Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol53.Issue3-4.Feb2000:
Soil & Tillage Research 53 (2000) 245±254
Development and testing of a model for predicting tillage
effects on nitrate leaching from cracked clay soils
A.M. Matthewsa, A.C. Armstronga,*, P.B. Leeds-Harrisonb, G.L. Harrisa, J.A. Cattc
b
a
ADAS Gleadthorpe, Meden Vale, Mans®eld, Notts NG20 9PF, UK
Department of Agricultural Water Management, School of Agriculture, Food and Environment,
Cran®eld University, Silsoe, Bedfordshire MK45 4DT, UK
c
IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK
Accepted 23 March 1999
Abstract
Both water movement and nitrate leaching in structured soils are strongly in¯uenced by the nature of the macro-porosity.
That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential
impacts of tillage on rates of nitrate leaching from structured soils, a model speci®c to these soils, CRACK-NP was developed.
The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone
Farm, Oxfordshire, UK, is described. The model considers the soil as a series of aggregates whose size is also the spacing of
the macro-porosity. Water and solutes move in the macro-pores, but within the peds they move only by diffusion, internal
in®ltration and root uptake (evaporation). The model re¯ects the in¯uence of diffusion limitation in the release of solutes to
by-passing water. The model was then used to investigate the in¯uence of variable ped spacings which were created by
variations in tillage practices. The results both from the model and from the ®eld data demonstrated that ®ner soil structures,
which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction
between peds and the water moving around them, and so release more nitrates through the drainage waters. # 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Nitrate leaching models; Cracking clay soils; Tillage effects; Macro-porosity; Diffusion limitation
1. Introduction
The effects of tillage are perhaps most marked for
clay soils where the formation of aggregates and interaggregate spaces enables both water movement and
the penetration of plant roots. In clay soils, the domi*
Corresponding author. Tel.: 44-1623-844-331; fax: 441623-844-472.
E-mail address: adrian.armstrong@adas.co.uk (A.C. Armstrong).
nant hydrological ¯ow mechanism is the movement of
water through the inter-ped spaces, normally termed
macro-pores, and the ¯ow process is normally known
as macro-pore or by-pass ¯ow (Bouma, 1981; Beven
and Germann, 1982). Because tillage alters the nature
and distribution of aggregates and the spaces between
them, it can have a major in¯uence on soil hydrology
(Armstrong and Harris, 1996), and consequently the
pattern and amounts of solutes being leached (Addiscott and Dexter, 1994). Although some models have
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 1 0 9 - 9
246
A.M. Matthews et al. / Soil & Tillage Research 53 (2000) 245±254
considered the effect of tillage on the movement of
water and solutes through clay soils (e.g., LeedsHarrison et al., 1992), few have considered its impact
on the nitrogen budget of a site.
This paper reports a modelling study which considers the in¯uence of tillage practices on the leaching
of nitrate from structured clay soils. Because conventional leaching models do not perform well for heavily
cracked soils (Armstrong et al., 1995), this has
required the development of a model that explicitly
addresses the issue of water movement in such soils.
Leaching models range from very simple functional
models, which are often based on empirical relationships, to complex mechanistic models, which explicitly consider the physical processes of water and
solute movement but often require large parameter
sets (e.g., Addiscott and Wagenet, 1985, for a review
and classi®cation of leaching models). However, to
facilitate both the representation of tillage effects and
also to address the by-pass ¯ow in clay soils, it is
important that a model has a complete, detailed and
¯exible description of soil structure, and this is possible only by the use of relatively complex models. In
this paper, we describe the characterisation of tillage
effects within the nitrate leaching model CRACK-NP.
The ability of the model to reproduce differences in
short-term leaching behaviour, whilst maintaining a
realistic nitrogen budget over a longer period will be
tested against data collected from the Brimstone Farm
experimental site in Oxfordshire, UK.
2. Field context
Detailed data sets are required for the development
and application of models in order to supply the
necessary input parameters, and to provide the data
for the validation of simulation results (Armstrong et
al., 1996). The data used for this study were provided
by the Brimstone Farm experiment, described by
Cannell et al. (1984), Catt (1991), and Harris et al.
(1993). The site was established in 1978 at Coleshill,
near Faringdon in Oxfordshire on a heavy clay soil of
the Denchworth series, a Verti-Eutric Gleysol (FAO
classi®cation) typical of much of the lowland clay in
arable use in UK (Goss et al., 1988), and some basic
physical properties are summarised in Table 1. This
site has been intensively monitored for most aspects of
site agronomy and hydrology, and so provides an ideal
source of data and parameters for the validation of
leaching models.
The site has 20 plot-scale lysimeters, each of which
is 0.2 ha in area and therefore large enough for normal
agricultural operations to be undertaken. The initial
phase of experimentation (1978±1988) examined the
effects of different soil management practices on the
loss of water and nitrate from these plots. Half the
plots were conventionally ploughed to a depth of
20 cm and the other half were prepared by direct
drilling. Of the 10 plots in each tillage treatment, half
were drained by mole-and-pipe systems and half were
left undrained. The two treatments (tillage and drainage) were the subject of a full factorial design with
®vefold replication. Measurements available from the
drained plots include half-hourly rainfall, water table
and drain¯ow readings and nitrate concentrations in
the drains sampled during ¯ow events. In addition, full
measurements were made of the crop conditions, and
many detailed studies of soil water and the nitrogen
economy of the site were undertaken (Catt, 1991).
Large vertical cracks tend to occur during the
summer months as the soil dries and these can provide
Table 1
Typical soil properties for each horizon (percent for soil components) (information from Cannell et al., 1984)
Horizon (depth (cm))
Sand (60 mm to 2 mm)
Silt (2±60 mm)
Clay (
Development and testing of a model for predicting tillage
effects on nitrate leaching from cracked clay soils
A.M. Matthewsa, A.C. Armstronga,*, P.B. Leeds-Harrisonb, G.L. Harrisa, J.A. Cattc
b
a
ADAS Gleadthorpe, Meden Vale, Mans®eld, Notts NG20 9PF, UK
Department of Agricultural Water Management, School of Agriculture, Food and Environment,
Cran®eld University, Silsoe, Bedfordshire MK45 4DT, UK
c
IACR-Rothamsted, Harpenden, Herts AL5 2JQ, UK
Accepted 23 March 1999
Abstract
Both water movement and nitrate leaching in structured soils are strongly in¯uenced by the nature of the macro-porosity.
That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential
impacts of tillage on rates of nitrate leaching from structured soils, a model speci®c to these soils, CRACK-NP was developed.
The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone
Farm, Oxfordshire, UK, is described. The model considers the soil as a series of aggregates whose size is also the spacing of
the macro-porosity. Water and solutes move in the macro-pores, but within the peds they move only by diffusion, internal
in®ltration and root uptake (evaporation). The model re¯ects the in¯uence of diffusion limitation in the release of solutes to
by-passing water. The model was then used to investigate the in¯uence of variable ped spacings which were created by
variations in tillage practices. The results both from the model and from the ®eld data demonstrated that ®ner soil structures,
which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction
between peds and the water moving around them, and so release more nitrates through the drainage waters. # 2000 Elsevier
Science B.V. All rights reserved.
Keywords: Nitrate leaching models; Cracking clay soils; Tillage effects; Macro-porosity; Diffusion limitation
1. Introduction
The effects of tillage are perhaps most marked for
clay soils where the formation of aggregates and interaggregate spaces enables both water movement and
the penetration of plant roots. In clay soils, the domi*
Corresponding author. Tel.: 44-1623-844-331; fax: 441623-844-472.
E-mail address: adrian.armstrong@adas.co.uk (A.C. Armstrong).
nant hydrological ¯ow mechanism is the movement of
water through the inter-ped spaces, normally termed
macro-pores, and the ¯ow process is normally known
as macro-pore or by-pass ¯ow (Bouma, 1981; Beven
and Germann, 1982). Because tillage alters the nature
and distribution of aggregates and the spaces between
them, it can have a major in¯uence on soil hydrology
(Armstrong and Harris, 1996), and consequently the
pattern and amounts of solutes being leached (Addiscott and Dexter, 1994). Although some models have
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 1 0 9 - 9
246
A.M. Matthews et al. / Soil & Tillage Research 53 (2000) 245±254
considered the effect of tillage on the movement of
water and solutes through clay soils (e.g., LeedsHarrison et al., 1992), few have considered its impact
on the nitrogen budget of a site.
This paper reports a modelling study which considers the in¯uence of tillage practices on the leaching
of nitrate from structured clay soils. Because conventional leaching models do not perform well for heavily
cracked soils (Armstrong et al., 1995), this has
required the development of a model that explicitly
addresses the issue of water movement in such soils.
Leaching models range from very simple functional
models, which are often based on empirical relationships, to complex mechanistic models, which explicitly consider the physical processes of water and
solute movement but often require large parameter
sets (e.g., Addiscott and Wagenet, 1985, for a review
and classi®cation of leaching models). However, to
facilitate both the representation of tillage effects and
also to address the by-pass ¯ow in clay soils, it is
important that a model has a complete, detailed and
¯exible description of soil structure, and this is possible only by the use of relatively complex models. In
this paper, we describe the characterisation of tillage
effects within the nitrate leaching model CRACK-NP.
The ability of the model to reproduce differences in
short-term leaching behaviour, whilst maintaining a
realistic nitrogen budget over a longer period will be
tested against data collected from the Brimstone Farm
experimental site in Oxfordshire, UK.
2. Field context
Detailed data sets are required for the development
and application of models in order to supply the
necessary input parameters, and to provide the data
for the validation of simulation results (Armstrong et
al., 1996). The data used for this study were provided
by the Brimstone Farm experiment, described by
Cannell et al. (1984), Catt (1991), and Harris et al.
(1993). The site was established in 1978 at Coleshill,
near Faringdon in Oxfordshire on a heavy clay soil of
the Denchworth series, a Verti-Eutric Gleysol (FAO
classi®cation) typical of much of the lowland clay in
arable use in UK (Goss et al., 1988), and some basic
physical properties are summarised in Table 1. This
site has been intensively monitored for most aspects of
site agronomy and hydrology, and so provides an ideal
source of data and parameters for the validation of
leaching models.
The site has 20 plot-scale lysimeters, each of which
is 0.2 ha in area and therefore large enough for normal
agricultural operations to be undertaken. The initial
phase of experimentation (1978±1988) examined the
effects of different soil management practices on the
loss of water and nitrate from these plots. Half the
plots were conventionally ploughed to a depth of
20 cm and the other half were prepared by direct
drilling. Of the 10 plots in each tillage treatment, half
were drained by mole-and-pipe systems and half were
left undrained. The two treatments (tillage and drainage) were the subject of a full factorial design with
®vefold replication. Measurements available from the
drained plots include half-hourly rainfall, water table
and drain¯ow readings and nitrate concentrations in
the drains sampled during ¯ow events. In addition, full
measurements were made of the crop conditions, and
many detailed studies of soil water and the nitrogen
economy of the site were undertaken (Catt, 1991).
Large vertical cracks tend to occur during the
summer months as the soil dries and these can provide
Table 1
Typical soil properties for each horizon (percent for soil components) (information from Cannell et al., 1984)
Horizon (depth (cm))
Sand (60 mm to 2 mm)
Silt (2±60 mm)
Clay (