Nitrate leaching from livestock in the Wellington Region, New Zealand

Nitrogen leachate: 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.

In the absence of data on management practice for all farms in New Zealand, the nutrient leaching maps assume average farmer practices, which it is assumed to include effective nutrient and dairy effluent management and avoidance of direct excreta connectivity to waterways. The Nitrate leaching per stock unit was calculated by assuming that stocking rates are an average for that particular type of land and land use.

http://www.mfe.govt.nz/publications/fresh-water/spatial-nitrate-leaching-extent-update-environmental-reporting

The time series is from 1990 to 2017, with no data in years where no APS survey was conducted (1997, 1998, 2000, 2001). In addition, the time series for two regions (Tasman, and Marlborough) begins in 1993. When calculating totals, values for these two regions have been imputed for 1990–92. APS values suppressed for confidentiality have been imputed for use in the nitrate leaching model. Differing historical land use is not taken into account, and we have assumed that leaching rates per animal are independent of stock density.

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This dataset reports on trends in nitrate-nitrogen from livestock that has leached from soil per year across New Zealand since 1990.

Nitrogen is an essential nutrient for plant growth. It occurs naturally, but in agricultural systems more nitrogen is commonly added to soils as fertiliser or as urine or dung from livestock. Not all the additional nitrogen can be used by plants and microorganisms, so some nitrate-nitrogen may leach (drain) from the soil. Livestock urine is the dominant source of nitrate-nitrogen leached from soil. Leached nitrate-nitrogen can enter groundwater and waterways, potentially causing ecological harm. The amount of nitrate-nitrogen leaching from the soil varies around the country as a result of different land uses, climates, and soils.

Data is sourced from Manaaki Whenua, and compiled following the methodology of Dymond et al (2013). model takes a bottom-up approach, estimating nitrate-nitrogen leaching rates per animal in each area in New Zealand taking into account differences in climate, soil type, and irrigation.
The time series of nitrate-nitrogen leaching by region was produced by multiplying the 2017 average nitrate-nitrogen leaching per animal type for each region by the number of animals in that region each year (from the Stats NZ Agricultural Production Survey (APS)).