Trends in nitrogen leaching from agriculture

  • Image, Trends in nitrogen leaching from agriculuture.

    Nitrogen is an essential nutrient for plant growth. It occurs naturally in the environment but is added in agricultural processes (typically as fertiliser) to boost production. Although much of the applied nitrogen is taken up by plants, livestock waste returns a considerable amount to the soil. Nitrate formed from this waste easily drains (leaches) from the soil before plants can absorb it, and it can enter waterways, potentially harming ecosystems.

    We classified Trends in nitrogen leaching from agriculture as a national indicator.

    Key findings

     Increasing trend (increasing pressure)

    From 1990 to 2012, the quantity of nitrogen leached from agriculture annually increased by an estimated 28.6 percent (30.4 million kilograms).

    • Over the 1990–2012 period, nitrate leaching increased at an average of 1.2 percent a year.
    • In 2012, 1,963 million kilograms of nitrogen were applied to agricultural land as fertiliser.
    • Of the total nitrogen applied in 2012, an estimated 137 million kilograms leached from the soil. Only 19 percent of the loss was directly from fertiliser; the remainder was through livestock waste.

    Note: Other – deer, goats, horses, swine, and any other livestock not described.

    Definition and methodology

    The amount of nitrogen that leaches (called ‘nitrogen leachate’) is only a fraction of the total amount added to land. The amount leached depends on the rate of plant uptake, the amount of rainfall, and the texture and type of soil (McDowell et al, 2008). It is also affected by individual farm characteristics, such as the stocking rate of grazing animals.

    We assume 7 percent of the nitrogen applied nationally is lost as leachate. This is based on the New Zealand-specific value adopted for greenhouse gas reporting (Ministry for the Environment, 2014; Thomas et al, 2005).

    Nitrogen leachate is defined as the mass of nitrogen drained through the soil and below the plant root zone. Typically, nitrogen leachate is in the form of nitrate, which drains away easily through soil compared with other forms of nitrogen. As it leaves the plant root zone, nitrate can enter groundwater, eventually feeding into rivers, streams, lakes, and, ultimately, the sea.

    Estimates of livestock’s contribution of nitrogen to the soil are based on modelled livestock productivity and nitrogen-excretion characteristics for dairy cattle, beef cattle, sheep, and deer. Other livestock sources include goats, horses, and swine. Animal population numbers are derived from Statistics New Zealand’s Agricultural Production Survey.

    The amount of nitrogen fertiliser applied to soils is estimated from Fertiliser Association of New Zealand sales records. 

    Data quality

     

    Topic

    Classification 

     Relevance

    Accuracy 

     Sediment, discharges and waste

     National indicator

     

    Direct

    High

    See Data quality information for more detail.

    References

    McDowell, RW, Houlbrooke, DJ, Muirhead, RW, Müller, K, Shepherd, M, & Cuttle, SP (2008). Grazed pastures and surface water quality. New York: Nova Science Publishers.

    Ministry for the Environment (2014). New Zealand’s greenhouse gas inventory 1990–2012. Available from www.mfe.govt.nz.

    Statistics New Zealand (nd). Agricultural Production Survey. Available from www.stats.govt.nz.

    Thomas, SM, Ledgard, SF, & Francis, GS (2005). Improving estimates of nitrate leaching for quantifying New Zealand’s indirect nitrous oxide emissions. Nutrient Cycling in Agroecosystems, 73, 213–226.

    Archived pages

    See Trends in nitrogen leaching from agriculture (archived April 2017).

    Updated 27 April 2017

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