Agricultural land area is 38 percent of the global land surface, about one-third of this is used as cropland [1]. Land is 29% of Earth [2]. So we use 1/3 * 0.

Agricultural land area is 38 percent of the global land surface, about one-third of this is used as cropland [1]. Land is 29% of Earth [2]. So we use 1/3 * 0.38 * 0.29 = 3.6% of Earth's surface for farming crops and 7% for cattle. Most of that is fertilized every year, and then the surplus fertilizer is flushed to rivers and seas and ends up in the oceans fertilizing algae there. [3] Basically we're all in the food chain, it doesn't matter where you add the new nitrogen - it goes everywhere. [1] https://www.fao.org/sustainability/news/detail/en/c/1274219/ https://www.fao.org/sustainability/news/detail/en/c/1274219/ [2] https://www.nationsonline.org/oneworld/earth.htm https://www.nationsonline.org/oneworld/earth.htm [3] https://www.nytimes.com/2017/07/27/climate/nitrogen-fertilizers-climate-change-pollution-waterways-global-warming.html https://www.nytimes.com/2017/07/27/climate/nitrogen-fertiliz... EDIT: there are also negative effects, so it's not so simple. I'd still estimate that the global biomass should increase over time.