground and applying it to land surfaces for agriculture, 25 million tons a year. But a large amount of this phosphorus is being mobilized in the Earth system through the watercourses. It goes into our rivers and receiving oceans. It also contributes to nutrient enrichment. Nutrient enrichment doesn’t really sound like a problem, but it is a very serious one. Locally it’s a major problem for public health and environment, and it’s an economic issue as well.
In nutrient-enriched water systems biodiversity typically decreases. And when the algal blooms die and rot away they starve the water of oxygen. We end up with dead zones, and this is another example of tipping points that we see in the environment. It’s the risk of large and irreversible dead zones in the oceans that really calls for a planetary boundary for phosphorus.
The changes in both the nitrogen and phosphorus cycles are largely driven by the same human activity. In other words, fertilizer application for food production. The global impact arises from regional activities, so a really major area for research and policy at the moment is to improve the global assessment of both these nutrient elements.
The planetary boundaries for human interference in the global nutrient cycles now reflect these regional differences. The ecological impacts of the eutrophication of surface waters, and increasingly we’re working on trying to improve the links between the elemental cycles because they’re linked in nature.