Nitrogen is the seventh element in the periodic table, constitutes 78% of the Earth’s atmosphere, and without it life as we know it could not exist. It is a vital component in protein, DNA, and many cellular signalling compounds.
In its basic forms, it can be nitrogen gas (N2), ammonia (NH3), ammonium (NH4+), nitrous oxide (NO2) or nitrate (NO3-). These have many functions in society such as cleaners, industrial gases and fertilisers.
As one of the most abundantly available elements in nature, it’s not surprising that nitrogen has an important role in ecosystems. As land use changes, the timing and scale of nitrogen movement though systems can also change.
Nitrogen’s importance in dairy farming:
Where protein is created, nitrogen is needed. Given that the main income sources on a dairy farm are milk – where protein forms the lions’ share of the milk price, meat – valued for its protein content, and cattle – full of protein, nitrogen is continually leaving dairy farms.
Nitrogen can also leave dairy farms as a waste product. It can volatilise into the atmosphere as nitrous oxide, mainly from effluent storage ponds and nitrogen containing fertiliser sitting on the surface of the soil. It can also leach into water bodies.
This needs to be replenished in the soil, so it is available for pastures to grow and supply more protein to the cows. This is mainly done by the fixation of atmospheric nitrogen by legumes, like clover, and by applying nitrogen fertilisers, such as urea, ammonium nitrate, ammonium sulfate and various organic products.
Cow urine is very high in nitrogen compounds, mainly urea and uric acids, and urine patches create localised high concentrations of nitrogen on pastures that they graze. This makes effluent collected from dairies and contained feedpads and wintering barns high in nitrogen.
What is nitrogen leaching?
Nitrogen in the form of nitrate is very soluble in water and is easily carried away through drainage into waterways, a process known as leaching. There are several factors that affect how much nitrogen is leached into the soil.
• Nitrate concentration: The higher the concentration of nitrate in a particular area of soil, the more readily it will be dissolved into water that passes through that area of soil. Urine patches create high local concentrations of nitrogen, which generates a high degree of leaching.
• Plant uptake: A rapid uptake of nitrogen by pastures can decrease the nitrogen lost by leaching. The faster the plant grows, the quicker nutrients will be removed from the soil. Soil temperature has a big effect on this.
• Rainfall: Higher rainfall will collect more nutrients from the soil to runoff to water bodies. Leaching is much more pronounced when soils are saturated.
• Soil structure: Free draining soils will transfer water to waterways faster than soils that hold water for longer. The longer the water is retained in the soils, the more opportunity there is for plants to use the nutrients that are carried in it.
• Anion/cation exchange capacity: This is a characteristic of soils that dictates how well they bind nutrients in the soil, preventing them from being lost through water. Soils high in clay and organic matter will tend to hold nutrients better than sandy soils.
• Soil damage-drainage: Soil damage from pugging or artificial drainage can provide a free channel for water to travel to waterways, reducing water retention and accelerating nutrient loss. This also reduces the capacity for nutrients to be absorbed into the soil to be made available to pastures.
• Form of nitrogen: Other forms of nitrogen are less soluble than nitrate and forms such as ammonium bind more strongly with organic material in the soil, preventing leaching. Nitrification inhibitors such a dicyandiamide (DCD) have been used to slow the conversion of soil nitrogen to nitrate, though its use is currently withdrawn after residues of the chemical were detected at very low levels in milk.
Timing is an important variable in many of these mechanisms. Rainfall and soil temperature are strongly linked to seasonal variation. This means the effect of any nitrogen application can vary significantly depending on the time of year it is applied.
How is nitrogen measured?
Nutrient budgeting software Overseer is being used by many regional councils to assess nitrogen leaching at the farm level. It does not measure nitrogen loss, but approximates the loss through a series of models generated from data collected in plot and field experiments. By inputting physical farm data such as stocking rate, soil type, rainfall, feed imported, nitrogen applied and milk produced, Overseer compares these variables to information in its database to predict the nitrogen loss on a dairy farm.
Measuring nitrogen loss from a dairy farm is much more difficult than from a point source discharge, such as a wastewater plant, because there are several ways it can enter the waterways.
It can directly run off from the surface, run through natural or artificial drains, through seepage or into groundwater. Measurement is more often done at the larger catchment scale.
Overseer is a useful tool for determining the effects of changes to farming systems, but the margin of error in the results is about 30%. It is, however, one of the most cost-effective methods to estimate diffuse leaching from dairy farms.
Nitrate in the water is most commonly measured by taking water samples to be tested at laboratories. Different forms of nitrogen require different chemical tests.
Another method used to monitor nitrate levels is a spectrophotometer, which measures changes in light as it is passed through a sample of water. This method can be used to continuously sample a site, though the cost and logistics deters extensive use.
Lysimeters are an important tool in calculating nitrogen loss, because they measure how much water is lost through evapotranspiration.
By deducting this loss from rainfall, the amount of water moving to waterways can be calculated. Knowing the flow and concentration, the total nitrogen loading in a water body can be calculated.
What is done with the data?
There are several groups that collect data related to nitrogen management, including regional councils, DairyNZ, universities and crown research institutes such as NIWA and AgResearch.
The two main uses of the data are to model what is happening and to determine trends.
Overseer is a prime example of the end use of such data. Each revision of the Overseer programme incorporates new data to more accurately predict outcomes on farm. Such pooled data is used in forecasts that evaluate where the state of farming and freshwater might be in the future.
In contrast, trends show the effects of previous activity. This is where “state of the environment” type reports are developed, and are a reasonable indicator of the direction of change.
The Land, Air, Water Aotearoa project is a publicly available collation of such data showing the state and trends of environmental quality from hundreds of different sites in New Zealand.
Both are important in determining the direction of environmental change as a result of dairy farming.
The challenge is marrying the two together. Nitrogen, depending on the catchment, can have a residence time in waterways and soils of anywhere between one month and twenty years. Developing a cohesive model takes time and resources.
The other primary use of data is to develop solutions to improve nitrogen efficiency.
By analysing the timing and method of nitrogen loss, innovative minds can come up with profitable solutions to minimise the loss.
Examples of such solutions are timing autumn nitrogen applications to minimise winter flushing and altering feed compositions across lactation to minimise excess nitrogen in cows’ diets.
Next month the focus will shift to why and how nitrogen is a problem in the environment, and the final instalment will look at the best solutions currently available to mitigate nitrogen losses.
Further Resources:
www.niwa.org.nz National Institute for Water and Atmospheric Research
www.lawa.org.nz Land, Air, Water Aotearoa
www.dairynz.co.nz/environment DairyNZ
