Gorse is a nitrogen fixing legume often present on undeveloped blocks.
In May 2017 the "Industry Overseer Working Group" was asked the question, "How do I model gorse and broom, given it’s a plant pest I’m required to control; and if I model as “Trees and Scrub” the low N loss does not allow me to convert to farming without increasing N loss which may breach my limit".
The IOWG Decision Register records the advice -
Model gorse and broom as low productivity dryland sheep.
This will result in the N loss being slightly higher than if modelled as “Trees and Scrub” and will allow land to be cleared and lightly grazed without exceeding N loss limits.
This advice was developed following review of the two papers attached Magesan & Wang "Nitrogen Leaching from gorse - Final Report"; and Liu et al "Gorse is a "facultive" N2 fixer".
Overseer Limited is not aware of any ongoing work into the subject of N leaching from Gorse, and so accepts the advice of the IOWG in the development of a proxy for land containing gorse.