Nutrient cycles  (Page 5/24)

Art connection

Which of the following statements about the nitrogen cycle is false?

1. Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH ₄ ⁺ ).
2. Denitrification by bacteria converts nitrates (NO ₃ ⁻ ) to nitrogen gas (N ₂ ).
3. Nitrification by bacteria converts nitrates (NO ₃ ⁻ ) to nitrites (NO ₂ ⁻ ).
4. Nitrogen fixing bacteria convert nitrogen gas (N ₂ ) into organic compounds.

Human activity can release nitrogen into the environment by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers in agriculture, which are then washed into lakes, streams, and rivers by surface runoff. Atmospheric nitrogen is associated with several effects on Earth’s ecosystems including the production of acid rain (as nitric acid, HNO ₃ ) and greenhouse gas (as nitrous oxide, N ₂ O) potentially causing climate change. A major effect from fertilizer runoff is saltwater and freshwater eutrophication    , a process whereby nutrient runoff causes the excess growth of microorganisms, depleting dissolved oxygen levels and killing ecosystem fauna.

A similar process occurs in the marine nitrogen cycle, where the ammonification, nitrification, and denitrification processes are performed by marine bacteria. Some of this nitrogen falls to the ocean floor as sediment, which can then be moved to land in geologic time by uplift of the Earth’s surface and thereby incorporated into terrestrial rock. Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle. ^{Scott L. Morford, Benjamin Z. Houlton, and Randy A. Dahlgren, “Increased Forest Ecosystem Carbon and Nitrogen Storage from Nitrogen Rich Bedrock,” Nature 477, no. 7362 (2011): 78–81.}

The phosphorus cycle

Phosphorus is an essential nutrient for living processes; it is a major component of nucleic acid and phospholipids, and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is often the limiting nutrient (necessary for growth) in aquatic ecosystems. Although the cycle in ( [link] ) suggests there is an atmospheric component to this nutrient cycle, the amount that occurs in the atmosphere is so negligible that for our intents and purposes, we will assume there is not atmospheric phase. It is this that sets the phosphorus cycle apart fron the other cycles we already looked at.

Phosphorus occurs in nature as the phosphate ion (PO ₄ ³⁻ ). In addition to phosphate runoff as a result of human activity, natural surface runoff occurs when it is leached from phosphate-containing rock by weathering, thus sending phosphates into rivers, lakes, and the ocean. This rock has its origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of ocean organisms and from their excretions. However, in remote regions, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources. This sediment then is moved to land over geologic time by the uplifting of areas of the Earth’s surface.