B. Hansen et al., Nitrogen leaching from conventional versus organic farming systems - a systems modelling approach, EUR J AGRON, 13(1), 2000, pp. 65-82
The level of nitrogen leaching from organic compared to conventional farmin
g was evaluated by using a systems modelling approach. Two different method
s were used for estimating and evaluating nitrate leaching. A simple functi
on was used in which nitrate leaching is dependent on percolation, soil cla
y content, average nitrogen input and crop sequence. A nitrogen balance mod
el was used to estimate the long-term potential for nitrate leaching. These
methods were applied to models of both current conventional farming system
s in Denmark in 1996 and of well-managed organic fanning systems. On averag
e, the total estimated nitrogen input to the organic systems was lower (104
-216 kg N ha(-1) year(-1)) than to the conventional farming systems (146-31
1 kg N ha(-1) year(-1)). The N-balances in the organic fields showed a surp
lus of nitrogen (net input of nitrogen) in to the root zone of 60-143 kg N
ha(-1) year(-1). In the conventional systems the surplus varied from 25 to
155 kg N ha(-1) year(-1). The modelled nitrogen leaching from the organic s
ystems varied from 19 to 30 kg N ha(-1) year(-1) on loamy soils to 36-65 kg
N ha(-1) year(-1) on sandy soils. The modelled nitrogen leaching from the
organic systems was always lower than from the comparable conventional agri
cultural systems due to: (I) the lower total input of nitrogen to the organ
ic systems; and (II) the composition of the organic crop rotations includin
g extensive use of catch crops. However, the modelling of nitrogen leaching
has many uncertainties, principally due to difficulties in predicting the
nitrogen leaching from different types of grass fields. Comparison of the r
esults from two methods: (i) modelling of nitrogen leaching; and (ii) N-bal
ances for the root zones, showed that organic arable crop production and da
iry/beef farming on sandy soils are farming systems with a clear potential
for lower nitrogen leaching than from the selected conventional systems. It
is still uncertain whether the nitrogen leaching is lower or higher from o
rganic arable crop production systems on loamy soil and organic pig product
ion on loamy and sandy soil than from the same conventional systems in Denm
ark. The results point to the need for future research in the following are
as: (i) the ability to build up soil organic nitrogen in organic farming sy
stems and the consequences for both the level of crop production and nitrog
en leaching in the long term; (ii) the effects of catch crops in organic cr
op rotations; and (iii) a better operational understanding of nitrogen leac
hing from different types of organically managed grass and grass-clover fie
lds. (C) 2000 Elsevier Science B.V. All rights reserved.