Nutrient use in agriculture drastically changed over the course of the 20th century. Results from this project have shown that the legacy phosphorus (P) present in the croplands of developed regions has increased drastically since 1900, with a 31% increase in Europe and a 15% increase in North America. Other regions have also experienced a large increase in the cropland legacy P, such as Asia (+17 %), and Oceania (+17%); while South America has seen a minor increase (2%) and Africa has remained relatively stable.
Our results suggest that, during this period, the global nutrient delivery to streams has reached 64 Tg N yr−1 and 9 Tg P yr−1 almost double the values at the beginning of the 20th century. The increase in nutrient delivery to rivers leads to a total export of the oceans of 37 Tg N yr−1 and 4 Tg P yr−1, despite in-stream retention and removal which grew from 14 to 27 Tg N yr−1 and 3 to 5 Tg P yr−1 during the 1900-2000 period.
Toward the future, we have also estimated that synthetic fertilizer inputs will increase from 14.5 Tg P yr-1 in 2005 to 22-27 Tg P yr-1 in croplands in 2050, and that 4-12 Tg P yr-1 would be needed in 2050 in global intensively managed grasslands to maintain fertility. Likewise N synthetic fertilization must increase to 85 – 260 Tg N yr-1, with the lower estimate highly dependent on future agronomical improvements and a decrease in the consumption of animal protein.