Nitrogen fixation: Anthropogenic enhancement‐environmental response

TLDR

In natural systems, biotic fixation supplies ~90–130 Tg N yr⁻¹, while human activities add ~140 Tg N yr⁻¹ through energy production, fertilizer manufacture, and crop cultivation, yet the fate of this anthropogenic nitrogen remains uncertain due to variable sinks and atmospheric accumulation. The study predicts that anthropogenic N‑fixation will rise by ~60 % by 2020, mainly driven by increased fertilizer use and fossil‑fuel combustion. The model indicates that two‑thirds of this rise will occur in Asia, which will then contribute more than half of global anthropogenic N‑fixation by 2020.

Abstract

In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90–130 Tg N yr −1 (Tg = 10 12 g) on the continents. Human activities have resulted in the fixation of an additional ≈140 Tg N yr −1 by energy production (≈20 Tg N yr −1 ), fertilizer production (≈80 Tg N yr −1 ), and cultivation of crops (e.g., legumes, rice) (≈40 Tg N yr −1 ). We can only account for part of this anthropogenic N. N 2 O is accumulating in the atmosphere at a rate of 3 Tg N yr −1 . Coastal oceans receive another 41 Tg N yr −1 via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr −1 by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO 3 − , N 2 ). The remaining 80 Tg N yr −1 are either retained on continents in groundwater, soils, or vegetation or denitrified to N 2 . Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N‐fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil‐fuel combustion. About two‐thirds of the increase will occur in Asia, which by 2020 will account for over half of the global anthropogenic N fixation.