VVM - N-tab fra rodzonen

Summary
Danish farmers, who wish to increase their livestock, have to apply the local authority for approval.
Environmental impact assessments are undertaken as part of the approval procedure,
which previously were undertaken by 14 counties. With the structural reform of the Danish
public sector effective from 1 January 2007, the management of environmental impact assessments
will be transferred to 98 new municipalities. In the further guidance of the municipalities
and to ensure consistency in their future administration, this report proposes a conceptual
model-based tool to calculate nitrate leaching from arable land.
Concept for calculating nitrate leaching from the root zone
The concept is based on a two-step procedure starting with calculating the farm-gate nitrogen
(N) balance (surplus) of the applicant farm, followed by a comparison of the surplus with an
aggregated loss estimate based on the use of four simple, empirical models. Calculation of the
farm-gate N balance are based on national norms for feeding, yield, crop nitrogen requirements,
animal manure production, etc. Farm-specific data for calculating the norm-based
farm-gate N balance can be sourced from the compulsory fertilizer accounts to which the
norms are attached. A study using a larger dataset indicate that N surpluses estimated from
norm-based farm-gate balances are generally in good agreement with the actual N surpluses
from agriculture, where the largest uncertainty factor apparently is the determination of net
yields in crops grown for roughage. The farm-gate balance concept can also be used for calculating
a phosphorus balance.
As part of the calculations of the norm-based farm-gate N balance, we present a summary of
the farm-gate N budget, which provides data for calculation of N losses using four models:
ammonia (emission coefficients), denitrification (SIMDEN), nitrate leaching from the root
zone (N-LES3) and changes to the soil N pool (C-TOOL). The N-LES3 model has been thoroughly
validated, while the other models primarily have been compared with Danish and foreign
research results. As the figure for the norm-based farm-gate N balance is considered to
have a high level of certainty, a comparison of the farm-gate N balance and the sum of the
four losses will give an evaluation of the reliability of the loss estimates. The farm-gate N balance
and the four loss-models will be combined in the programme FarmNTool, including algorithms
to handle minor differences between farm-gate N balance and the aggregated estimate
of N loss with the purpose of respecting mass-conservation of N.
Factors affecting the nitrate reduction
In the management of environmental impact assessment it is assumed that there will be restrictions
on maximum nitrogen loadings for individual recipients (groundwater, fresh and
marine surface waters), as can be expected with the implementation of the EU Water Framework
Directive. In the proposed concept nitrate load to the recipient is calculated as the nitrate
leaching from the root zone of arable land adjusted for nitrate reduction between the root zone
and the recipient. It is advisable to compile maps with indications of expected nitrate reduc-
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