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.
The data provider assumes 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).
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.