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CropSyst Training Course,CropSyst Training Course,Piracicaba, Brasil, 2010Piracicaba, Brasil, 2010
Modelling the Nitrogen Balance in CropSyst
Marcello Donatelli CRA-ISCI, Italy
Claudio Stockle BSE, Washington State University, USA
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CropSyst Course, 2010, Piracicaba, BrasilCropSyst Course, 2010, Piracicaba, Brasil
Modelling the Nitrogen Balance
Nitrogen is an important nutrient whose availability is primordial for crops to reach their potential growth
To model N acquisition by crops, many components of the soil root zone N balance must be considered
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organic organic NN
inorganic inorganic NN
X.NHX.NH44++
cropcropresiduesresidues
N inorg. fertilizerN inorg. fertilizer
N org. fertilizerN org. fertilizer
NHNH44++ NONO33
--
mineralization
immobilization
denitrification
crop uptake(- fixation)
nitrification
leaching
volatilization
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CropSyst Course, 2010, Piracicaba, BrasilCropSyst Course, 2010, Piracicaba, Brasil
Mineral N Balance Components
Fertilization Mineralization N fixation Crop Residues Irrigation water Atmospheric N
Volatilization Denitrification Immobilization Plant uptake Leaching
Input: Output:
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CropSyst Course, 2010, Piracicaba, BrasilCropSyst Course, 2010, Piracicaba, Brasil
Fertilization Mineral or organic N Many sources available Quantity, main N species involved (nitrate or
ammonium), form (organic or inorganic; solid or liquid), and application method must be specified for simulation purposes
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N Symbiotic Fixation Symbiotic N2 fixation converts atmospheric N2 gas
into plant N The quantification of the process is not well
developed The fraction of crop N demand supplied by N
fixation increases with crop development, reaching a maximum at flowering, and decreasing again thereafter
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N Symbiotic Fixation (cont.) N fixation is reduced as the soil dries, becoming
negligible when the soil water content is below the middle range of the water holding capacity
The rate of bacterial fixation is reduced when N is already present (about 50 kg ha-1), becoming negligible at high soil N content (300 kg ha-1)
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N in the Irrigation Water
This quantity can vary greatly from site to site (10 to 150 kg ha-1 year-1)
It is determined as the product of the irrigation volume and the N concentration of the water
Atmospheric N This quantity can vary greatly from site to site (10
to 150 kg ha-1 year-1)
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N transformations in the Soil Net mineralization (mineralization - immobilization) Nitrification (no direct effect on balance) Denitrification Microbiologically-mediated processes Usually modelled assuming that they obey irreversible
first-order kinetics
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Nitrogen Transformations
Nt = Amount transformed during time interval t (kg m-2 t-1)
N0 = Amount available for transformation (kg m-2 t-1)
K = Rate constant (t-1)
Nt = N0 [1 - e (-K t)]
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Nitrogen Transformations
Note that Note that KK = = f f ( ( Temp, SWCTemp, SWC))
N0 = 100 kg/ha
0
20
40
60
80
100
0 25 50 75 100days
N tr
ansf
orm
ed
(kg/
ha)
K=0.1 K=0.01
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Nitrogen Volatilization Important when N is applied as ammonium and
is not incorporated to the soil Can be simulated mechanistically based on
gas concentration gradients and resistances A simpler approach subtracts a fixed fraction
which depends on application conditions
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Nitrogen Leaching Related to the movement of water in the soil and the
mobility of the N species of interest (nitrate or ammonium)
N transport can be simulated for both the cascading and the finite difference approaches
N transport in the soil is also important to determine where the N is available in the soil profile
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Nitrogen Leaching N transport in the soil may be obtained by multiplying
water fluxes between layers by the N concentration of the water in the layer originating the flow
The effect of diffusion and hydrodynamic dispersion may also be added
While nitrates are not retained by the soil, the movement of ammonium is restricted due to its absorption by the solid matrix
The amount retained and the amount in concentration are related by a Langmuir relation
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Ammonium Sorption
X.NH4 = Ammonium in exchage sites (kg kg-1)
Soil.NH4 = Total amount in the soil (kg m-3)
w = Gravimetric soil water content (kg kg-1)b = Bulk density (kg m-3)
K, Q = Constants (kg kg-1)
Soil.NH4 = [X.NH4 + w NH4] b
X.NH4 =K Q [NH4 ]
1 + K [NH4 ]
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Crop Nitrogen Uptake N uptake is the component of the balance that
relates directly with the simulation of crop growth Usually is calculated once the concentration of
nitrate and ammonium in the soil solution is known for each soil layer
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Crop Nitrogen Uptake
bulk soilbulk soilrootsroots
rizosphererizosphere
uptakeuptakemass flowmass flow
diffusiondiffusion
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Potential N Crop Uptake
Nup = Potential N uptake per unit root length
Numax = Maximum N uptake per unit root length
Nr = N concentration in the rizosphere
K = Half-rate constant
Nup =Numax [Nr ]
K + [Nr ]
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CropSyst Course, 2010, Piracicaba, BrasilCropSyst Course, 2010, Piracicaba, Brasil
Potential N Crop Uptake
Nup = Potential N uptake per unit root length
Numax = Maximum N uptake per unit root length
Navail = Availability factor [ e fn(N in bulk soil)]
PAW = Plant available water
Nup = Numax Navail PAW 2
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Growth Limited by Nitrogen
growth limitedby nitrogen
min
1ppcrit
ppcritN NN
NNBB
B = growth limited by radiation and waterNpcrit = critical plant N concentrationNp = plant N concentrationNpmin = minimum plant N concentration
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Crop N uptake: Demand Maximum plant N concentration Critical plant N concentration Minimum plant N concentration
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Crop N uptake: Demand
Nd = Plant N demand
Npmax = Maximum plant N concentration
Np = Current plant N concentration
Bc = Current cumulative biomass
Bt = Potential biomass to be produced today
Nd = (Npmax- Np) Bc + Npmax Bt
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Actual Crop N Uptake
Note that Nup for the entire soil profile is
determined by the sum of the product of the potential N uptake and the root length for each layer
Nact = MIN ( Nup , ND )
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Modelling the Nitrogen Balance Other N input/output components exist. For example,
some of the plant N (2 to 8%) is lost to the atmosphere (as ammonia and volatile amines) during plant senescence.
The N balance has more uncertainties and it is more complex to study and model than the water balance