cropsyst training course, piracicaba, brasil, 2010 modelling the nitrogen balance in cropsyst...

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

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


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


Mineral N Balance Components

Fertilization Mineralization N fixation Crop Residues Irrigation water Atmospheric N

Volatilization Denitrification Immobilization Plant uptake Leaching

Input: Output:


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


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


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)


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)


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


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)]


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


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


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


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


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 ]


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


Crop Nitrogen Uptake

bulk soilbulk soilrootsroots

rizosphererizosphere

uptakeuptakemass flowmass flow

diffusiondiffusion


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 ]


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


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


Crop N uptake: Demand Maximum plant N concentration Critical plant N concentration Minimum plant N concentration


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


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 )


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

cropsyst training course, piracicaba, brasil, 2010 modelling the nitrogen balance in cropsyst...

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