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Nutrient Cycling

What goes in must come out . In farming, one of those outputs is urine. A 525kg cow can produce  more than 23 litres of urine per day. Nitrate from the urine can enter waterways, either directly or via groundwater, with undesirable environmental outcomes. A number of research projects now focus on the impact of urine's nitrate content on pastures, on ways to reduce its movement through the water table, and on monitoring and mitigating nitrate's effects on waterways.

Nutrient Cycles | The Carbon Cycle | The Phosphorus CycleThe Nitrogen Cycle

Nutrient Cycles

The Earth has a limited quantity of chemical elements from when it was formed; and the only way more elements are obtained is from occasional meteorites striking the Earth from outer space. Because the chemicals on Earth function in a closed system, neither significantly increasing nor decreasing in quantity, they are recycled throughout the Earth’s biological and geological cycles. These cycles include both the living biosphere, and the nonliving lithosphere
, atmosphere, and hydrosphere.

The main biogeochemical cycles describe the movement of carbon, nitrogen and phosphorus. Over time, these elements cycle through the biosphere, lithosphere, hydrosphere and atmosphere (the latter three are also called geospheres).They can be taken up by living things and used for growth and reproduction before either passing on to another organism or returning to one of the geospheres. They can be present in the atmosphere (except for phosphorus) as gases such as CO2
, N2; in the hydrosphere as dissolved nutrients and gases such as PO43-, NO3- and  CO2; or in the form of minerals such as carbonates, sulphates or phosphates in sedimentary and volcanic rocks.

Biogeochemical cycles can be broken down into two types: local cycles such as the phosphorus cycle
, which involve elements with no mechanisms for long distance transfer; and global cycles, which involve an interchange between the atmosphere and the ecosystem. It is these global nutrient cycles, such as the nitrogen cycle and carbon cycle, that unite the Earth and its living organisms into one giant interconnected ecosystem called the biosphere.


Drs Dave Campbell and Louis Schipper, along with technical staff Jacinta Parenzee and Craig Hosking and a team of research students (Susanna Rutledge, Paul Mudge, Tehani Kuske, Suzanne Lambie) are trying to determine how different land management practices affect the storage of carbon in soil organic matter. Organic matter is critical for
maintaining many important soil qualities such as water and nutrient
storage, being the major food source for soil organisms. Soil also
contains the largest terrestrial store of carbon. The team are excited
by their research because they are measuring gains and losses in carbon at paddock scales in real time. This research will provide important information for farmers to improve sustainability of their farm
management practices.

See also the Greenhouse gases pages of this site.


The Phosphorus Cycle

The phosphorus cycle differs from many other biogeochemical cycles because it does not involve the atmosphere in any significant way (although there is some evidnece of an atmospheric component in the form of PH3).  Phosphorus and phosphorus-based compounds are usually solids at normal temperatures and pressuresfound on Earth, and any phosphorus in the atmosphere is usually only present in the form of dust particles.  

Phosphorus - which is an essential nutrient - is usually found in the form of the phosphate ions (PO43- and HPO42-). It is an important component of  nucleic acid  molecules (DNA & RNA) and of the cellular energy carrier ATP. Phosphorus is also an important building block of bones and teeth, where it is found in the form of calcium phosphate. 

Most phosphates originate as salts in ocean sediments or in rocks. Over time, geological uplift brings these sediments to the surface, and weathering releases the phosphate ions. Plants can then absorb these phosphates from the soil and use it in cellular processes. Phosphate taken up by plants may then be passed on to animals when the plant is consumed by herbivore that, in turn, may be consumed by carnivores. After death, the animal or plant decays, and the phosphates are returned to the soil by way of bacterial decomposition. Runoff from the land may carry leached phosphate back to the ocean, where it eventually enters sediments and is reincorporated into rock.

Because there is generally only a small quantity of phosphorus available for uptake by plants, phosphorus is often a limiting factor for plant growth. This is why farmers often apply phosphate fertilisers to farmland to increase plant growth. 

Since phosphorus - in the form of phosphate from natural sources - is not very water-soluble it may also be a limiting factor for plant growth in marine and freshwater ecosystems. Runoff of phosphate fertilisers from farmland leads to eutrophication of waterways and can cause algal blooms in aquatic ecosystems, resulting in lower water quality.  

The Nitrogen Cycle

The nitrogen cycle is a gaseous cycle: it involves the movement of nitrogen between the soil, living things, and the atmosphere. The atmosphere is the major reservoir of nitrogen (in the form of nitrogen gas, N2, which makes up 73% of the Earth's atmosphere). All living things require nitrogen, using it to make DNA and RNA, and amino acids. (For an explanation of just why nitrogen is so common in the atmosphere, visit "Ask an Earth Scientist".)

Although nitrogen makes up such a large proportion of the atmosphere, it is often a limiting factor for plant growth.  This is because plants can absorb nitrogen only in the form of nitrate (NO3-) or ammonium (NH4+). (Animals must obtain their nitrogen by eating organic material - plants or other animals - containing nitrogen.) Because of this, nitrogen - in the form of nitrate - makes up a large proportion of most commercial fertilisers. Unfortunately, excessive amounts of nitrate in agricultural run-off can have harmful effects on aquatic ecosystems.

The nitrogen cycle

Image from Pidwirny, M. (2006)

Some atmospheric nitrogen is 'fixed' by lightning into NO3-, and then carried into the soil by rainwater. However, most of it is fixed by micro-organisms in the soil.

When an organism dies, nitrogen from their bodies - in the form of ammonia (NH3) is converted by decomposers into ammonium (NH4+), a process known as mineralisation. Nitrosomonas bacteria convert the ammonium into nitrite (NO2-), and the nitrite is in turn altered to form nitrate (NO3-) by Nitrobacter bacteria. These processes are called nitrification. The nitrate they produce is highly soluble in soil water and so readily available to plants. However, because it is so soluble, nitrate is also easily leached from the soil and into waterways. NItrogen returns to the atmosphere as a result of denitrification, in which nitrate is reduced by anaerobic bacteria into nitrogen gas (N2) or nitrous oxide (N2O).

Human activities have had a significant effect on this naturally-occurring nitrogen cycle. Nitrates can enter the soil - and the soil water - from septic tanks, dairy shed effluent, agricultural fertilisers and animal wastes, industrial waste waters, landfills, and sewage plants. For lakes such as Rotorua, run-off from agricultural land has been a major contributor to excessive levels of nitrate in the lake water

Most leaching of nitrate to waterways occurs during winter, when plant growth is slower, soils are wet, and rainfall is higher. This means that nitrate concentrations in groundwater are highest in winter and spring. However, during summer and autumn, when plants are growing and taking up nitrate from the soil, there is less leaching and nitrate concentrations in groundwater are lower. The amount of leaching that happens is also affected by when and how much fertiliser is applied to pasture; when, how, and how much the land is irrrigated; disposal of wastewater; and general weather patterns  - in other words, it's a very complex picture!

Jump to the Ecology page for more information on the impact of nitrate on our waterways, and on research that's being done on ways to reduce that impact..

  Image sources

Nitrogen cycle: 
Pidwirny, M. (2006). "The Nitrogen Cycle". Fundamentals of Physical Geography, 2nd Edition. Date Viewed 27 May 2008. 

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