How industrial agriculture is disturbing the nitrogen cycle and undermining conditions for life on Earth

Six of our nine planetary boundaries have now been crossed – and industrial agriculture are the main culprit. That is what a team of scientists under Johan Rockström reported in an article published in September..

First, a reminder: planetary boundaries are thresholds of disturbance beyond which our Earth’s system are put on an uncontrollable and irreversible path that undermines the very conditions for life. This notion of overstepping boundaries is clear in regard to the best-known limit of them all: that of climate change. In order to cap global warming at 1.5°C above pre-industrial levels, and prevent it from escalating beyond bearable conditions (even if this is not enough to stop climate disruption already underway), we need to keep the proportion of atmospheric CO₂ below a certain limit. And to achieve this, we need to reach carbon neutrality, quickly.

Regarding the climate, it is easy to see how such a global limit is relevant: the carbon cycle is open to the whole planet and CO₂ emitted (or captured) anywhere on Earth immediately affects the world’s atmosphere in its entirety. Yet in the case of the planetary boundary for nitrogen, exceeding the threshold is different, as it is the industrialisation of agriculture that is largely, and more complexly, responsible for breaking the limit.

But how can agriculture affect the nitrogen cycle? How has it managed to reach a breaking point? To feed the world, isn’t intensive agriculture best? Let’s take stock of the situation.

The natural nitrogen cycle
First, we need to understand the natural cycle of carbon and nitrogen – two of the main elements that form living matter. We can observe this cycle in forests, for example. To function, forests rely on a balance between plants growth – a process that turns mineral (inorganic) forms of carbon and nitrogen into biomass (organic) – and animals, fungi and micro-organisms decomposing this biomass, a process that remineralises it.

But whereas the inorganic form of carbon (CO₂) is present in the atmosphere, distributed evenly worldwide and absorbed by plants via their leaves, nitrogen is remineralised in the soil and absorbed by plant roots. So, the boundaries of the nitrogen cycle have to remain local: any loss of nitrogen brings about a risk of soil depletion, which jeopardises continued plant growth.