Biogeochemical flows

The disruption of nutrient cycles that regulate soil fertility and the health of water ecosystems, through excessive fertilizer use and pollution.

In the 20th century, the invention of industrial nitrogen fixation made it possible to convert molecular nitrogen from the atmosphere into reactive forms, such as those used in inorganic fertilisers. Combined with the mining of phosphate rock, this led to a drastic increase in fertilizer application on agricultural land. Because only a part of the fertilizer is actually taken up by crops, large quantities of both nutrients remain in the environment.

Nitrogen is released into the air and stored in groundwater and surface waters. Phosphorus accumulates in soils and is released into surface waters via soil erosion and surface runoff. Excess nutrients can have negative effects on biodiversity and the resilience of ecosystems on land, in freshwater and in the ocean, which are adapted to specific nutrient levels. Currently, losses of both nutrients to the environment are disrupting ecosystems beyond the safe level.

Control Variables

The Planetary Boundary for Biogeochemical Flows tracks the global cycles of nitrogen and phosphorus — two essential nutrients for life. The nitrogen boundary measures how much reactive nitrogen humans introduce each year, mainly through fertilizers and fossil fuel combustion. The phosphorus boundary has both global and regional dimensions; the regional one focuses on how much phosphorus flows from land into freshwater systems, while the global one concerns the transport of phosphorus into the ocean. Both boundaries have been substantially exceeded.

Impacts

Exceeding safe limits for nitrogen and phosphorus has wide-reaching consequences for ecosystems, climate, and human well-being. Once released into the environment, these nutrients move through water, air, and soil, creating knock-on effects across multiple Earth system processes. They pollute waterways, alter species composition, and even contribute to global warming and ozone depletion. Because nutrient cycles are so tightly connected to other boundaries — including Climate Change, Biosphere Integrity, and Land-System Change — transgressing them has cascading effects across the Earth system.