The nitrogen cycle and dairy farming

Nitrogen undergoes a number of transformations as it moves between the atmosphere, the land and living things. This simplified nitrogen cycle focuses on some of the transformations associated with dairy farming.

To use this interactive, move your mouse or finger over any of the labelled boxes and click to obtain more information.

Transcript

Life processes

Nitrogen is a crucially important component for all life. It is part of many cells and processes, such as amino acids and proteins. Cows obtain nitrogen as they graze pasture plants and use it for their life processes and in milk production.

Image: Cows feeding on maize silage. © DairyNZ.

Fertiliser

Fertilisers are added to soil to make it more fertile. They supply plants with the elements that may be missing or in short supply in a form that can be used by the plants for faster growth. Fertilisers enable dairy farmers to replace nutrients that are removed from the soil when pasture is grazed by cows.

Farmers need to carefully balance fertiliser applications to optimise plant growth and reduce nutrient loss to run-off or leaching.

Image: Truck spreading urea, from Good Fertiliser Practices.

Pasture legumes

Legumes like clover form root nodules that harbour bacteria known as rhizobia. Rhizobia convert atmospheric nitrogen gas to forms that plants can use. White clover is a common pasture species in Aotearoa New Zealand. It is nutritious feed for cows and it’s beneficial to soil fertility.

Image: White clover nodules. © DairyNZ and The University of Waikato Te Whare Wānanga o Waikato.

Dung, urine and effluent

Nitrogen from the pasture plants is absorbed by the cow’s body, and excess nitrogen is excreted as urine and dung.

Urine is of particular concern as a considerable amount of nitrogen is deposited in a small area. Pasture plants are not able to utilise all of the nitrogen, and it can be lost to leaching.

Dairy shed effluent is a mix of wash water, dung and urine. It is collected and treated on the farm and irrigated to the land, returning and reusing the nutrients.

Image: © The University of Waikato Te Whare Wānanga o Waikato.

Pasture run-off

Ideally, rain or irrigated water and/or effluent infiltrates the soil and moves into the plant root systems. If the soil is saturated or if the rain or irrigated water is more than the soil can handle, water can move off the site and take nutrients with it. The nutrients are lost to the pasture system.

Image: Farmland during the 2015 Whanganui district floods. © Whanganui District Council.

Gaseous losses

Nitrogen within the soil is lost to the atmosphere through volatilisation and denitrification. Volatilisation is the conversion of dissolved ammonia from urine patches into ammonia gas (NH3). Denitrification is the conversion of nitrate to nitrous oxide (N2O) and gaseous nitrogen (N2). Although these are natural processes, the conversions are considered losses as the nutrients are no longer available for plant uptake. Nitrous oxide is a potent greenhouse gas. Image: © DairyNZ

Soil transformations

Bacteria in the soil are responsible for several transformations within the nitrogen cycle. Ammonium (NH4+) is converted to nitrite (NO2-) and then to nitrate (NO3-). This process is known as nitrification. Other processes that occur in the soil are mineralisation and denitrification. Mineralisation is the decomposition of litter in soil and the release of nitrogen into forms that plants can take up. Denitrification is the conversion of nitrate to nitrous oxide (N2O) and gaseous nitrogen (N2). Image: © The University of Waikato Te Whare Wānanga o Waikato

Uptake by plants and microorganisms

Plants and microorganisms take up different types of nitrogen. The main types are ammonium (NH4+) and nitrate (NO2-). Plants use nitrogen for many life processes. One is to make chlorophyll – the green pigment in plants that enables energy from sunlight to be transformed into chemical energy. Image: Plantain in pasture. © DairyNZ.

Leaching

Leaching is when water moves through the soil. As the water moves, it can carry nitrate with it. If nitrate moves below the root zone, it is no longer available for plants to use and can move into groundwater and then eventually into surface water. Image: © The University of Waikato Te Whare Wānanga o Waikato.

Acknowledgement

This resource has been produced with the support of DairyNZ.