Biogeochemical cycles

Story hook

In 1909, the German chemist Fritz Haber worked in his Karlsruhe laboratory with a 75-cm-tall steel reactor. Inside it, he pressed nitrogen and hydrogen gases at 200 atmospheres and 500 °C in the presence of an osmium catalyst. What emerged was ammonia — synthesised, for the first time in human history, from the atmosphere itself. Until that moment, all the reactive nitrogen on Earth had been produced by lightning, by nitrogen-fixing bacteria, or by mining Chilean saltpetre. Within a decade, the Haber-Bosch process scaled industrially. Today, roughly 50 % of the nitrogen atoms in your body came not from biology but from a Haber-Bosch reactor.

It was a triumph and a catastrophe. The synthetic fertiliser revolution fed billions of people and is credited with sustaining ~50 % of the current world population. But humans now fix more nitrogen than all of nature combined. The excess nitrogen runs off into rivers and oceans, creating 400+ dead zones worldwide — including the Bay of Bengal's expanding hypoxic zone. We have broken the nitrogen cycle.

This file is about the great planetary cycles — carbon, nitrogen, phosphorus, water — and how humans have rewired each of them. Understanding these cycles is the foundation for understanding climate change, pollution, soil degradation, and ocean acidification.

Why this matters for UPSC

Biogeochemical cycles are tested every UPSC year at the Prelims level — factual questions on which organism does nitrogen fixation, the steps of the nitrogen/carbon cycle, eutrophication, the Haber-Bosch process. Mains ties cycles to pollution policy, fertiliser subsidies, and climate mitigation. Interview probes connect cycles to soil health and food security. 1-2 Prelims questions per year; medium Mains weight.