Which ecosystem is much more phosphorus limited than nitrogen limited and which Earth sphere has the largest supply of phosphorus?
In the sense that 80% of the atmosphere is Nitrogen and there are both biological and chemical catalyzed fixation nitrogen is not really limited. Growing legumes and using natural gas can be a cost of sort but Nitrogen globally is present everywhere.
The situation with Phosphorus is that the mineral is not found as abundantly and is not circulating through a gas form.
Morrocco has been the largest P resource but the new discovery in Norway might be even greater.
In absolute terms the ability of Phosphorus to limit food and feed is probably more a long term issue than Nitrogen.
The ability to extend the benefitical use of Phosphorus would be alleviated if the manures and excreta could be economically recycled better and if the biological use of mycorrhiza and cropping combinations can be used to improve the efficiency of it use.
For instance with effective mycorrhizal plants the P to optmize productive yield can be one quarter of a sole P optimization in non mycorrhizal plants.
Peak Phosphorus is still an active concern and the prospectives for peak Nitrogen are of lesser long term concern.
Generally, temperate and boreal ecosystems are N-limited due to slow N mineralization, while tropical forests and savannah ecosystems are P-limited in their highly weathered soil. Because phosphorus is not ex- changing between the ocean and an atmospheric reservoir as nitrogen does, the delivery of phosphorus-not nitrogen- limits net production (and sedimentation) of organic material in the ocean as a whole.Nitrogen (N) and phosphorus (P) limitation constrains the magnitude of terrestrial carbon uptake in response to elevated carbon dioxide and climate change. One of the reasons for this is the high reactivity of phosphorus. It tends to bind to soil particles and complex with metals in the soil. This makes it unavailable to plants even if it is present in the soil. The ocean sediments are therefore by far the greatest reservoirs of phosphorus. In terrestrial ecosystems, much of the available phosphorus moves in a closed cycle between living organisms and the organic debris in the soil. The largest source of phosphorus on Earth is in sediments and sedimentary rocks. The phosphorus cycle on land is very closed in such a way that any phosphorus available on the surface of the land is not stored for long but used by living things. The largest source or reservoir of phosphorus on earth is sediments, typically sedimentary rocks. There is a significant amount of phosphorous is ocean sediments as well. Phosphorous can also be found in plants, animals, and the soil.