Nutrient limitation in terrestrial and freshwater ecosystems?

Question

In terms of primary production, it is often described in textbooks that nitrogen is the most limiting nutrient in terrestrial ecosystems, while phosphorus is the most limiting nutrient in freshwater ecosystems. What creates this difference?

Answer 1

One thing we have to consider is that both nitrogen AND phosphorous are limiting nutrients in both terrestrial and aquatic ecosystems, but they play different roles. For example, phosphorous is less naturally abundant in aquatic ecosystems than nitrogen because it is not very water soluble. This makes it hard for organisms to utilize it. On the other side of the coin, aquatic ecosystems can experience too much phosphorous because of something called eutrophication. This means pollution, agricultural runoff, etc. can introduce excessive phosphorous. When this happens, algae that isn't used to having free-floating phosphorous will thrive. This algae can use up too much oxygen in the water and block out the sun. Other organisms cannot thrive in these conditions, therefor phosphorous balance is essential to maintaining a freshwater ecosystem. Nitrogen is more readily available in aquatic ecosystems as the nitrogen cycle converts biological waste (ammonia) into nitrogen.

In terrestrial ecosystems, nitrogen is very limiting. This can be a surprising concept for some people considering our atmosphere is made of 78% percent oxygen! Most organisms on Earth cannot directly use this nitrogen, though. Atmospheric nitrogen has to be converted into other forms, like ammonia, before it can be usable. This process is called nitrogen-fixing. Nitrogen levels in soil can fluctuate greatly depending on a number of environmental factors such as temperature and organic matter content. Additionally, plants in some areas may use nitrogen faster than it is replenished in the soil. (This is why it is limiting! More Nitrogen = more plants, but when they use up all the nitrogen, no more plants. The ecosystem is limited by the availability of nitrogen.)

While both phosphorous and nitrogen are limiting in both ecosystems, they play different roles, meaning these limitations have different consequences in each ecosystem, therefor they create different limitations.