Description of the Nitrogen Model
The Model
program estimates the response of twenty four different C3 arable crops to
N-fertilizer and the way it is affected by time, by soil type, by cultural
practice and by weather. It also calculates crop nitrogen and nitrate contents,
the distributions of water and nitrate down the soil profile and the amounts of
nitrate leached below different depths from the soil surface. Weather files
have been prepared to represent the daily mean, temperature, rainfall, and potential
evaporation for different parts of the world. To run the model it is necessary
to select the most appropriate weather file and make any adjustments to monthly
rainfall. Although it should therefore be possible to simulate N-response of
crops grown in many countries, it must be emphasised that the validity of the
model has only been tested in
In the model the
soil is visualized as consisting of 20 consecutive 5 cm thick layers. Roots
develop laterally and vertically in the soil. The volume of soil from which
they can extract mineral-N increases with plant mass until a stage is reached
at which further root development ceases or until the roots reach a hard pan or
other barrier to root penetration.
Fertilizer and
crop debris are incorporated in the uppermost layers of soil. Microbial
breakdown of the endogenous soil organic matter always increases soil
mineral-N. But mineral-N is either produced or immobilized during the
decomposition of crop debris, depending on its C/N ratio. When mineral- N is
released it is first converted to ammonium-N which is then nitrified to
nitrate-N, which it is assumed, is not absorbed on soil. It can be taken up by
plant roots, can be leached downwards during rain or can move upwards during
evaporation from the soil surface.
Crucial to the
estimation of plant growth are two crop-nitrogen parameters, the critical plant
N concentration (the minimum concentration for maximum growth rate and the
maximum possible N concentration, and the way they decline with increasing
plant growth. This information is provided by a file ancillary to the program.
Plant growth is calculated from the daily mean air temperatures, from the plant
mass per unit area, and from its N concentration relative to the critical
concentration for a plant of the same weight.
To calculate
crop uptake first the potential demand of the crop for nitrogen is calculated
from its mass per unit area, from its N content, from the maximum possible
concentration for a plant of the same mass and from the potential maximum
increment in weight. Then the maximum amount of N which the plant roots can
extract from soil is estimated. If this is less than crop demand then it
determines uptake.
Transpiration is
calculated from fractional crop cover (derived from the plant mass), the rate
of loss of water from an open water surface (a component of the weather file)
and the amount of water in the rooting zone. Evaporation from bare soil
likewise depends soil water distribution down the profile and on the
evaporative conditions.
The foregoing
processes are represented by equations and the calculations are repeated for
each day and for each layer of soil.
The effects of
water stress are assumed to be taken account of by the input of the maximum
yield. It is assumed that crop growth and thus crop demand for N is more
sensitive to dry conditions than the ability of the roots to extract that
demand from soil. In consequence the model does not include routines for the
effects of low soil moisture content on the transport of mineral-N to the
roots.
The model has
been developed for use of ammonium and nitrate based fertilizers. It does not
include equations for any adverse effects of fertilizer-N on plant growth such
as those caused by high osmotic stress or ammonia toxicity. Nor does it deal
explicitly with ammonia volatilization.
Flow Diagram of the Nitrogen
Model
Most of the
algorithms in the model are given in
The main
exception is that most of the critical-N and maximum possible N-concentrations
in the plants and the way they decline with increase in plant mass are taken
from
Estimation of
mineralization rate of soil organic matter
The
Nitrogen Model assumes that mineralization rate is dependent on temperature in
degrees centigrade. But even when account is taken of temperature,
mineralization varies greatly from soil to soil, partly as a consequence of
differences in organic matter content and in soil texture. Most arable soils
have an organic carbon content of less than 4% .and a C/N ratio of between 9
and 12 in the top 30 cm. For these soils the mineralization rate (MNRLT) of
soil organic matter in kg of N/ha/day at 15 degrees centigrade can be
calculated from the percentage organic-C in soil (CORG) and the percentage by
weight of soil mineral particles that are less than 20 micro metres
( x ), a value that can be obtained by interpolation of standard soil
mechanical analyses. First the decomposable %C (CDEC) is calculated as which
ever is the larger
CDEC = 0.1 or CDEC = CORG -0.017 * x + 0.001 * EXP(0.075 *
x)
and
then MNRLT is calculated as
MNRLT = 674 * CDEC * [ 0.0069 * exp(-4.294 * CDEC) +
0.0012]
The
value 674 is a calibration coefficient
for a site in the
The above
formulae are derived from:
RUHLMANN, J. (1999). A new approach
to estimating the pool of stable organic matter in soil using data from long
term field experiments. Plant and Soil 243, 149-162.
RUHLMANN, J. (1999). Calculation of
net mineralization from the decomposable soil organic matter pool. Acta
Horticulturae 506, 167-173.
These estimates
can only be very approximate as mineralization rates are also very dependent,
in ways that are not fully understood, on factors such the type of soil organic
matter, previous cropping, wetting and drying cycles and cultivation practices.
NON-OBVIOUS
APPLICATIONS OF THE N MODEL
- Estimation of daily weather for
any of 134 regions throughout the world.
- Select the region
- Click its link to get the
'Simple Input' page
- Scroll down and click on the
RUN button and wait for output
- Click on the 'Detailed output'
link
- Click on the 'Weather input'
link
The displayed page shows four columns, namely: the day
number of the year, the mean of the minimum and maximum temperature (deg. C),
the evaporation from an open water surface (mm) and the rainfall (mm). You can
save the page as a weather file or print it directly from your browser.
- Estimation of maximum yield
(total dry matter excluding fibrous roots) for 25 crops grown in any of
the 134 climatic regions.
- Select the region
- Click its link to get the
'Simple Input' page
- Click the link for 'Advanced
Input'
- Select a crop
- Enter a sowing/planting date
- Enter a harvest date
- Enter zero for every month in
the rainfall table
- Click on the RUN button and
wait for output
A drought warning then appears which gives an estimate of
the maximum plant dry weight assuming that the crop is healthy and growth is
never limited by shortage of either water or nutrients.
- Estimation of the rates of
processes in fallow soils.
By setting the maximum yield to the low value of 0.1 t/ha
(thus minimizing crop-N uptake) and running the model in the usual way the
following may be estimated from the 10-day outputs:
- Evaporation from the soil
surface.
- The amount of nitrate-N
leached below the maximum depth of rooting and 90 cm from the soil
surface.
- The amount of mineral-N that
is either mineralized or immobilized during the decomposition of crop
debris. If the carbon/nitrogen ratio of the crop debris is high the
breakdown results in a disaappearance of soil-mineral by immobilization
into newly formed organic materials. The rate of breakdown is then
limited by the amounts of soil mineral-N. If the carbon/nitrogen ratio is
low decomposition results in the release of mineral-N into soil. So the
carbon/nitrogen ratio of the crop residues has a decisive influence on
determining the effects of their decomposition on the N-economy of soil.
- To determine the amounts of
mineral-N either released or immobilized during decomposition the model
should be run with the required soil mineral-N (and fertilizer-N
application) first with crop debris and then without crop debris. The
difference in mineral -N between the two outputs gives the
release/immobilization of mineral-N during decompositoion.
- Effect of soil type
and rainfall on leaching losses.
This can be estimated for cropped soils by modifying the
volumetric water content at field capacity according to the soil type (see
table below) and by modifying the input monthly rainfall. It can also be
estimated for fallow soils by making, in addition to the above changes, the
maximum plant dry weight equal to 0.1 t/ha thereby minimizing the effect of
plant growth on the calculated N-economy of the soil.
|
SOIL
TEXTURE AND FIELD CAPACITY |
|
|
Soil Texture |
Volumetric
water content |
|
Loamy
sand |
0.18 |
|
|
0.22 |
|
Loam |
0.34 |
|
Silty
clay loam |
0.46 |
|
Clay |
0.42 |
The volumetric water contents in the table above are median values of
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BURNS,
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